diff --git a/Lib/test/test_tstring.py b/Lib/test/test_tstring.py
index e91bf3f8b4e..a1f686c8f56 100644
--- a/Lib/test/test_tstring.py
+++ b/Lib/test/test_tstring.py
@@ -4,7 +4,6 @@
 
 
 class TestTString(unittest.TestCase, TStringBaseCase):
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; + Template(strings=('Hello',), interpolations=())
     def test_string_representation(self):
         # Test __repr__
         t = t"Hello"
diff --git a/crates/common/src/str.rs b/crates/common/src/str.rs
index 79c407909ff..38e73a683f2 100644
--- a/crates/common/src/str.rs
+++ b/crates/common/src/str.rs
@@ -258,6 +258,7 @@ impl StrData {
         &self.data
     }
 
+    // TODO: rename to to_str
     #[inline]
     pub fn as_str(&self) -> Option<&str> {
         self.kind
@@ -429,13 +430,13 @@ pub fn zfill(bytes: &[u8], width: usize) -> Vec<u8> {
 
 /// Convert a string to ascii compatible, escaping unicode-s into escape
 /// sequences.
-pub fn to_ascii(value: &str) -> AsciiString {
+pub fn to_ascii(value: &Wtf8) -> AsciiString {
     let mut ascii = Vec::new();
-    for c in value.chars() {
-        if c.is_ascii() {
-            ascii.push(c as u8);
+    for cp in value.code_points() {
+        if cp.is_ascii() {
+            ascii.push(cp.to_u32() as u8);
         } else {
-            let c = c as i64;
+            let c = cp.to_u32();
             let hex = if c < 0x100 {
                 format!("\\x{c:02x}")
             } else if c < 0x10000 {
diff --git a/crates/stdlib/src/_asyncio.rs b/crates/stdlib/src/_asyncio.rs
index d7b5dad3c9d..2733e801251 100644
--- a/crates/stdlib/src/_asyncio.rs
+++ b/crates/stdlib/src/_asyncio.rs
@@ -6,6 +6,7 @@ pub(crate) use _asyncio::module_def;
 
 #[pymodule]
 pub(crate) mod _asyncio {
+    use crate::common::wtf8::{Wtf8Buf, wtf8_concat};
     use crate::{
         common::lock::PyRwLock,
         vm::{
@@ -859,7 +860,7 @@ pub(crate) mod _asyncio {
         }
     }
 
-    fn get_future_repr_info(future: &PyObject, vm: &VirtualMachine) -> PyResult<String> {
+    fn get_future_repr_info(future: &PyObject, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // Try to use asyncio.base_futures._future_repr_info
         // Import from sys.modules if available, otherwise try regular import
         let sys_modules = vm.sys_module.get_attr("modules", vm)?;
@@ -892,29 +893,34 @@ pub(crate) mod _asyncio {
             Err(_) => return get_future_repr_info_fallback(future, vm),
         };
 
-        let parts: Vec<String> = list
-            .borrow_vec()
-            .iter()
-            .filter_map(|x: &PyObjectRef| x.str(vm).ok().map(|s| s.as_str().to_string()))
-            .collect();
-        Ok(parts.join(" "))
+        let mut result = Wtf8Buf::new();
+        let parts = list.borrow_vec();
+        for (i, x) in parts.iter().enumerate() {
+            if i > 0 {
+                result.push_str(" ");
+            }
+            if let Ok(s) = x.str(vm) {
+                result.push_wtf8(s.as_wtf8());
+            }
+        }
+        Ok(result)
     }
 
-    fn get_future_repr_info_fallback(future: &PyObject, vm: &VirtualMachine) -> PyResult<String> {
+    fn get_future_repr_info_fallback(future: &PyObject, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // Fallback: build repr from properties directly
         if let Ok(Some(state)) =
             vm.get_attribute_opt(future.to_owned(), vm.ctx.intern_str("_state"))
         {
-            let state_str = state
+            let s = state
                 .str(vm)
-                .map(|s| s.as_str().to_lowercase())
-                .unwrap_or_else(|_| "unknown".to_string());
-            return Ok(state_str);
+                .map(|s| s.as_wtf8().to_lowercase())
+                .unwrap_or_else(|_| Wtf8Buf::from("unknown"));
+            return Ok(s);
         }
-        Ok("state=unknown".to_string())
+        Ok(Wtf8Buf::from("state=unknown"))
     }
 
-    fn get_task_repr_info(task: &PyObject, vm: &VirtualMachine) -> PyResult<String> {
+    fn get_task_repr_info(task: &PyObject, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // vm.import returns the top-level module, get base_tasks submodule
         match vm
             .import("asyncio.base_tasks", 0)
@@ -927,12 +933,15 @@ pub(crate) mod _asyncio {
                         let list: PyListRef = info.downcast().map_err(|_| {
                             vm.new_type_error("_task_repr_info should return a list")
                         })?;
-                        let parts: Vec<String> = list
-                            .borrow_vec()
-                            .iter()
-                            .map(|x: &PyObjectRef| x.str(vm).map(|s| s.as_str().to_string()))
-                            .collect::<PyResult<Vec<_>>>()?;
-                        Ok(parts.join(" "))
+                        let mut result = Wtf8Buf::new();
+                        let parts = list.borrow_vec();
+                        for (i, x) in parts.iter().enumerate() {
+                            if i > 0 {
+                                result.push_str(" ");
+                            }
+                            result.push_wtf8(x.str(vm)?.as_wtf8());
+                        }
+                        Ok(result)
                     }
                     _ => get_future_repr_info(task, vm),
                 }
@@ -1928,40 +1937,28 @@ pub(crate) mod _asyncio {
     }
 
     impl Representable for PyTask {
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             let class_name = zelf.class().name().to_string();
 
             if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
                 // Try to use _task_repr_info if available
                 if let Ok(info) = get_task_repr_info(zelf.as_object(), vm)
-                    && info != "state=unknown"
+                    && info.as_bytes() != b"state=unknown"
                 {
-                    return Ok(format!("<{} {}>", class_name, info));
+                    return Ok(wtf8_concat!("<", class_name, " ", info, ">"));
                 }
 
                 // Fallback: build repr from task properties directly
                 let state = zelf.base.fut_state.load().as_str().to_lowercase();
-                let name = zelf
-                    .task_name
-                    .read()
-                    .as_ref()
-                    .and_then(|n| n.str(vm).ok())
-                    .map(|s| s.as_str().to_string())
-                    .unwrap_or_else(|| "?".to_string());
-                let coro_repr = zelf
-                    .task_coro
-                    .read()
-                    .as_ref()
-                    .and_then(|c| c.repr(vm).ok())
-                    .map(|s| s.as_str().to_string())
-                    .unwrap_or_else(|| "?".to_string());
-
-                Ok(format!(
-                    "<{} {} name='{}' coro={}>",
-                    class_name, state, name, coro_repr
+                let name = zelf.task_name.read().as_ref().and_then(|n| n.str(vm).ok());
+                let coro_repr = zelf.task_coro.read().as_ref().and_then(|c| c.repr(vm).ok());
+                let name = name.as_ref().map_or("?".as_ref(), |s| s.as_wtf8());
+                let coro_repr = coro_repr.as_ref().map_or("?".as_ref(), |s| s.as_wtf8());
+                Ok(wtf8_concat!(
+                    "<", class_name, " ", state, " name='", name, "' coro=", coro_repr, ">"
                 ))
             } else {
-                Ok(format!("<{} ...>", class_name))
+                Ok(Wtf8Buf::from(format!("<{class_name} ...>")))
             }
         }
     }
@@ -2151,10 +2148,8 @@ pub(crate) mod _asyncio {
         // Check if task awaits on itself
         let task_obj: PyObjectRef = task.clone().into();
         if result.is(&task_obj) {
-            let msg = format!(
-                "Task cannot await on itself: {}",
-                task_obj.repr(vm)?.as_str()
-            );
+            let task_repr = task_obj.repr(vm)?;
+            let msg = format!("Task cannot await on itself: {}", task_repr.as_wtf8());
             task.base.fut_state.store(FutureState::Finished);
             *task.base.fut_exception.write() = Some(vm.new_runtime_error(msg).into());
             PyTask::schedule_callbacks(task, vm)?;
@@ -2254,7 +2249,8 @@ pub(crate) mod _asyncio {
                 vm.call_method(&loop_obj, "call_soon", (step_wrapper,))?;
             }
         } else {
-            let msg = format!("Task got bad yield: {}", result.repr(vm)?.as_str());
+            let result_repr = result.repr(vm)?;
+            let msg = format!("Task got bad yield: {}", result_repr.as_wtf8());
             task.base.fut_state.store(FutureState::Finished);
             *task.base.fut_exception.write() = Some(vm.new_runtime_error(msg).into());
             PyTask::schedule_callbacks(task, vm)?;
diff --git a/crates/stdlib/src/_sqlite3.rs b/crates/stdlib/src/_sqlite3.rs
index f7ae445fe81..dbb13577514 100644
--- a/crates/stdlib/src/_sqlite3.rs
+++ b/crates/stdlib/src/_sqlite3.rs
@@ -89,7 +89,7 @@ mod _sqlite3 {
                 $(
                     #[allow(dead_code)]
                     fn [<new_ $x:snake>](vm: &VirtualMachine, msg: String) -> PyBaseExceptionRef {
-                        vm.new_exception_msg([<$x:snake _type>]().to_owned(), msg)
+                        vm.new_exception_msg([<$x:snake _type>]().to_owned(), msg.into())
                     }
                     fn [<$x:snake _type>]() -> &'static Py<PyType> {
                         [<$x:snake:upper>].get().expect("exception type not initialize")
@@ -723,7 +723,7 @@ mod _sqlite3 {
         converter: ArgCallable,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        let name = typename.as_str().to_uppercase();
+        let name = typename.expect_str().to_uppercase();
         converters().set_item(&name, converter.into(), vm)
     }
 
@@ -2194,8 +2194,8 @@ mod _sqlite3 {
                     let Some(obj) = obj.downcast_ref::<PyStr>() else {
                         break;
                     };
-                    let a_iter = name.as_str().chars().flat_map(|x| x.to_uppercase());
-                    let b_iter = obj.as_str().chars().flat_map(|x| x.to_uppercase());
+                    let a_iter = name.expect_str().chars().flat_map(|x| x.to_uppercase());
+                    let b_iter = obj.expect_str().chars().flat_map(|x| x.to_uppercase());
 
                     if a_iter.eq(b_iter) {
                         return self.data.getitem_by_index(vm, i);
@@ -2918,7 +2918,7 @@ mod _sqlite3 {
             };
             let mut s = Vec::with_capacity(16);
             s.extend(b"BEGIN ");
-            s.extend(isolation_level.as_str().bytes());
+            s.extend(isolation_level.expect_str().bytes());
             s.push(b'\0');
             self._exec(&s, vm)
         }
@@ -3469,7 +3469,7 @@ mod _sqlite3 {
             return e;
         }
 
-        vm.new_exception_msg_dict(typ, msg, dict)
+        vm.new_exception_msg_dict(typ, msg.into(), dict)
     }
 
     static BEGIN_STATEMENTS: &[&[u8]] = &[
diff --git a/crates/stdlib/src/array.rs b/crates/stdlib/src/array.rs
index 9877d0dcc4f..656b5028623 100644
--- a/crates/stdlib/src/array.rs
+++ b/crates/stdlib/src/array.rs
@@ -19,7 +19,7 @@ mod array {
             builtins::{
                 PositionIterInternal, PyByteArray, PyBytes, PyBytesRef, PyDictRef, PyFloat,
                 PyGenericAlias, PyInt, PyList, PyListRef, PyStr, PyStrRef, PyTupleRef, PyType,
-                PyTypeRef, builtins_iter,
+                PyTypeRef, PyUtf8StrRef, builtins_iter,
             },
             class_or_notimplemented,
             convert::{ToPyObject, ToPyResult, TryFromBorrowedObject, TryFromObject},
@@ -559,7 +559,7 @@ mod array {
 
     impl ArrayElement for WideChar {
         fn try_into_from_object(vm: &VirtualMachine, obj: PyObjectRef) -> PyResult<Self> {
-            PyStrRef::try_from_object(vm, obj)?
+            PyUtf8StrRef::try_from_object(vm, obj)?
                 .as_str()
                 .chars()
                 .exactly_one()
@@ -625,7 +625,7 @@ mod array {
     #[derive(FromArgs)]
     pub struct ArrayNewArgs {
         #[pyarg(positional)]
-        spec: PyStrRef,
+        spec: PyUtf8StrRef,
         #[pyarg(positional, optional)]
         init: OptionalArg<PyObjectRef>,
     }
@@ -884,7 +884,7 @@ mod array {
             if not_enough_bytes {
                 Err(vm.new_exception_msg(
                     vm.ctx.exceptions.eof_error.to_owned(),
-                    "read() didn't return enough bytes".to_owned(),
+                    "read() didn't return enough bytes".into(),
                 ))
             } else {
                 Ok(())
@@ -1425,7 +1425,7 @@ mod array {
         #[pyarg(positional)]
         arraytype: PyTypeRef,
         #[pyarg(positional)]
-        typecode: PyStrRef,
+        typecode: PyUtf8StrRef,
         #[pyarg(positional)]
         mformat_code: MachineFormatCode,
         #[pyarg(positional)]
@@ -1568,7 +1568,7 @@ mod array {
         Ok(typ)
     }
 
-    fn check_type_code(spec: PyStrRef, vm: &VirtualMachine) -> PyResult<ArrayContentType> {
+    fn check_type_code(spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<ArrayContentType> {
         let spec = spec.as_str().chars().exactly_one().map_err(|_| {
             vm.new_type_error(
                 "_array_reconstructor() argument 2 must be a unicode character, not str",
diff --git a/crates/stdlib/src/binascii.rs b/crates/stdlib/src/binascii.rs
index ee55d482e4c..9af56d4eea7 100644
--- a/crates/stdlib/src/binascii.rs
+++ b/crates/stdlib/src/binascii.rs
@@ -849,7 +849,7 @@ mod decl {
 struct Base64DecodeError(base64::DecodeError);
 
 fn new_binascii_error(msg: String, vm: &VirtualMachine) -> PyBaseExceptionRef {
-    vm.new_exception_msg(decl::error_type(vm), msg)
+    vm.new_exception_msg(decl::error_type(vm), msg.into())
 }
 
 impl ToPyException for Base64DecodeError {
diff --git a/crates/stdlib/src/contextvars.rs b/crates/stdlib/src/contextvars.rs
index 883ef6f6820..1fc98edb5a7 100644
--- a/crates/stdlib/src/contextvars.rs
+++ b/crates/stdlib/src/contextvars.rs
@@ -15,7 +15,7 @@ mod _contextvars {
         AsObject, Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine, atomic_func,
         builtins::{PyGenericAlias, PyList, PyStrRef, PyType, PyTypeRef},
         class::StaticType,
-        common::hash::PyHash,
+        common::{hash::PyHash, wtf8::Wtf8Buf},
         function::{ArgCallable, FuncArgs, OptionalArg},
         protocol::{PyMappingMethods, PySequenceMethods},
         types::{AsMapping, AsSequence, Constructor, Hashable, Iterable, Representable},
@@ -333,7 +333,7 @@ mod _contextvars {
             if vars.swap_remove(zelf).is_none() {
                 // TODO:
                 // PyErr_SetObject(PyExc_LookupError, (PyObject *)var);
-                let msg = zelf.as_object().repr(vm)?.as_str().to_owned();
+                let msg = zelf.as_object().repr(vm)?.as_wtf8().to_owned();
                 return Err(vm.new_lookup_error(msg));
             }
 
@@ -409,7 +409,7 @@ mod _contextvars {
                 default.clone()
             } else {
                 let msg = zelf.as_object().repr(vm)?;
-                return Err(vm.new_lookup_error(msg.as_str().to_owned()));
+                return Err(vm.new_lookup_error(msg.as_wtf8().to_owned()));
             };
             Ok(Some(value))
         }
@@ -611,11 +611,14 @@ mod _contextvars {
 
     impl Representable for ContextToken {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             let used = if zelf.used.get() { " used" } else { "" };
-            let var = Representable::repr_str(&zelf.var, vm)?;
+            let var = Representable::repr_wtf8(&zelf.var, vm)?;
             let ptr = zelf.as_object().get_id() as *const u8;
-            Ok(format!("<Token{used} var={var} at {ptr:p}>"))
+            let mut result = Wtf8Buf::from(format!("<Token{used} var="));
+            result.push_wtf8(&var);
+            result.push_str(&format!(" at {ptr:p}>"));
+            Ok(result)
         }
     }
 
diff --git a/crates/stdlib/src/csv.rs b/crates/stdlib/src/csv.rs
index b898dc8c106..a1147fd1cbb 100644
--- a/crates/stdlib/src/csv.rs
+++ b/crates/stdlib/src/csv.rs
@@ -6,7 +6,7 @@ mod _csv {
     use crate::vm::{
         AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject,
         VirtualMachine,
-        builtins::{PyBaseExceptionRef, PyInt, PyNone, PyStr, PyType, PyTypeRef},
+        builtins::{PyBaseExceptionRef, PyInt, PyNone, PyStr, PyType, PyTypeRef, PyUtf8StrRef},
         function::{ArgIterable, ArgumentError, FromArgs, FuncArgs, OptionalArg},
         protocol::{PyIter, PyIterReturn},
         raise_if_stop,
@@ -16,7 +16,7 @@ mod _csv {
     use csv_core::Terminator;
     use itertools::{self, Itertools};
     use parking_lot::Mutex;
-    use rustpython_common::lock::LazyLock;
+    use rustpython_common::{lock::LazyLock, wtf8::Wtf8Buf};
     use rustpython_vm::{match_class, sliceable::SliceableSequenceOp};
     use std::collections::HashMap;
 
@@ -50,8 +50,8 @@ mod _csv {
     });
     static GLOBAL_FIELD_LIMIT: LazyLock<Mutex<isize>> = LazyLock::new(|| Mutex::new(131072));
 
-    fn new_csv_error(vm: &VirtualMachine, msg: String) -> PyBaseExceptionRef {
-        vm.new_exception_msg(super::_csv::error(vm), msg)
+    fn new_csv_error(vm: &VirtualMachine, msg: impl Into<Wtf8Buf>) -> PyBaseExceptionRef {
+        vm.new_exception_msg(super::_csv::error(vm), msg.into())
     }
 
     #[pyattr]
@@ -138,7 +138,7 @@ mod _csv {
         } else {
             match_class!(match obj.to_owned() {
                 s @ PyStr => {
-                    Ok(s.as_str().bytes().exactly_one().map_err(|_| {
+                    Ok(s.as_bytes().iter().copied().exactly_one().map_err(|_| {
                         vm.new_type_error(format!(
                             r#""delimiter" must be a unicode character, not a string of length {}"#,
                             s.len()
@@ -159,19 +159,16 @@ mod _csv {
     fn parse_quotechar_from_obj(vm: &VirtualMachine, obj: &PyObject) -> PyResult<Option<u8>> {
         match_class!(match obj.get_attr("quotechar", vm)? {
             s @ PyStr => {
-                Ok(Some(s.as_str().bytes().exactly_one().map_err(|_| {
-                    vm.new_exception_msg(
-                        super::_csv::error(vm),
-                        format!(r#""quotechar" must be a unicode character or None, not a string of length {}"#, s.len()),
-                    )
+                Ok(Some(s.as_bytes().iter().copied().exactly_one().map_err(|_| {
+                    new_csv_error(vm, format!(r#""quotechar" must be a unicode character or None, not a string of length {}"#, s.len()))
                 })?))
             }
             _n @ PyNone => {
                 Ok(None)
             }
             attr => {
-                Err(vm.new_exception_msg(
-                    super::_csv::error(vm),
+                Err(new_csv_error(
+                    vm,
                     format!(
                         r#""quotechar" must be a unicode character or None, not {}"#,
                         attr.class()
@@ -183,9 +180,9 @@ mod _csv {
     fn parse_escapechar_from_obj(vm: &VirtualMachine, obj: &PyObject) -> PyResult<Option<u8>> {
         match_class!(match obj.get_attr("escapechar", vm)? {
             s @ PyStr => {
-                Ok(Some(s.as_str().bytes().exactly_one().map_err(|_| {
-                    vm.new_exception_msg(
-                        super::_csv::error(vm),
+                Ok(Some(s.as_bytes().iter().copied().exactly_one().map_err(|_| {
+                    new_csv_error(
+                        vm,
                         format!(r#""escapechar" must be a unicode character or None, not a string of length {}"#, s.len()),
                     )
                 })?))
@@ -213,10 +210,7 @@ mod _csv {
                     // only capture the first character
                     csv_core::Terminator::Any(*t)
                 } else {
-                    return Err(vm.new_exception_msg(
-                        super::_csv::error(vm),
-                        r#""lineterminator" must be a string"#.to_owned(),
-                    ));
+                    return Err(new_csv_error(vm, r#""lineterminator" must be a string"#));
                 })
             }
             attr => {
@@ -278,9 +272,10 @@ mod _csv {
         mut _rest: FuncArgs,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        let Some(name) = name.downcast_ref::<PyStr>() else {
-            return Err(vm.new_type_error("argument 0 must be a string"));
-        };
+        let name = name
+            .downcast::<PyStr>()
+            .map_err(|_| vm.new_type_error("argument 0 must be a string"))?;
+        let name: PyUtf8StrRef = name.try_into_utf8(vm)?;
         let dialect = match dialect {
             OptionalArg::Present(d) => PyDialect::try_from_object(vm, d)
                 .map_err(|_| vm.new_type_error("argument 1 must be a dialect object"))?,
@@ -299,17 +294,18 @@ mod _csv {
         mut _rest: FuncArgs,
         vm: &VirtualMachine,
     ) -> PyResult<PyDialect> {
-        let Some(name) = name.downcast_ref::<PyStr>() else {
-            return Err(vm.new_exception_msg(
-                super::_csv::error(vm),
-                format!("argument 0 must be a string, not '{}'", name.class()),
-            ));
-        };
+        let name = name.downcast::<PyStr>().map_err(|obj| {
+            new_csv_error(
+                vm,
+                format!("argument 0 must be a string, not '{}'", obj.class()),
+            )
+        })?;
+        let name: PyUtf8StrRef = name.try_into_utf8(vm)?;
         let g = GLOBAL_HASHMAP.lock();
         if let Some(dialect) = g.get(name.as_str()) {
             return Ok(*dialect);
         }
-        Err(vm.new_exception_msg(super::_csv::error(vm), "unknown dialect".to_string()))
+        Err(new_csv_error(vm, "unknown dialect"))
     }
 
     #[pyfunction]
@@ -318,17 +314,18 @@ mod _csv {
         mut _rest: FuncArgs,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        let Some(name) = name.downcast_ref::<PyStr>() else {
-            return Err(vm.new_exception_msg(
-                super::_csv::error(vm),
-                format!("argument 0 must be a string, not '{}'", name.class()),
-            ));
-        };
+        let name = name.downcast::<PyStr>().map_err(|obj| {
+            new_csv_error(
+                vm,
+                format!("argument 0 must be a string, not '{}'", obj.class()),
+            )
+        })?;
+        let name: PyUtf8StrRef = name.try_into_utf8(vm)?;
         let mut g = GLOBAL_HASHMAP.lock();
         if let Some(_removed) = g.remove(name.as_str()) {
             return Ok(());
         }
-        Err(vm.new_exception_msg(super::_csv::error(vm), "unknown dialect".to_string()))
+        Err(new_csv_error(vm, "unknown dialect"))
     }
 
     #[pyfunction]
@@ -537,7 +534,8 @@ mod _csv {
     ) -> Result<DialectItem, ArgumentError> {
         match_class!(match obj {
             s @ PyStr => {
-                Ok(DialectItem::Str(s.as_str().to_string()))
+                let s = s.try_into_utf8(vm).map_err(ArgumentError::Exception)?;
+                Ok(DialectItem::Str(s.as_str().to_owned()))
             }
             PyNone => {
                 Err(ArgumentError::InvalidKeywordArgument("dialect".to_string()))
@@ -581,10 +579,11 @@ mod _csv {
 
             if let Some(escapechar) = args.kwargs.swap_remove("escapechar") {
                 res.escapechar = match_class!(match escapechar {
-                    s @ PyStr => Some(s.as_str().bytes().exactly_one().map_err(|_| {
-                        let msg = r#""escapechar" must be a 1-character string"#;
-                        vm.new_type_error(msg.to_owned())
-                    })?),
+                    s @ PyStr =>
+                        Some(s.as_bytes().iter().copied().exactly_one().map_err(|_| {
+                            let msg = r#""escapechar" must be a 1-character string"#;
+                            vm.new_type_error(msg.to_owned())
+                        })?),
                     _ => None,
                 })
             };
@@ -626,10 +625,12 @@ mod _csv {
             };
             if let Some(quotechar) = args.kwargs.swap_remove("quotechar") {
                 res.quotechar = match_class!(match quotechar {
-                    s @ PyStr => Some(Some(s.as_str().bytes().exactly_one().map_err(|_| {
-                        let msg = r#""quotechar" must be a 1-character string"#;
-                        vm.new_type_error(msg.to_owned())
-                    })?)),
+                    s @ PyStr => Some(Some(s.as_bytes().iter().copied().exactly_one().map_err(
+                        |_| {
+                            let msg = r#""quotechar" must be a 1-character string"#;
+                            vm.new_type_error(msg.to_owned())
+                        }
+                    )?)),
                     PyNone => {
                         if let Some(QuoteStyle::All) = res.quoting {
                             let msg = "quotechar must be set if quoting enabled";
diff --git a/crates/stdlib/src/grp.rs b/crates/stdlib/src/grp.rs
index c1a52eee62e..eec901b0e57 100644
--- a/crates/stdlib/src/grp.rs
+++ b/crates/stdlib/src/grp.rs
@@ -5,7 +5,7 @@ pub(crate) use grp::module_def;
 mod grp {
     use crate::vm::{
         PyObjectRef, PyResult, VirtualMachine,
-        builtins::{PyIntRef, PyListRef, PyStrRef},
+        builtins::{PyIntRef, PyListRef, PyUtf8StrRef},
         convert::{IntoPyException, ToPyObject},
         exceptions,
         types::PyStructSequence,
@@ -67,7 +67,7 @@ mod grp {
     }
 
     #[pyfunction]
-    fn getgrnam(name: PyStrRef, vm: &VirtualMachine) -> PyResult<GroupData> {
+    fn getgrnam(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<GroupData> {
         let gr_name = name.as_str();
         if gr_name.contains('\0') {
             return Err(exceptions::cstring_error(vm));
diff --git a/crates/stdlib/src/hashlib.rs b/crates/stdlib/src/hashlib.rs
index 5097d804a79..441b8f44815 100644
--- a/crates/stdlib/src/hashlib.rs
+++ b/crates/stdlib/src/hashlib.rs
@@ -11,7 +11,7 @@ pub mod _hashlib {
     use crate::vm::{
         Py, PyObjectRef, PyPayload, PyResult, VirtualMachine,
         builtins::{
-            PyBaseExceptionRef, PyBytes, PyFrozenSet, PyStr, PyStrRef, PyTypeRef, PyValueError,
+            PyBaseExceptionRef, PyBytes, PyFrozenSet, PyStr, PyTypeRef, PyUtf8StrRef, PyValueError,
         },
         class::StaticType,
         convert::ToPyObject,
@@ -79,7 +79,7 @@ pub mod _hashlib {
     #[allow(unused)]
     struct NewHashArgs {
         #[pyarg(positional)]
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         #[pyarg(any, optional)]
         data: OptionalArg<ArgBytesLike>,
         #[pyarg(named, default = true)]
@@ -188,7 +188,7 @@ pub mod _hashlib {
     #[allow(unused)]
     struct Pbkdf2HmacArgs {
         #[pyarg(any)]
-        hash_name: PyStrRef,
+        hash_name: PyUtf8StrRef,
         #[pyarg(any)]
         password: ArgBytesLike,
         #[pyarg(any)]
@@ -218,18 +218,21 @@ pub mod _hashlib {
     }
 
     fn resolve_digestmod(digestmod: &PyObjectRef, vm: &VirtualMachine) -> PyResult<String> {
-        if let Some(name) = digestmod.downcast_ref::<PyStr>() {
-            return Ok(name.as_str().to_lowercase());
+        if let Some(name) = digestmod.downcast_ref::<PyStr>()
+            && let Some(name_str) = name.to_str()
+        {
+            return Ok(name_str.to_lowercase());
         }
         if let Ok(name_obj) = digestmod.get_attr("__name__", vm)
             && let Some(name) = name_obj.downcast_ref::<PyStr>()
-            && let Some(algo) = name.as_str().strip_prefix("openssl_")
+            && let Some(name_str) = name.to_str()
+            && let Some(algo) = name_str.strip_prefix("openssl_")
         {
             return Ok(algo.to_owned());
         }
         Err(vm.new_exception_msg(
             UnsupportedDigestmodError::static_type().to_owned(),
-            "unsupported digestmod".to_owned(),
+            "unsupported digestmod".into(),
         ))
     }
 
@@ -254,7 +257,7 @@ pub mod _hashlib {
     fn unsupported_hash(name: &str, vm: &VirtualMachine) -> PyBaseExceptionRef {
         vm.new_exception_msg(
             UnsupportedDigestmodError::static_type().to_owned(),
-            format!("unsupported hash type {name}"),
+            format!("unsupported hash type {name}").into(),
         )
     }
 
diff --git a/crates/stdlib/src/json.rs b/crates/stdlib/src/json.rs
index 3baeba629c8..41c3dead090 100644
--- a/crates/stdlib/src/json.rs
+++ b/crates/stdlib/src/json.rs
@@ -6,7 +6,7 @@ mod _json {
     use super::machinery;
     use crate::vm::{
         AsObject, Py, PyObjectRef, PyPayload, PyResult, VirtualMachine,
-        builtins::{PyBaseExceptionRef, PyStrRef, PyType},
+        builtins::{PyBaseExceptionRef, PyStrRef, PyType, PyUtf8StrRef},
         convert::ToPyResult,
         function::{IntoFuncArgs, OptionalArg},
         protocol::PyIterReturn,
@@ -91,14 +91,14 @@ mod _json {
     impl JsonScanner {
         fn parse(
             &self,
-            pystr: PyStrRef,
+            pystr: PyUtf8StrRef,
             char_idx: usize,
             byte_idx: usize,
             scan_once: PyObjectRef,
             vm: &VirtualMachine,
         ) -> PyResult<PyIterReturn> {
             flame_guard!("JsonScanner::parse");
-            let bytes = pystr.as_str().as_bytes();
+            let bytes = pystr.as_bytes();
             let wtf8 = pystr.as_wtf8();
 
             let first_byte = match bytes.get(byte_idx) {
@@ -115,7 +115,7 @@ mod _json {
                     // Parse string - pass slice starting after the quote
                     let (wtf8_result, chars_consumed, _bytes_consumed) =
                         machinery::scanstring(&wtf8[byte_idx + 1..], char_idx + 1, self.strict)
-                            .map_err(|e| py_decode_error(e, pystr.clone(), vm))?;
+                            .map_err(|e| py_decode_error(e, pystr.clone().into_wtf8(), vm))?;
                     let end_char_idx = char_idx + 1 + chars_consumed;
                     return Ok(PyIterReturn::Return(
                         vm.new_tuple((wtf8_result, end_char_idx)).into(),
@@ -228,7 +228,7 @@ mod _json {
         /// Returns (parsed_object, end_char_index, end_byte_index).
         fn parse_object(
             &self,
-            pystr: PyStrRef,
+            pystr: PyUtf8StrRef,
             start_char_idx: usize,
             start_byte_idx: usize,
             scan_once: &PyObjectRef,
@@ -237,7 +237,7 @@ mod _json {
         ) -> PyResult<(PyObjectRef, usize, usize)> {
             flame_guard!("JsonScanner::parse_object");
 
-            let bytes = pystr.as_str().as_bytes();
+            let bytes = pystr.as_bytes();
             let wtf8 = pystr.as_wtf8();
             let mut char_idx = start_char_idx;
             let mut byte_idx = start_byte_idx;
@@ -275,7 +275,7 @@ mod _json {
                 // Parse key string using scanstring with byte slice
                 let (key_wtf8, chars_consumed, bytes_consumed) =
                     machinery::scanstring(&wtf8[byte_idx..], char_idx, self.strict)
-                        .map_err(|e| py_decode_error(e, pystr.clone(), vm))?;
+                        .map_err(|e| py_decode_error(e, pystr.clone().into_wtf8(), vm))?;
 
                 char_idx += chars_consumed;
                 byte_idx += bytes_consumed;
@@ -389,7 +389,7 @@ mod _json {
         /// Returns (parsed_array, end_char_index, end_byte_index).
         fn parse_array(
             &self,
-            pystr: PyStrRef,
+            pystr: PyUtf8StrRef,
             start_char_idx: usize,
             start_byte_idx: usize,
             scan_once: &PyObjectRef,
@@ -398,7 +398,7 @@ mod _json {
         ) -> PyResult<(PyObjectRef, usize, usize)> {
             flame_guard!("JsonScanner::parse_array");
 
-            let bytes = pystr.as_str().as_bytes();
+            let bytes = pystr.as_bytes();
             let mut char_idx = start_char_idx;
             let mut byte_idx = start_byte_idx;
 
@@ -507,15 +507,15 @@ mod _json {
         fn call_scan_once(
             &self,
             scan_once: &PyObjectRef,
-            pystr: PyStrRef,
+            pystr: PyUtf8StrRef,
             char_idx: usize,
             byte_idx: usize,
             memo: &mut HashMap<String, PyStrRef>,
             vm: &VirtualMachine,
         ) -> PyResult<(PyObjectRef, usize, usize)> {
-            let s = pystr.as_str();
-            let bytes = s.as_bytes();
+            let bytes = pystr.as_bytes();
             let wtf8 = pystr.as_wtf8();
+            let s = pystr.as_str();
 
             let first_byte = match bytes.get(byte_idx) {
                 Some(&b) => b,
@@ -527,7 +527,7 @@ mod _json {
                     // String - pass slice starting after the quote
                     let (wtf8_result, chars_consumed, bytes_consumed) =
                         machinery::scanstring(&wtf8[byte_idx + 1..], char_idx + 1, self.strict)
-                            .map_err(|e| py_decode_error(e, pystr.clone(), vm))?;
+                            .map_err(|e| py_decode_error(e, pystr.clone().into_wtf8(), vm))?;
                     let py_str = vm.ctx.new_str(wtf8_result.to_string());
                     Ok((
                         py_str.into(),
@@ -620,12 +620,12 @@ mod _json {
         fn make_decode_error(
             &self,
             msg: &str,
-            s: PyStrRef,
+            s: PyUtf8StrRef,
             pos: usize,
             vm: &VirtualMachine,
         ) -> PyBaseExceptionRef {
             let err = machinery::DecodeError::new(msg, pos);
-            py_decode_error(err, s, vm)
+            py_decode_error(err, s.into_wtf8(), vm)
         }
     }
 
@@ -636,6 +636,7 @@ mod _json {
                 return Err(vm.new_value_error("idx cannot be negative"));
             }
             let char_idx = char_idx as usize;
+            let pystr = pystr.try_into_utf8(vm)?;
             let s = pystr.as_str();
 
             // Calculate byte index from char index (O(char_idx) but only at entry point)
@@ -652,14 +653,8 @@ mod _json {
                 }
             };
 
-            zelf.parse(
-                pystr.clone(),
-                char_idx,
-                byte_idx,
-                zelf.to_owned().into(),
-                vm,
-            )
-            .and_then(|x| x.to_pyresult(vm))
+            zelf.parse(pystr, char_idx, byte_idx, zelf.to_owned().into(), vm)
+                .and_then(|x| x.to_pyresult(vm))
         }
     }
 
@@ -674,12 +669,12 @@ mod _json {
     }
 
     #[pyfunction]
-    fn encode_basestring(s: PyStrRef) -> String {
+    fn encode_basestring(s: PyUtf8StrRef) -> String {
         encode_string(s.as_str(), false)
     }
 
     #[pyfunction]
-    fn encode_basestring_ascii(s: PyStrRef) -> String {
+    fn encode_basestring_ascii(s: PyUtf8StrRef) -> String {
         encode_string(s.as_str(), true)
     }
 
diff --git a/crates/stdlib/src/locale.rs b/crates/stdlib/src/locale.rs
index 496325b5038..a22c6afe57e 100644
--- a/crates/stdlib/src/locale.rs
+++ b/crates/stdlib/src/locale.rs
@@ -45,7 +45,7 @@ mod _locale {
     use core::{ffi::CStr, ptr};
     use rustpython_vm::{
         PyObjectRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PyIntRef, PyListRef, PyStrRef, PyTypeRef},
+        builtins::{PyDictRef, PyIntRef, PyListRef, PyTypeRef, PyUtf8StrRef},
         convert::ToPyException,
         function::OptionalArg,
     };
@@ -149,14 +149,14 @@ mod _locale {
     }
 
     #[pyfunction]
-    fn strcoll(string1: PyStrRef, string2: PyStrRef, vm: &VirtualMachine) -> PyResult {
+    fn strcoll(string1: PyUtf8StrRef, string2: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult {
         let cstr1 = CString::new(string1.as_str()).map_err(|e| e.to_pyexception(vm))?;
         let cstr2 = CString::new(string2.as_str()).map_err(|e| e.to_pyexception(vm))?;
         Ok(vm.new_pyobj(unsafe { libc::strcoll(cstr1.as_ptr(), cstr2.as_ptr()) }))
     }
 
     #[pyfunction]
-    fn strxfrm(string: PyStrRef, vm: &VirtualMachine) -> PyResult {
+    fn strxfrm(string: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult {
         // https://github.com/python/cpython/blob/eaae563b6878aa050b4ad406b67728b6b066220e/Modules/_localemodule.c#L390-L442
         let n1 = string.byte_len() + 1;
         let mut buff = vec![0u8; n1];
@@ -227,7 +227,7 @@ mod _locale {
         #[pyarg(any)]
         category: i32,
         #[pyarg(any, optional)]
-        locale: OptionalArg<Option<PyStrRef>>,
+        locale: OptionalArg<Option<PyUtf8StrRef>>,
     }
 
     /// Maximum code page encoding name length on Windows
@@ -262,34 +262,34 @@ mod _locale {
     fn setlocale(args: LocaleArgs, vm: &VirtualMachine) -> PyResult {
         let error = error(vm);
         if cfg!(windows) && (args.category < LC_ALL || args.category > LC_TIME) {
-            return Err(vm.new_exception_msg(error, String::from("unsupported locale setting")));
+            return Err(vm.new_exception_msg(error, "unsupported locale setting".into()));
         }
         unsafe {
             let result = match args.locale.flatten() {
                 None => libc::setlocale(args.category, ptr::null()),
                 Some(locale) => {
+                    let locale_str = locale.as_str();
                     // On Windows, validate encoding name length
                     #[cfg(windows)]
                     {
                         let valid = if args.category == LC_ALL {
-                            check_locale_name_all(locale.as_str())
+                            check_locale_name_all(locale_str)
                         } else {
-                            check_locale_name(locale.as_str())
+                            check_locale_name(locale_str)
                         };
                         if !valid {
-                            return Err(vm.new_exception_msg(
-                                error,
-                                String::from("unsupported locale setting"),
-                            ));
+                            return Err(
+                                vm.new_exception_msg(error, "unsupported locale setting".into())
+                            );
                         }
                     }
                     let c_locale: CString =
-                        CString::new(locale.as_str()).map_err(|e| e.to_pyexception(vm))?;
+                        CString::new(locale_str).map_err(|e| e.to_pyexception(vm))?;
                     libc::setlocale(args.category, c_locale.as_ptr())
                 }
             };
             if result.is_null() {
-                return Err(vm.new_exception_msg(error, String::from("unsupported locale setting")));
+                return Err(vm.new_exception_msg(error, "unsupported locale setting".into()));
             }
             pystr_from_raw_cstr(vm, result)
         }
diff --git a/crates/stdlib/src/lzma.rs b/crates/stdlib/src/lzma.rs
index 80e4ce80755..d9f19f0f24e 100644
--- a/crates/stdlib/src/lzma.rs
+++ b/crates/stdlib/src/lzma.rs
@@ -82,7 +82,8 @@ mod _lzma {
     }
 
     fn new_lzma_error(message: impl Into<String>, vm: &VirtualMachine) -> PyBaseExceptionRef {
-        vm.new_exception_msg(vm.class("lzma", "LZMAError"), message.into())
+        let msg: String = message.into();
+        vm.new_exception_msg(vm.class("lzma", "LZMAError"), msg.into())
     }
 
     #[pyfunction]
diff --git a/crates/stdlib/src/multiprocessing.rs b/crates/stdlib/src/multiprocessing.rs
index cad20972ac1..fe52cbd19fc 100644
--- a/crates/stdlib/src/multiprocessing.rs
+++ b/crates/stdlib/src/multiprocessing.rs
@@ -224,7 +224,7 @@ mod _multiprocessing {
                 if !ismine!(self) {
                     return Err(vm.new_exception_msg(
                         vm.ctx.exceptions.assertion_error.to_owned(),
-                        "attempt to release recursive lock not owned by thread".to_owned(),
+                        "attempt to release recursive lock not owned by thread".into(),
                     ));
                 }
                 if self.count.load(Ordering::Acquire) > 1 {
@@ -796,7 +796,7 @@ mod _multiprocessing {
                 if !ismine!(self) {
                     return Err(vm.new_exception_msg(
                         vm.ctx.exceptions.assertion_error.to_owned(),
-                        "attempt to release recursive lock not owned by thread".to_owned(),
+                        "attempt to release recursive lock not owned by thread".into(),
                     ));
                 }
                 // if (self->count > 1) { --self->count; Py_RETURN_NONE; }
diff --git a/crates/stdlib/src/overlapped.rs b/crates/stdlib/src/overlapped.rs
index 779fe31efe5..1243a1297ea 100644
--- a/crates/stdlib/src/overlapped.rs
+++ b/crates/stdlib/src/overlapped.rs
@@ -309,7 +309,7 @@ mod _overlapped {
                 let mut addr: SOCKADDR_IN = unsafe { core::mem::zeroed() };
                 addr.sin_family = AF_INET;
 
-                let host_wide: Vec<u16> = host.as_str().encode_utf16().chain([0]).collect();
+                let host_wide: Vec<u16> = host.as_wtf8().encode_wide().chain([0]).collect();
                 let mut addr_len = core::mem::size_of::<SOCKADDR_IN>() as i32;
 
                 let ret = unsafe {
@@ -348,7 +348,7 @@ mod _overlapped {
                 let mut addr: SOCKADDR_IN6 = unsafe { core::mem::zeroed() };
                 addr.sin6_family = AF_INET6;
 
-                let host_wide: Vec<u16> = host.as_str().encode_utf16().chain([0]).collect();
+                let host_wide: Vec<u16> = host.as_wtf8().encode_wide().chain([0]).collect();
                 let mut addr_len = core::mem::size_of::<SOCKADDR_IN6>() as i32;
 
                 let ret = unsafe {
diff --git a/crates/stdlib/src/posixshmem.rs b/crates/stdlib/src/posixshmem.rs
index 2a142d8b6f3..f5481619bba 100644
--- a/crates/stdlib/src/posixshmem.rs
+++ b/crates/stdlib/src/posixshmem.rs
@@ -8,13 +8,15 @@ mod _posixshmem {
 
     use crate::{
         common::os::errno_io_error,
-        vm::{FromArgs, PyResult, VirtualMachine, builtins::PyStrRef, convert::IntoPyException},
+        vm::{
+            FromArgs, PyResult, VirtualMachine, builtins::PyUtf8StrRef, convert::IntoPyException,
+        },
     };
 
     #[derive(FromArgs)]
     struct ShmOpenArgs {
         #[pyarg(any)]
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         #[pyarg(any)]
         flags: libc::c_int,
         #[pyarg(any, default = 0o600)]
@@ -37,7 +39,7 @@ mod _posixshmem {
     }
 
     #[pyfunction]
-    fn shm_unlink(name: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
+    fn shm_unlink(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<()> {
         let name = CString::new(name.as_str()).map_err(|e| e.into_pyexception(vm))?;
         // SAFETY: `name` is a valid NUL-terminated string and `shm_unlink` only reads it.
         let ret = unsafe { libc::shm_unlink(name.as_ptr()) };
diff --git a/crates/stdlib/src/pyexpat.rs b/crates/stdlib/src/pyexpat.rs
index 7d603c72ed5..40418f54c30 100644
--- a/crates/stdlib/src/pyexpat.rs
+++ b/crates/stdlib/src/pyexpat.rs
@@ -41,7 +41,7 @@ macro_rules! create_bool_property {
 mod _pyexpat {
     use crate::vm::{
         Context, Py, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject, VirtualMachine,
-        builtins::{PyBytesRef, PyException, PyModule, PyStr, PyStrRef, PyType},
+        builtins::{PyBytesRef, PyException, PyModule, PyStr, PyStrRef, PyType, PyUtf8StrRef},
         extend_module,
         function::{ArgBytesLike, Either, IntoFuncArgs, OptionalArg},
         types::Constructor,
@@ -390,7 +390,7 @@ mod _pyexpat {
         #[pyarg(any, optional)]
         encoding: Option<PyStrRef>,
         #[pyarg(any, optional)]
-        namespace_separator: Option<PyStrRef>,
+        namespace_separator: Option<PyUtf8StrRef>,
         #[pyarg(any, optional)]
         intern: Option<PyObjectRef>,
     }
@@ -403,11 +403,9 @@ mod _pyexpat {
         // Validate namespace_separator: must be at most one character
         let ns_sep = match args.namespace_separator {
             Some(ref s) => {
-                let chars: Vec<char> = s.as_str().chars().collect();
-                if chars.len() > 1 {
+                if s.as_str().chars().count() > 1 {
                     return Err(vm.new_value_error(
-                        "namespace_separator must be at most one character, omitted, or None"
-                            .to_owned(),
+                        "namespace_separator must be at most one character, omitted, or None",
                     ));
                 }
                 Some(s.as_str().to_owned())
diff --git a/crates/stdlib/src/pystruct.rs b/crates/stdlib/src/pystruct.rs
index d3be417edb3..9c5d67f396c 100644
--- a/crates/stdlib/src/pystruct.rs
+++ b/crates/stdlib/src/pystruct.rs
@@ -19,6 +19,7 @@ pub(crate) mod _struct {
         types::{Constructor, IterNext, Iterable, Representable, SelfIter},
     };
     use crossbeam_utils::atomic::AtomicCell;
+    use rustpython_common::wtf8::{Wtf8Buf, wtf8_concat};
 
     #[derive(Traverse)]
     struct IntoStructFormatBytes(PyStrRef);
@@ -305,8 +306,8 @@ pub(crate) mod _struct {
 
     impl Representable for PyStruct {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
-            Ok(format!("Struct('{}')", zelf.format.as_str()))
+        fn repr_wtf8(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            Ok(wtf8_concat!("Struct('", zelf.format.as_wtf8(), "')"))
         }
     }
 
diff --git a/crates/stdlib/src/socket.rs b/crates/stdlib/src/socket.rs
index 8c307eb54ec..dce1f27d1ce 100644
--- a/crates/stdlib/src/socket.rs
+++ b/crates/stdlib/src/socket.rs
@@ -12,6 +12,7 @@ mod _socket {
         AsObject, Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
         builtins::{
             PyBaseExceptionRef, PyListRef, PyModule, PyOSError, PyStrRef, PyTupleRef, PyTypeRef,
+            PyUtf8StrRef,
         },
         common::os::ErrorExt,
         convert::{IntoPyException, ToPyObject, TryFromBorrowedObject, TryFromObject},
@@ -1224,6 +1225,7 @@ mod _socket {
                             obj.class().name()
                         ))
                     })?;
+                    let interface = interface.try_into_utf8(vm).map_err(IoOrPyException::from)?;
                     let ifname = interface.as_str();
 
                     // Get interface index
@@ -1291,6 +1293,8 @@ mod _socket {
                         ))
                     })?;
 
+                    let alg_type = alg_type.try_into_utf8(vm).map_err(IoOrPyException::from)?;
+                    let alg_name = alg_name.try_into_utf8(vm).map_err(IoOrPyException::from)?;
                     let type_str = alg_type.as_str();
                     let name_str = alg_name.as_str();
 
@@ -2134,7 +2138,7 @@ mod _socket {
                     && let Ok(addr) = sock.local_addr()
                     && let Ok(repr) = get_addr_tuple(&addr, vm).repr(vm)
                 {
-                    format!(", laddr={}", repr.as_str())
+                    format!(", laddr={}", repr.as_wtf8())
                 } else {
                     String::new()
                 };
@@ -2484,7 +2488,7 @@ mod _socket {
     }
 
     struct Address {
-        host: PyStrRef,
+        host: PyUtf8StrRef,
         port: u16,
     }
 
@@ -2509,6 +2513,7 @@ mod _socket {
     impl Address {
         fn from_tuple(tuple: &[PyObjectRef], vm: &VirtualMachine) -> PyResult<Self> {
             let host = PyStrRef::try_from_object(vm, tuple[0].clone())?;
+            let host = host.try_into_utf8(vm)?;
             let port = i32::try_from_borrowed_object(vm, &tuple[1])?;
             let port = port
                 .to_u16()
@@ -2625,12 +2630,12 @@ mod _socket {
 
     #[cfg(all(unix, not(target_os = "redox")))]
     #[pyfunction]
-    fn sethostname(hostname: PyStrRef) -> nix::Result<()> {
+    fn sethostname(hostname: PyUtf8StrRef) -> nix::Result<()> {
         nix::unistd::sethostname(hostname.as_str())
     }
 
     #[pyfunction]
-    fn inet_aton(ip_string: PyStrRef, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
+    fn inet_aton(ip_string: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
         ip_string
             .as_str()
             .parse::<Ipv4Addr>()
@@ -2904,7 +2909,7 @@ mod _socket {
 
     #[pyfunction]
     fn gethostbyaddr(
-        addr: PyStrRef,
+        addr: PyUtf8StrRef,
         vm: &VirtualMachine,
     ) -> Result<(String, PyListRef, PyListRef), IoOrPyException> {
         let addr = get_addr(vm, addr, c::AF_UNSPEC)?;
@@ -2919,7 +2924,7 @@ mod _socket {
     }
 
     #[pyfunction]
-    fn gethostbyname(name: PyStrRef, vm: &VirtualMachine) -> Result<String, IoOrPyException> {
+    fn gethostbyname(name: PyUtf8StrRef, vm: &VirtualMachine) -> Result<String, IoOrPyException> {
         let addr = get_addr(vm, name, c::AF_INET)?;
         match addr {
             SocketAddr::V4(ip) => Ok(ip.ip().to_string()),
@@ -2929,7 +2934,7 @@ mod _socket {
 
     #[pyfunction]
     fn gethostbyname_ex(
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         vm: &VirtualMachine,
     ) -> Result<(String, PyListRef, PyListRef), IoOrPyException> {
         let addr = get_addr(vm, name, c::AF_INET)?;
@@ -2944,7 +2949,7 @@ mod _socket {
     }
 
     #[pyfunction]
-    fn inet_pton(af_inet: i32, ip_string: PyStrRef, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
+    fn inet_pton(af_inet: i32, ip_string: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Vec<u8>> {
         static ERROR_MSG: &str = "illegal IP address string passed to inet_pton";
         let ip_addr = match af_inet {
             c::AF_INET => ip_string
@@ -3015,10 +3020,10 @@ mod _socket {
             protocol: 0,
         };
         let service = addr.port.to_string();
-        let mut res =
-            dns_lookup::getaddrinfo(Some(addr.host.as_str()), Some(&service), Some(hints))
-                .map_err(|e| convert_socket_error(vm, e, SocketError::GaiError))?
-                .filter_map(Result::ok);
+        let host_str = addr.host.as_str();
+        let mut res = dns_lookup::getaddrinfo(Some(host_str), Some(&service), Some(hints))
+            .map_err(|e| convert_socket_error(vm, e, SocketError::GaiError))?
+            .filter_map(Result::ok);
         let mut ainfo = res.next().unwrap();
         if res.next().is_some() {
             return Err(vm
@@ -3183,7 +3188,7 @@ mod _socket {
 
     fn get_addr(
         vm: &VirtualMachine,
-        pyname: PyStrRef,
+        pyname: PyUtf8StrRef,
         af: i32,
     ) -> Result<SocketAddr, IoOrPyException> {
         let name = pyname.as_str();
@@ -3236,7 +3241,7 @@ mod _socket {
         let name = vm
             .state
             .codec_registry
-            .encode_text(pyname, "idna", None, vm)?;
+            .encode_text(pyname.into_wtf8(), "idna", None, vm)?;
         let name = core::str::from_utf8(name.as_bytes())
             .map_err(|_| vm.new_runtime_error("idna output is not utf8"))?;
         let mut res = dns_lookup::getaddrinfo(Some(name), None, Some(hints))
diff --git a/crates/stdlib/src/ssl.rs b/crates/stdlib/src/ssl.rs
index a1b089078a8..fefca04443f 100644
--- a/crates/stdlib/src/ssl.rs
+++ b/crates/stdlib/src/ssl.rs
@@ -40,7 +40,10 @@ mod _ssl {
         vm::{
             AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject,
             VirtualMachine,
-            builtins::{PyBaseExceptionRef, PyBytesRef, PyListRef, PyStrRef, PyType, PyTypeRef},
+            builtins::{
+                PyBaseExceptionRef, PyBytesRef, PyListRef, PyStrRef, PyType, PyTypeRef,
+                PyUtf8StrRef,
+            },
             convert::IntoPyException,
             function::{
                 ArgBytesLike, ArgMemoryBuffer, Either, FuncArgs, OptionalArg, PyComparisonValue,
@@ -57,10 +60,16 @@ mod _ssl {
     };
     use alloc::sync::Arc;
     use core::{
+        hash::{Hash, Hasher},
         sync::atomic::{AtomicUsize, Ordering},
         time::Duration,
     };
-    use std::{collections::HashMap, time::SystemTime};
+    use rustls::crypto::aws_lc_rs::ALL_CIPHER_SUITES;
+    use std::{
+        collections::{HashMap, hash_map::DefaultHasher},
+        io::BufRead,
+        time::SystemTime,
+    };
 
     // Rustls imports
     use parking_lot::{Mutex as ParkingMutex, RwLock as ParkingRwLock};
@@ -597,8 +606,6 @@ mod _ssl {
     /// - "ALL" or "DEFAULT" → all available
     /// - "!MD5" → exclusion (ignored, rustls doesn't support weak ciphers anyway)
     fn parse_cipher_string(cipher_str: &str) -> Result<Vec<rustls::SupportedCipherSuite>, String> {
-        use rustls::crypto::aws_lc_rs::ALL_CIPHER_SUITES;
-
         if cipher_str.is_empty() {
             return Err("No cipher can be selected".to_string());
         }
@@ -801,7 +808,7 @@ mod _ssl {
         sock: PyObjectRef,
         server_side: bool,
         #[pyarg(positional, optional)]
-        server_hostname: OptionalArg<Option<PyStrRef>>,
+        server_hostname: OptionalArg<Option<PyUtf8StrRef>>,
         #[pyarg(named, optional)]
         owner: OptionalArg<PyObjectRef>,
         #[pyarg(named, optional)]
@@ -815,7 +822,7 @@ mod _ssl {
         #[pyarg(named, optional)]
         server_side: OptionalArg<bool>,
         #[pyarg(named, optional)]
-        server_hostname: OptionalArg<Option<PyStrRef>>,
+        server_hostname: OptionalArg<Option<PyUtf8StrRef>>,
         #[pyarg(named, optional)]
         owner: OptionalArg<PyObjectRef>,
         #[pyarg(named, optional)]
@@ -1080,7 +1087,7 @@ mod _ssl {
 
                 // Convert callable result to string
                 let password_from_callable = if let Ok(pwd_str) =
-                    PyStrRef::try_from_object(vm, pwd_result.clone())
+                    PyUtf8StrRef::try_from_object(vm, pwd_result.clone())
                 {
                     pwd_str.as_str().to_owned()
                 } else if let Ok(pwd_bytes_like) = ArgBytesLike::try_from_object(vm, pwd_result) {
@@ -1501,7 +1508,7 @@ mod _ssl {
         }
 
         #[pymethod]
-        fn set_ciphers(&self, ciphers: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
+        fn set_ciphers(&self, ciphers: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<()> {
             let cipher_str = ciphers.as_str();
 
             // Parse cipher string and store selected ciphers
@@ -1520,7 +1527,6 @@ mod _ssl {
         fn get_ciphers(&self, vm: &VirtualMachine) -> PyResult<PyListRef> {
             // Dynamically generate cipher list from rustls ALL_CIPHER_SUITES
             // This automatically includes all cipher suites supported by the current rustls version
-            use rustls::crypto::aws_lc_rs::ALL_CIPHER_SUITES;
 
             let cipher_list = ALL_CIPHER_SUITES
                 .iter()
@@ -1741,7 +1747,7 @@ mod _ssl {
             }
 
             // Validate filepath is str or bytes
-            let path_str = if let Ok(s) = PyStrRef::try_from_object(vm, filepath.clone()) {
+            let path_str = if let Ok(s) = PyUtf8StrRef::try_from_object(vm, filepath.clone()) {
                 s.as_str().to_owned()
             } else if let Ok(b) = ArgBytesLike::try_from_object(vm, filepath) {
                 String::from_utf8(b.borrow_buf().to_vec())
@@ -1796,7 +1802,7 @@ mod _ssl {
             }
 
             // Validate name is str or bytes
-            let curve_name = if let Ok(s) = PyStrRef::try_from_object(vm, name.clone()) {
+            let curve_name = if let Ok(s) = PyUtf8StrRef::try_from_object(vm, name.clone()) {
                 s.as_str().to_owned()
             } else if let Ok(b) = ArgBytesLike::try_from_object(vm, name) {
                 String::from_utf8(b.borrow_buf().to_vec())
@@ -1995,7 +2001,7 @@ mod _ssl {
             vm: &VirtualMachine,
         ) -> PyResult<String> {
             match arg {
-                Either::A(s) => Ok(s.as_str().to_owned()),
+                Either::A(s) => Ok(s.clone().try_into_utf8(vm)?.as_str().to_owned()),
                 Either::B(b) => String::from_utf8(b.borrow_buf().to_vec())
                     .map_err(|_| vm.new_value_error("path contains invalid UTF-8".to_owned())),
             }
@@ -2013,7 +2019,7 @@ mod _ssl {
             match password {
                 OptionalArg::Present(p) => {
                     // Try string first
-                    if let Ok(pwd_str) = PyStrRef::try_from_object(vm, p.clone()) {
+                    if let Ok(pwd_str) = PyUtf8StrRef::try_from_object(vm, p.clone()) {
                         Ok((Some(pwd_str.as_str().to_owned()), None))
                     }
                     // Try bytes-like
@@ -2171,10 +2177,10 @@ mod _ssl {
         fn parse_cadata_arg(
             &self,
             arg: &Either<PyStrRef, ArgBytesLike>,
-            _vm: &VirtualMachine,
+            vm: &VirtualMachine,
         ) -> PyResult<Vec<u8>> {
             match arg {
-                Either::A(s) => Ok(s.as_str().as_bytes().to_vec()),
+                Either::A(s) => Ok(s.clone().try_into_utf8(vm)?.as_str().as_bytes().to_vec()),
                 Either::B(b) => Ok(b.borrow_buf().to_vec()),
             }
         }
@@ -3577,7 +3583,7 @@ mod _ssl {
                             Some(conn) => conn,
                             None => return Err(create_ssl_eof_error(vm).upcast()),
                         };
-                        use std::io::BufRead;
+
                         let mut reader = conn.reader();
                         reader.fill_buf().map(|buf| buf.len()).unwrap_or(0)
                     };
@@ -3606,7 +3612,7 @@ mod _ssl {
                             Some(conn) => conn,
                             None => return Err(create_ssl_zero_return_error(vm).upcast()),
                         };
-                        use std::io::BufRead;
+
                         let mut reader = conn.reader();
                         reader.fill_buf().map(|buf| buf.len()).unwrap_or(0)
                     };
@@ -3674,7 +3680,6 @@ mod _ssl {
             // Use rustls Reader's fill_buf() to check buffered plaintext
             // fill_buf() returns a reference to buffered data without consuming it
             // This matches OpenSSL's SSL_pending() behavior
-            use std::io::BufRead;
             let mut reader = conn.reader();
             match reader.fill_buf() {
                 Ok(buf) => Ok(buf.len()),
@@ -3894,7 +3899,7 @@ mod _ssl {
         #[pygetset(setter)]
         fn set_server_hostname(
             &self,
-            value: Option<PyStrRef>,
+            value: Option<PyUtf8StrRef>,
             vm: &VirtualMachine,
         ) -> PyResult<()> {
             // Check if handshake is already done
@@ -3905,11 +3910,14 @@ mod _ssl {
             }
 
             // Validate hostname
-            if let Some(hostname_str) = &value {
-                validate_hostname(hostname_str.as_str(), vm)?;
-            }
+            let hostname_string = value
+                .map(|s| {
+                    validate_hostname(s.as_str(), vm)?;
+                    Ok::<String, _>(s.as_str().to_owned())
+                })
+                .transpose()?;
 
-            *self.server_hostname.write() = value.map(|s| s.as_str().to_string());
+            *self.server_hostname.write() = hostname_string;
             Ok(())
         }
 
@@ -4183,7 +4191,7 @@ mod _ssl {
                                         // Timeout reached - raise TimeoutError
                                         return Err(vm.new_exception_msg(
                                             vm.ctx.exceptions.timeout_error.to_owned(),
-                                            "The read operation timed out".to_owned(),
+                                            "The read operation timed out".into(),
                                         ));
                                     }
                                     Some(dl - now)
@@ -4203,7 +4211,7 @@ mod _ssl {
                                     // Raise TimeoutError
                                     return Err(vm.new_exception_msg(
                                         vm.ctx.exceptions.timeout_error.to_owned(),
-                                        "The read operation timed out".to_owned(),
+                                        "The read operation timed out".into(),
                                     ));
                                 }
 
@@ -4529,7 +4537,7 @@ mod _ssl {
         #[pymethod]
         fn get_channel_binding(
             &self,
-            cb_type: OptionalArg<PyStrRef>,
+            cb_type: OptionalArg<PyUtf8StrRef>,
             vm: &VirtualMachine,
         ) -> PyResult<Option<PyBytesRef>> {
             let cb_type_str = cb_type.as_ref().map_or("tls-unique", |s| s.as_str());
@@ -4628,7 +4636,7 @@ mod _ssl {
                 if !is_contiguous {
                     return Err(vm.new_exception_msg(
                         vm.ctx.exceptions.buffer_error.to_owned(),
-                        "non-contiguous buffer is not supported".to_owned(),
+                        "non-contiguous buffer is not supported".into(),
                     ));
                 }
             }
@@ -4724,8 +4732,6 @@ mod _ssl {
         #[pygetset]
         fn id(&self, vm: &VirtualMachine) -> PyBytesRef {
             // Return session ID (hash of session data for uniqueness)
-            use core::hash::{Hash, Hasher};
-            use std::collections::hash_map::DefaultHasher;
 
             let mut hasher = DefaultHasher::new();
             self.session_data.hash(&mut hasher);
@@ -4755,7 +4761,7 @@ mod _ssl {
 
     #[derive(FromArgs)]
     struct Txt2ObjArgs {
-        txt: PyStrRef,
+        txt: PyUtf8StrRef,
         #[pyarg(named, optional)]
         name: OptionalArg<bool>,
     }
@@ -4901,13 +4907,13 @@ mod _ssl {
     /// This is a simplified wrapper around cert_der_to_dict_helper that handles
     /// file reading and PEM/DER auto-detection. Used by test suite.
     #[pyfunction]
-    fn _test_decode_cert(path: PyStrRef, vm: &VirtualMachine) -> PyResult<PyObjectRef> {
+    fn _test_decode_cert(path: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<PyObjectRef> {
         // Read certificate file
-        let cert_data = std::fs::read(path.as_str()).map_err(|e| {
+        let path_str = path.as_str();
+        let cert_data = std::fs::read(path_str).map_err(|e| {
             vm.new_os_error(format!(
                 "Failed to read certificate file {}: {}",
-                path.as_str(),
-                e
+                path_str, e
             ))
         })?;
 
@@ -4944,11 +4950,9 @@ mod _ssl {
     }
 
     #[pyfunction]
-    fn PEM_cert_to_DER_cert(pem_cert: PyStrRef, vm: &VirtualMachine) -> PyResult<PyBytesRef> {
-        let pem_str = pem_cert.as_str();
-
+    fn PEM_cert_to_DER_cert(pem_cert: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<PyBytesRef> {
         // Parse PEM format
-        let mut cursor = std::io::Cursor::new(pem_str.as_bytes());
+        let mut cursor = std::io::Cursor::new(pem_cert.as_bytes());
         let mut certs = rustls_pemfile::certs(&mut cursor);
 
         if let Some(Ok(cert)) = certs.next() {
@@ -4961,22 +4965,27 @@ mod _ssl {
     // Windows-specific certificate store enumeration functions
     #[cfg(windows)]
     #[pyfunction]
-    fn enum_certificates(store_name: PyStrRef, vm: &VirtualMachine) -> PyResult<Vec<PyObjectRef>> {
+    fn enum_certificates(
+        store_name: PyUtf8StrRef,
+        vm: &VirtualMachine,
+    ) -> PyResult<Vec<PyObjectRef>> {
         use schannel::{RawPointer, cert_context::ValidUses, cert_store::CertStore};
         use windows_sys::Win32::Security::Cryptography;
 
+        let store_name_str = store_name.as_str();
+
         // Try both Current User and Local Machine stores
         let open_fns = [CertStore::open_current_user, CertStore::open_local_machine];
         let stores = open_fns
             .iter()
-            .filter_map(|open| open(store_name.as_str()).ok())
+            .filter_map(|open| open(store_name_str).ok())
             .collect::<Vec<_>>();
 
         // If no stores could be opened, raise OSError
         if stores.is_empty() {
             return Err(vm.new_os_error(format!(
                 "failed to open certificate store {:?}",
-                store_name.as_str()
+                store_name_str
             )));
         }
 
@@ -5011,14 +5020,14 @@ mod _ssl {
 
     #[cfg(windows)]
     #[pyfunction]
-    fn enum_crls(store_name: PyStrRef, vm: &VirtualMachine) -> PyResult<Vec<PyObjectRef>> {
+    fn enum_crls(store_name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Vec<PyObjectRef>> {
         use windows_sys::Win32::Security::Cryptography::{
             CRL_CONTEXT, CertCloseStore, CertEnumCRLsInStore, CertOpenSystemStoreW,
             X509_ASN_ENCODING,
         };
 
-        let store_name_wide: Vec<u16> = store_name
-            .as_str()
+        let store_name_str = store_name.as_str();
+        let store_name_wide: Vec<u16> = store_name_str
             .encode_utf16()
             .chain(core::iter::once(0))
             .collect();
@@ -5029,7 +5038,7 @@ mod _ssl {
         if store.is_null() {
             return Err(vm.new_os_error(format!(
                 "failed to open certificate store {:?}",
-                store_name.as_str()
+                store_name_str
             )));
         }
 
@@ -5135,9 +5144,6 @@ mod _ssl {
     // Implement Hashable trait for PySSLCertificate
     impl Hashable for PySSLCertificate {
         fn hash(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<PyHash> {
-            use core::hash::{Hash, Hasher};
-            use std::collections::hash_map::DefaultHasher;
-
             let mut hasher = DefaultHasher::new();
             zelf.der_bytes.hash(&mut hasher);
             Ok(hasher.finish() as PyHash)
diff --git a/crates/stdlib/src/unicodedata.rs b/crates/stdlib/src/unicodedata.rs
index a575c1ae7e5..5664fd0c36e 100644
--- a/crates/stdlib/src/unicodedata.rs
+++ b/crates/stdlib/src/unicodedata.rs
@@ -20,14 +20,12 @@ enum NormalizeForm {
 impl<'a> TryFromBorrowedObject<'a> for NormalizeForm {
     fn try_from_borrowed_object(vm: &VirtualMachine, obj: &'a PyObject) -> PyResult<Self> {
         obj.try_value_with(
-            |form: &PyStr| {
-                Ok(match form.as_str() {
-                    "NFC" => Self::Nfc,
-                    "NFKC" => Self::Nfkc,
-                    "NFD" => Self::Nfd,
-                    "NFKD" => Self::Nfkd,
-                    _ => return Err(vm.new_value_error("invalid normalization form")),
-                })
+            |form: &PyStr| match form.as_bytes() {
+                b"NFC" => Ok(Self::Nfc),
+                b"NFKC" => Ok(Self::Nfkc),
+                b"NFD" => Ok(Self::Nfd),
+                b"NFKD" => Ok(Self::Nfkd),
+                _ => Err(vm.new_value_error("invalid normalization form")),
             },
             vm,
         )
@@ -36,6 +34,7 @@ impl<'a> TryFromBorrowedObject<'a> for NormalizeForm {
 
 #[pymodule]
 mod unicodedata {
+    use super::NormalizeForm::*;
     use crate::vm::{
         Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
         builtins::{PyModule, PyStrRef},
@@ -126,7 +125,8 @@ mod unicodedata {
 
         #[pymethod]
         fn lookup(&self, name: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
-            if let Some(character) = unicode_names2::character(name.as_str())
+            if let Some(name_str) = name.to_str()
+                && let Some(character) = unicode_names2::character(name_str)
                 && self.check_age(character.into())
             {
                 return Ok(character.to_string());
@@ -188,7 +188,6 @@ mod unicodedata {
 
         #[pymethod]
         fn normalize(&self, form: super::NormalizeForm, unistr: PyStrRef) -> PyResult<Wtf8Buf> {
-            use super::NormalizeForm::*;
             let text = unistr.as_wtf8();
             let normalized_text = match form {
                 Nfc => text.map_utf8(|s| s.nfc()).collect(),
@@ -201,7 +200,6 @@ mod unicodedata {
 
         #[pymethod]
         fn is_normalized(&self, form: super::NormalizeForm, unistr: PyStrRef) -> PyResult<bool> {
-            use super::NormalizeForm::*;
             let text = unistr.as_wtf8();
             let normalized: Wtf8Buf = match form {
                 Nfc => text.map_utf8(|s| s.nfc()).collect(),
diff --git a/crates/stdlib/src/zlib.rs b/crates/stdlib/src/zlib.rs
index 40269f12bbf..35a617ed152 100644
--- a/crates/stdlib/src/zlib.rs
+++ b/crates/stdlib/src/zlib.rs
@@ -466,7 +466,8 @@ mod zlib {
     }
 
     fn new_zlib_error(message: impl Into<String>, vm: &VirtualMachine) -> PyBaseExceptionRef {
-        vm.new_exception_msg(vm.class("zlib", "error"), message.into())
+        let msg: String = message.into();
+        vm.new_exception_msg(vm.class("zlib", "error"), msg.into())
     }
 
     struct Level(Option<flate2::Compression>);
diff --git a/crates/vm/src/buffer.rs b/crates/vm/src/buffer.rs
index 33670f1c30a..db58c909bca 100644
--- a/crates/vm/src/buffer.rs
+++ b/crates/vm/src/buffer.rs
@@ -725,5 +725,6 @@ pub fn struct_error_type(vm: &VirtualMachine) -> &'static PyTypeRef {
 pub fn new_struct_error(vm: &VirtualMachine, msg: impl Into<String>) -> PyBaseExceptionRef {
     // can't just STRUCT_ERROR.get().unwrap() cause this could be called before from buffer
     // machinery, independent of whether _struct was ever imported
+    let msg: String = msg.into();
     vm.new_exception_msg(struct_error_type(vm).clone(), msg.into())
 }
diff --git a/crates/vm/src/builtins/bool.rs b/crates/vm/src/builtins/bool.rs
index 24ded08ab10..dff2e642b93 100644
--- a/crates/vm/src/builtins/bool.rs
+++ b/crates/vm/src/builtins/bool.rs
@@ -1,4 +1,4 @@
-use super::{PyInt, PyStrRef, PyType, PyTypeRef};
+use super::{PyInt, PyStrRef, PyType, PyTypeRef, PyUtf8StrRef};
 use crate::common::format::FormatSpec;
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyResult, TryFromBorrowedObject, VirtualMachine,
@@ -101,7 +101,7 @@ impl Constructor for PyBool {
 #[pyclass(with(Constructor, AsNumber, Representable), flags(_MATCH_SELF))]
 impl PyBool {
     #[pymethod]
-    fn __format__(obj: PyObjectRef, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn __format__(obj: PyObjectRef, spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<String> {
         let new_bool = obj.try_to_bool(vm)?;
         FormatSpec::parse(spec.as_str())
             .and_then(|format_spec| format_spec.format_bool(new_bool))
diff --git a/crates/vm/src/builtins/classmethod.rs b/crates/vm/src/builtins/classmethod.rs
index d2f1377be04..22df6f27475 100644
--- a/crates/vm/src/builtins/classmethod.rs
+++ b/crates/vm/src/builtins/classmethod.rs
@@ -211,11 +211,11 @@ impl Representable for PyClassMethod {
             class
                 .__qualname__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|n| n.as_str()),
+                .map(|n| n.as_wtf8()),
             class
                 .__module__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|m| m.as_str()),
+                .map(|m| m.as_wtf8()),
         ) {
             (None, _) => return Err(vm.new_type_error("Unknown qualified name")),
             (Some(qualname), Some(module)) if module != "builtins" => {
diff --git a/crates/vm/src/builtins/code.rs b/crates/vm/src/builtins/code.rs
index 790f3fd8695..fcaff687946 100644
--- a/crates/vm/src/builtins/code.rs
+++ b/crates/vm/src/builtins/code.rs
@@ -459,7 +459,7 @@ impl Constructor for PyCode {
                 let s = obj.downcast_ref::<super::pystr::PyStr>().ok_or_else(|| {
                     vm.new_type_error("names must be tuple of strings".to_owned())
                 })?;
-                Ok(vm.ctx.intern_str(s.as_str()))
+                Ok(vm.ctx.intern_str(s.as_wtf8()))
             })
             .collect::<PyResult<Vec<_>>>()?
             .into_boxed_slice();
@@ -471,7 +471,7 @@ impl Constructor for PyCode {
                 let s = obj.downcast_ref::<super::pystr::PyStr>().ok_or_else(|| {
                     vm.new_type_error("varnames must be tuple of strings".to_owned())
                 })?;
-                Ok(vm.ctx.intern_str(s.as_str()))
+                Ok(vm.ctx.intern_str(s.as_wtf8()))
             })
             .collect::<PyResult<Vec<_>>>()?
             .into_boxed_slice();
@@ -483,7 +483,7 @@ impl Constructor for PyCode {
                 let s = obj.downcast_ref::<super::pystr::PyStr>().ok_or_else(|| {
                     vm.new_type_error("cellvars must be tuple of strings".to_owned())
                 })?;
-                Ok(vm.ctx.intern_str(s.as_str()))
+                Ok(vm.ctx.intern_str(s.as_wtf8()))
             })
             .collect::<PyResult<Vec<_>>>()?
             .into_boxed_slice();
@@ -495,7 +495,7 @@ impl Constructor for PyCode {
                 let s = obj.downcast_ref::<super::pystr::PyStr>().ok_or_else(|| {
                     vm.new_type_error("freevars must be tuple of strings".to_owned())
                 })?;
-                Ok(vm.ctx.intern_str(s.as_str()))
+                Ok(vm.ctx.intern_str(s.as_wtf8()))
             })
             .collect::<PyResult<Vec<_>>>()?
             .into_boxed_slice();
@@ -551,15 +551,15 @@ impl Constructor for PyCode {
             posonlyarg_count: args.posonlyargcount,
             arg_count: args.argcount,
             kwonlyarg_count: args.kwonlyargcount,
-            source_path: vm.ctx.intern_str(args.filename.as_str()),
+            source_path: vm.ctx.intern_str(args.filename.as_wtf8()),
             first_line_number: if args.firstlineno > 0 {
                 OneIndexed::new(args.firstlineno as usize)
             } else {
                 None
             },
             max_stackdepth: args.stacksize,
-            obj_name: vm.ctx.intern_str(args.name.as_str()),
-            qualname: vm.ctx.intern_str(args.qualname.as_str()),
+            obj_name: vm.ctx.intern_str(args.name.as_wtf8()),
+            qualname: vm.ctx.intern_str(args.qualname.as_wtf8()),
             cell2arg: None, // TODO: reuse `fn cell2arg`
             constants,
             names,
diff --git a/crates/vm/src/builtins/complex.rs b/crates/vm/src/builtins/complex.rs
index dd68061557d..ab656c974ee 100644
--- a/crates/vm/src/builtins/complex.rs
+++ b/crates/vm/src/builtins/complex.rs
@@ -1,9 +1,9 @@
 use super::{PyStr, PyType, PyTypeRef, float};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-    builtins::PyStrRef,
+    builtins::PyUtf8StrRef,
     class::PyClassImpl,
-    common::format::FormatSpec,
+    common::{format::FormatSpec, wtf8::Wtf8Buf},
     convert::{IntoPyException, ToPyObject, ToPyResult},
     function::{FuncArgs, OptionalArg, PyComparisonValue},
     protocol::PyNumberMethods,
@@ -276,13 +276,14 @@ impl PyComplex {
     }
 
     #[pymethod]
-    fn __format__(zelf: &Py<Self>, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn __format__(zelf: &Py<Self>, spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // Empty format spec: equivalent to str(self)
         if spec.is_empty() {
-            return Ok(zelf.as_object().str(vm)?.as_str().to_owned());
+            return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
         FormatSpec::parse(spec.as_str())
             .and_then(|format_spec| format_spec.format_complex(&zelf.value))
+            .map(Wtf8Buf::from_string)
             .map_err(|err| err.into_pyexception(vm))
     }
 }
diff --git a/crates/vm/src/builtins/dict.rs b/crates/vm/src/builtins/dict.rs
index 1d79a5a5906..0cc9ee66f3f 100644
--- a/crates/vm/src/builtins/dict.rs
+++ b/crates/vm/src/builtins/dict.rs
@@ -27,6 +27,7 @@ use crate::{
 };
 use alloc::fmt;
 use rustpython_common::lock::PyMutex;
+use rustpython_common::wtf8::Wtf8Buf;
 
 pub type DictContentType = dict_inner::Dict;
 
@@ -543,14 +544,19 @@ impl Representable for PyDict {
     #[inline]
     fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyStrRef> {
         let s = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-            let mut str_parts = Vec::with_capacity(zelf.__len__());
+            let mut result = Wtf8Buf::from("{");
+            let mut first = true;
             for (key, value) in zelf {
-                let key_repr = &key.repr(vm)?;
-                let value_repr = value.repr(vm)?;
-                str_parts.push(format!("{key_repr}: {value_repr}"));
+                if !first {
+                    result.push_str(", ");
+                }
+                first = false;
+                result.push_wtf8(key.repr(vm)?.as_wtf8());
+                result.push_str(": ");
+                result.push_wtf8(value.repr(vm)?.as_wtf8());
             }
-
-            vm.ctx.new_str(format!("{{{}}}", str_parts.join(", ")))
+            result.push_char('}');
+            vm.ctx.new_str(result)
         } else {
             vm.ctx.intern_str("{...}").to_owned()
         };
@@ -848,13 +854,17 @@ macro_rules! dict_view {
             #[inline]
             fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyStrRef> {
                 let s = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-                    let mut str_parts = Vec::with_capacity(zelf.__len__());
+                    let mut result = Wtf8Buf::from(format!("{}([", Self::NAME));
+                    let mut first = true;
                     for (key, value) in zelf.dict().clone() {
-                        let s = &Self::item(vm, key, value).repr(vm)?;
-                        str_parts.push(s.as_str().to_owned());
+                        if !first {
+                            result.push_str(", ");
+                        }
+                        first = false;
+                        result.push_wtf8(Self::item(vm, key, value).repr(vm)?.as_wtf8());
                     }
-                    vm.ctx
-                        .new_str(format!("{}([{}])", Self::NAME, str_parts.join(", ")))
+                    result.push_str("])");
+                    vm.ctx.new_str(result)
                 } else {
                     vm.ctx.intern_str("{...}").to_owned()
                 };
diff --git a/crates/vm/src/builtins/float.rs b/crates/vm/src/builtins/float.rs
index 89e42ec0f39..23d04d36998 100644
--- a/crates/vm/src/builtins/float.rs
+++ b/crates/vm/src/builtins/float.rs
@@ -1,11 +1,12 @@
 use super::{
-    PyByteArray, PyBytes, PyInt, PyIntRef, PyStr, PyStrRef, PyType, PyTypeRef, try_bigint_to_f64,
+    PyByteArray, PyBytes, PyInt, PyIntRef, PyStr, PyType, PyTypeRef, PyUtf8StrRef,
+    try_bigint_to_f64,
 };
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult,
     TryFromBorrowedObject, TryFromObject, VirtualMachine,
     class::PyClassImpl,
-    common::{float_ops, format::FormatSpec, hash},
+    common::{float_ops, format::FormatSpec, hash, wtf8::Wtf8Buf},
     convert::{IntoPyException, ToPyObject, ToPyResult},
     function::{
         ArgBytesLike, FuncArgs, OptionalArg, OptionalOption, PyArithmeticValue::*,
@@ -214,18 +215,19 @@ fn float_from_string(val: PyObjectRef, vm: &VirtualMachine) -> PyResult<f64> {
 )]
 impl PyFloat {
     #[pymethod]
-    fn __format__(zelf: &Py<Self>, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn __format__(zelf: &Py<Self>, spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // Empty format spec: equivalent to str(self)
         if spec.is_empty() {
-            return Ok(zelf.as_object().str(vm)?.as_str().to_owned());
+            return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
         FormatSpec::parse(spec.as_str())
             .and_then(|format_spec| format_spec.format_float(zelf.value))
+            .map(Wtf8Buf::from_string)
             .map_err(|err| err.into_pyexception(vm))
     }
 
     #[pystaticmethod]
-    fn __getformat__(spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn __getformat__(spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<String> {
         if !matches!(spec.as_str(), "double" | "float") {
             return Err(
                 vm.new_value_error("__getformat__() argument 1 must be 'double' or 'float'")
@@ -340,7 +342,7 @@ impl PyFloat {
     }
 
     #[pyclassmethod]
-    fn fromhex(cls: PyTypeRef, string: PyStrRef, vm: &VirtualMachine) -> PyResult {
+    fn fromhex(cls: PyTypeRef, string: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult {
         let result = crate::literal::float::from_hex(string.as_str().trim())
             .ok_or_else(|| vm.new_value_error("invalid hexadecimal floating-point string"))?;
         PyType::call(&cls, vec![vm.ctx.new_float(result).into()].into(), vm)
diff --git a/crates/vm/src/builtins/function.rs b/crates/vm/src/builtins/function.rs
index 43615711e5d..f8ff44e8a72 100644
--- a/crates/vm/src/builtins/function.rs
+++ b/crates/vm/src/builtins/function.rs
@@ -14,6 +14,7 @@ use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
     bytecode,
     class::PyClassImpl,
+    common::wtf8::{Wtf8Buf, wtf8_concat},
     frame::Frame,
     function::{FuncArgs, OptionalArg, PyComparisonValue, PySetterValue},
     scope::Scope,
@@ -1074,28 +1075,6 @@ impl PyBoundMethod {
     fn __module__(&self, vm: &VirtualMachine) -> Option<PyObjectRef> {
         self.function.get_attr("__module__", vm).ok()
     }
-
-    #[pygetset]
-    fn __qualname__(&self, vm: &VirtualMachine) -> PyResult {
-        if self
-            .function
-            .fast_isinstance(vm.ctx.types.builtin_function_or_method_type)
-        {
-            // Special case: we work with `__new__`, which is not really a method.
-            // It is a function, so its `__qualname__` is just `__new__`.
-            // We need to add object's part manually.
-            let obj_name = vm.get_attribute_opt(self.object.clone(), "__qualname__")?;
-            let obj_name: Option<PyStrRef> = obj_name.and_then(|o| o.downcast().ok());
-            return Ok(vm
-                .ctx
-                .new_str(format!(
-                    "{}.__new__",
-                    obj_name.as_ref().map_or("?", |s| s.as_str())
-                ))
-                .into());
-        }
-        self.function.get_attr("__qualname__", vm)
-    }
 }
 
 impl PyPayload for PyBoundMethod {
@@ -1107,21 +1086,23 @@ impl PyPayload for PyBoundMethod {
 
 impl Representable for PyBoundMethod {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-        let func_name =
-            if let Some(qname) = vm.get_attribute_opt(zelf.function.clone(), "__qualname__")? {
-                Some(qname)
-            } else {
-                vm.get_attribute_opt(zelf.function.clone(), "__name__")?
-            };
-        let func_name: Option<PyStrRef> = func_name.and_then(|o| o.downcast().ok());
-        let formatted_func_name = match func_name {
-            Some(name) => name.to_string(),
-            None => "?".to_string(),
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+        let func_name = if let Some(qname) =
+            vm.get_attribute_opt(zelf.function.clone(), identifier!(vm, __qualname__))?
+        {
+            Some(qname)
+        } else {
+            vm.get_attribute_opt(zelf.function.clone(), identifier!(vm, __name__))?
         };
+        let func_name: Option<PyStrRef> = func_name.and_then(|o| o.downcast().ok());
         let object_repr = zelf.object.repr(vm)?;
-        Ok(format!(
-            "<bound method {formatted_func_name} of {object_repr}>",
+        let name = func_name.as_ref().map_or("?".as_ref(), |s| s.as_wtf8());
+        Ok(wtf8_concat!(
+            "<bound method ",
+            name,
+            " of ",
+            object_repr.as_wtf8(),
+            ">"
         ))
     }
 }
diff --git a/crates/vm/src/builtins/function/jit.rs b/crates/vm/src/builtins/function/jit.rs
index a28335900da..9d3803759cf 100644
--- a/crates/vm/src/builtins/function/jit.rs
+++ b/crates/vm/src/builtins/function/jit.rs
@@ -41,7 +41,7 @@ impl ToPyObject for AbiValue {
 
 pub fn new_jit_error(msg: String, vm: &VirtualMachine) -> PyBaseExceptionRef {
     let jit_error = vm.ctx.exceptions.jit_error.to_owned();
-    vm.new_exception_msg(jit_error, msg)
+    vm.new_exception_msg(jit_error, msg.into())
 }
 
 fn get_jit_arg_type(dict: &Py<PyDict>, name: &str, vm: &VirtualMachine) -> PyResult<JitType> {
diff --git a/crates/vm/src/builtins/genericalias.rs b/crates/vm/src/builtins/genericalias.rs
index 8aabca1ae10..678fca606ff 100644
--- a/crates/vm/src/builtins/genericalias.rs
+++ b/crates/vm/src/builtins/genericalias.rs
@@ -635,14 +635,14 @@ impl Hashable for PyGenericAlias {
 
 impl GetAttr for PyGenericAlias {
     fn getattro(zelf: &Py<Self>, attr: &Py<PyStr>, vm: &VirtualMachine) -> PyResult {
-        let attr_str = attr.as_str();
+        let attr_str = attr.as_wtf8();
         for exc in &ATTR_EXCEPTIONS {
-            if *exc == attr_str {
+            if attr_str == *exc {
                 return zelf.as_object().generic_getattr(attr, vm);
             }
         }
         for blocked in &ATTR_BLOCKED {
-            if *blocked == attr_str {
+            if attr_str == *blocked {
                 return zelf.as_object().generic_getattr(attr, vm);
             }
         }
diff --git a/crates/vm/src/builtins/int.rs b/crates/vm/src/builtins/int.rs
index bbbc7d17673..0bf7372bbb3 100644
--- a/crates/vm/src/builtins/int.rs
+++ b/crates/vm/src/builtins/int.rs
@@ -2,13 +2,14 @@ use super::{PyByteArray, PyBytes, PyStr, PyType, PyTypeRef, float};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyRefExact, PyResult,
     TryFromBorrowedObject, VirtualMachine,
-    builtins::PyStrRef,
+    builtins::PyUtf8StrRef,
     bytes_inner::PyBytesInner,
     class::PyClassImpl,
     common::{
         format::FormatSpec,
         hash,
         int::{bigint_to_finite_float, bytes_to_int, true_div},
+        wtf8::Wtf8Buf,
     },
     convert::{IntoPyException, ToPyObject, ToPyResult},
     function::{
@@ -197,7 +198,7 @@ fn inner_truediv(i1: &BigInt, i2: &BigInt, vm: &VirtualMachine) -> PyResult {
     if float.is_infinite() {
         Err(vm.new_exception_msg(
             vm.ctx.exceptions.overflow_error.to_owned(),
-            "integer division result too large for a float".to_owned(),
+            "integer division result too large for a float".into(),
         ))
     } else {
         Ok(vm.ctx.new_float(float).into())
@@ -444,13 +445,14 @@ impl PyInt {
     }
 
     #[pymethod]
-    fn __format__(zelf: &Py<Self>, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn __format__(zelf: &Py<Self>, spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         // Empty format spec on a subclass: equivalent to str(self)
         if spec.is_empty() && !zelf.class().is(vm.ctx.types.int_type) {
-            return Ok(zelf.as_object().str(vm)?.as_str().to_owned());
+            return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
         FormatSpec::parse(spec.as_str())
             .and_then(|format_spec| format_spec.format_int(&zelf.value))
+            .map(Wtf8Buf::from_string)
             .map_err(|err| err.into_pyexception(vm))
     }
 
diff --git a/crates/vm/src/builtins/interpolation.rs b/crates/vm/src/builtins/interpolation.rs
index afdce51be9b..18fd10ba022 100644
--- a/crates/vm/src/builtins/interpolation.rs
+++ b/crates/vm/src/builtins/interpolation.rs
@@ -10,6 +10,8 @@ use crate::{
     function::{OptionalArg, PyComparisonValue},
     types::{Comparable, Constructor, Hashable, PyComparisonOp, Representable},
 };
+use itertools::Itertools;
+use rustpython_common::wtf8::Wtf8Buf;
 
 /// Interpolation object for t-strings (PEP 750).
 ///
@@ -42,11 +44,11 @@ impl PyInterpolation {
         let is_valid = vm.is_none(&conversion)
             || conversion
                 .downcast_ref::<PyStr>()
-                .is_some_and(|s| matches!(s.as_str(), "s" | "r" | "a"));
+                .is_some_and(|s| matches!(s.to_str(), Some("s") | Some("r") | Some("a")));
         if !is_valid {
             return Err(vm.new_exception_msg(
                 vm.ctx.exceptions.system_error.to_owned(),
-                "Interpolation() argument 'conversion' must be one of 's', 'a' or 'r'".to_owned(),
+                "Interpolation() argument 'conversion' must be one of 's', 'a' or 'r'".into(),
             ));
         }
         Ok(Self {
@@ -63,8 +65,13 @@ impl Constructor for PyInterpolation {
 
     fn py_new(_cls: &Py<PyType>, args: Self::Args, vm: &VirtualMachine) -> PyResult<Self> {
         let conversion: PyObjectRef = if let Some(s) = args.conversion {
-            let s_str = s.as_str();
-            if s_str.len() != 1 || !matches!(s_str.chars().next(), Some('s' | 'r' | 'a')) {
+            let has_flag = s
+                .as_bytes()
+                .iter()
+                .exactly_one()
+                .ok()
+                .is_some_and(|s| matches!(*s, b's' | b'r' | b'a'));
+            if !has_flag {
                 return Err(vm.new_value_error(
                     "Interpolation() argument 'conversion' must be one of 's', 'a' or 'r'",
                 ));
@@ -194,25 +201,26 @@ impl Hashable for PyInterpolation {
 
 impl Representable for PyInterpolation {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let value_repr = zelf.value.repr(vm)?;
         let expr_repr = zelf.expression.repr(vm)?;
+        let spec_repr = zelf.format_spec.repr(vm)?;
 
-        let conv_str = if vm.is_none(&zelf.conversion) {
-            "None".to_owned()
+        let mut result = Wtf8Buf::from("Interpolation(");
+        result.push_wtf8(value_repr.as_wtf8());
+        result.push_str(", ");
+        result.push_str(&expr_repr);
+        result.push_str(", ");
+        if vm.is_none(&zelf.conversion) {
+            result.push_str("None");
         } else {
-            zelf.conversion.repr(vm)?.as_str().to_owned()
-        };
-
-        let spec_repr = zelf.format_spec.repr(vm)?;
+            result.push_wtf8(zelf.conversion.repr(vm)?.as_wtf8());
+        }
+        result.push_str(", ");
+        result.push_str(&spec_repr);
+        result.push_char(')');
 
-        Ok(format!(
-            "Interpolation({}, {}, {}, {})",
-            value_repr.as_str(),
-            expr_repr.as_str(),
-            conv_str,
-            spec_repr.as_str()
-        ))
+        Ok(result)
     }
 }
 
diff --git a/crates/vm/src/builtins/list.rs b/crates/vm/src/builtins/list.rs
index 7e22f73f8ec..f74d4c4d62f 100644
--- a/crates/vm/src/builtins/list.rs
+++ b/crates/vm/src/builtins/list.rs
@@ -9,6 +9,7 @@ use crate::common::lock::{
 use crate::object::{Traverse, TraverseFn};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult,
+    builtins::PyStr,
     class::PyClassImpl,
     convert::ToPyObject,
     function::{ArgSize, FuncArgs, OptionalArg, PyComparisonValue},
@@ -516,19 +517,25 @@ impl Comparable for PyList {
 
 impl Representable for PyList {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-        let s = if zelf.__len__() == 0 {
-            "[]".to_owned()
-        } else if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+    fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyRef<PyStr>> {
+        if zelf.__len__() == 0 {
+            return Ok(vm.ctx.intern_str("[]").to_owned());
+        }
+        if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
             // Clone elements before calling repr to release the read lock.
             // Element repr may mutate the list (e.g., list.clear()), which
             // needs a write lock and would deadlock if read lock is held.
             let elements: Vec<PyObjectRef> = zelf.borrow_vec().to_vec();
-            collection_repr(None, "[", "]", elements.iter(), vm)?
+            Ok(vm
+                .ctx
+                .new_str(collection_repr(None, "[", "]", elements.iter(), vm)?))
         } else {
-            "[...]".to_owned()
-        };
-        Ok(s)
+            Ok(vm.ctx.intern_str("[...]").to_owned())
+        }
+    }
+
+    fn repr_str(_zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
+        unreachable!("repr() is overridden directly")
     }
 }
 
diff --git a/crates/vm/src/builtins/mappingproxy.rs b/crates/vm/src/builtins/mappingproxy.rs
index 11525c3f80a..7c852af081f 100644
--- a/crates/vm/src/builtins/mappingproxy.rs
+++ b/crates/vm/src/builtins/mappingproxy.rs
@@ -1,10 +1,9 @@
 use super::{PyDict, PyDictRef, PyGenericAlias, PyList, PyTuple, PyType, PyTypeRef};
-use crate::common::lock::LazyLock;
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
     atomic_func,
     class::PyClassImpl,
-    common::hash,
+    common::{hash, lock::LazyLock},
     convert::ToPyObject,
     function::{ArgMapping, OptionalArg, PyComparisonValue},
     object::{Traverse, TraverseFn},
@@ -14,6 +13,7 @@ use crate::{
         PyComparisonOp, Representable,
     },
 };
+use rustpython_common::wtf8::{Wtf8Buf, wtf8_concat};
 
 #[pyclass(module = false, name = "mappingproxy", traverse)]
 #[derive(Debug)]
@@ -287,9 +287,9 @@ impl Iterable for PyMappingProxy {
 
 impl Representable for PyMappingProxy {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let obj = zelf.to_object(vm)?;
-        Ok(format!("mappingproxy({})", obj.repr(vm)?))
+        Ok(wtf8_concat!("mappingproxy(", obj.repr(vm)?.as_wtf8(), ')'))
     }
 }
 
diff --git a/crates/vm/src/builtins/memory.rs b/crates/vm/src/builtins/memory.rs
index 329a6be6737..f29753c1f84 100644
--- a/crates/vm/src/builtins/memory.rs
+++ b/crates/vm/src/builtins/memory.rs
@@ -1,6 +1,6 @@
 use super::{
     PositionIterInternal, PyBytes, PyBytesRef, PyGenericAlias, PyInt, PyListRef, PySlice, PyStr,
-    PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef, iter::builtins_iter,
+    PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef, PyUtf8StrRef, iter::builtins_iter,
 };
 use crate::common::lock::LazyLock;
 use crate::{
@@ -805,8 +805,9 @@ impl PyMemoryView {
         Err(vm.new_value_error("memoryview.index(x): x not in memoryview"))
     }
 
-    fn cast_to_1d(&self, format: PyStrRef, vm: &VirtualMachine) -> PyResult<Self> {
-        let format_spec = Self::parse_format(format.as_str(), vm)?;
+    fn cast_to_1d(&self, format: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<Self> {
+        let format_str = format.as_str();
+        let format_spec = Self::parse_format(format_str, vm)?;
         let itemsize = format_spec.size();
         if !self.desc.len.is_multiple_of(itemsize) {
             return Err(vm.new_type_error("memoryview: length is not a multiple of itemsize"));
@@ -821,7 +822,7 @@ impl PyMemoryView {
                 len: self.desc.len,
                 readonly: self.desc.readonly,
                 itemsize,
-                format: format.to_string().into(),
+                format: format_str.to_owned().into(),
                 dim_desc: vec![(self.desc.len / itemsize, itemsize as isize, 0)],
             },
             hash: OnceCell::new(),
@@ -956,7 +957,7 @@ impl Py<PyMemoryView> {
 #[derive(FromArgs)]
 struct CastArgs {
     #[pyarg(any)]
-    format: PyStrRef,
+    format: PyUtf8StrRef,
     #[pyarg(any, optional)]
     shape: OptionalArg<Either<PyTupleRef, PyListRef>>,
 }
diff --git a/crates/vm/src/builtins/mod.rs b/crates/vm/src/builtins/mod.rs
index fa7ab1b854e..099787332c9 100644
--- a/crates/vm/src/builtins/mod.rs
+++ b/crates/vm/src/builtins/mod.rs
@@ -63,7 +63,7 @@ pub(crate) mod bool_;
 pub use bool_::PyBool;
 #[path = "str.rs"]
 pub(crate) mod pystr;
-pub use pystr::{PyStr, PyStrInterned, PyStrRef, PyUtf8Str, PyUtf8StrRef};
+pub use pystr::{PyStr, PyStrInterned, PyStrRef, PyUtf8Str, PyUtf8StrInterned, PyUtf8StrRef};
 #[path = "super.rs"]
 pub(crate) mod super_;
 pub use super_::PySuper;
diff --git a/crates/vm/src/builtins/module.rs b/crates/vm/src/builtins/module.rs
index a2221fb6b9a..ad90970e7b9 100644
--- a/crates/vm/src/builtins/module.rs
+++ b/crates/vm/src/builtins/module.rs
@@ -160,9 +160,10 @@ impl Py<PyModule> {
             .get_item_opt(identifier!(vm, __name__), vm)
             .ok()
             .flatten();
-        let mod_name_str = mod_name_obj
-            .as_ref()
-            .and_then(|n| n.downcast_ref::<PyStr>().map(|s| s.as_str().to_owned()));
+        let mod_name_str = mod_name_obj.as_ref().and_then(|n| {
+            n.downcast_ref::<PyStr>()
+                .map(|s| s.to_string_lossy().into_owned())
+        });
 
         // If __name__ is not set or not a string, use a simpler error message
         let mod_display = match mod_name_str.as_deref() {
@@ -424,7 +425,7 @@ impl Initializer for PyModule {
                 .flags
                 .has_feature(crate::types::PyTypeFlags::HAS_DICT)
         );
-        zelf.init_dict(vm.ctx.intern_str(args.name.as_str()), args.doc, vm);
+        zelf.init_dict(vm.ctx.intern_str(args.name.as_wtf8()), args.doc, vm);
         Ok(())
     }
 }
diff --git a/crates/vm/src/builtins/namespace.rs b/crates/vm/src/builtins/namespace.rs
index 2cc1693302a..191fbd804b6 100644
--- a/crates/vm/src/builtins/namespace.rs
+++ b/crates/vm/src/builtins/namespace.rs
@@ -1,4 +1,4 @@
-use super::{PyTupleRef, PyType, tuple::IntoPyTuple};
+use super::{PyStr, PyTupleRef, PyType, tuple::IntoPyTuple};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
     builtins::PyDict,
@@ -9,6 +9,7 @@ use crate::{
         Comparable, Constructor, DefaultConstructor, Initializer, PyComparisonOp, Representable,
     },
 };
+use rustpython_common::wtf8::Wtf8Buf;
 
 /// A simple attribute-based namespace.
 ///
@@ -138,7 +139,7 @@ impl Comparable for PyNamespace {
 
 impl Representable for PyNamespace {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let o = zelf.as_object();
         let name = if o.class().is(vm.ctx.types.namespace_type) {
             "namespace".to_owned()
@@ -148,16 +149,27 @@ impl Representable for PyNamespace {
 
         let repr = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
             let dict = zelf.as_object().dict().unwrap();
-            let mut parts = Vec::with_capacity(dict.__len__());
+            let mut result = Wtf8Buf::from(format!("{name}("));
+            let mut first = true;
             for (key, value) in dict {
-                let k = key.repr(vm)?;
-                let key_str = k.as_wtf8();
-                let value_repr = value.repr(vm)?;
-                parts.push(format!("{}={}", &key_str[1..key_str.len() - 1], value_repr));
+                let Some(key_str) = key.downcast_ref::<PyStr>() else {
+                    continue;
+                };
+                if key_str.as_wtf8().is_empty() {
+                    continue;
+                }
+                if !first {
+                    result.push_str(", ");
+                }
+                first = false;
+                result.push_wtf8(key_str.as_wtf8());
+                result.push_char('=');
+                result.push_wtf8(value.repr(vm)?.as_wtf8());
             }
-            format!("{}({})", name, parts.join(", "))
+            result.push_char(')');
+            result
         } else {
-            format!("{name}(...)")
+            Wtf8Buf::from(format!("{name}(...)"))
         };
         Ok(repr)
     }
diff --git a/crates/vm/src/builtins/object.rs b/crates/vm/src/builtins/object.rs
index e4c51061685..10b500619eb 100644
--- a/crates/vm/src/builtins/object.rs
+++ b/crates/vm/src/builtins/object.rs
@@ -1,4 +1,4 @@
-use super::{PyDictRef, PyList, PyStr, PyStrRef, PyType, PyTypeRef};
+use super::{PyDictRef, PyList, PyStr, PyStrRef, PyType, PyTypeRef, PyUtf8StrRef};
 use crate::common::hash::PyHash;
 use crate::types::PyTypeFlags;
 use crate::{
@@ -82,9 +82,12 @@ impl Constructor for PyBaseObject {
         if let Some(abs_methods) = cls.get_attr(identifier!(vm, __abstractmethods__))
             && let Some(unimplemented_abstract_method_count) = abs_methods.length_opt(vm)
         {
-            let methods: Vec<PyStrRef> = abs_methods.try_to_value(vm)?;
-            let methods: String =
-                Itertools::intersperse(methods.iter().map(|name| name.as_str()), "', '").collect();
+            let methods: Vec<PyUtf8StrRef> = abs_methods.try_to_value(vm)?;
+            let methods: String = Itertools::intersperse(
+                methods.iter().map(|name| name.as_str().to_owned()),
+                "', '".to_owned(),
+            )
+            .collect();
 
             let unimplemented_abstract_method_count = unimplemented_abstract_method_count?;
             let name = cls.name().to_string();
@@ -218,7 +221,7 @@ fn object_getstate_default(obj: &PyObject, required: bool, vm: &VirtualMachine)
         let has_weakref = if let Some(ref ext) = obj.class().heaptype_ext {
             match &ext.slots {
                 None => true, // Heap type without __slots__ has automatic weakref
-                Some(slots) => slots.iter().any(|s| s.as_str() == "__weakref__"),
+                Some(slots) => slots.iter().any(|s| s.as_bytes() == b"__weakref__"),
             }
         } else {
             let weakref_name = vm.ctx.intern_str("__weakref__");
@@ -255,7 +258,7 @@ fn object_getstate_default(obj: &PyObject, required: bool, vm: &VirtualMachine)
                 let Ok(value) = obj.get_attr(name, vm) else {
                     continue;
                 };
-                slots.set_item(name.as_str(), value, vm).unwrap();
+                slots.set_item(name.as_wtf8(), value, vm).unwrap();
             }
 
             if !slots.is_empty() {
@@ -385,11 +388,11 @@ impl PyBaseObject {
             class
                 .__qualname__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|n| n.as_str()),
+                .map(|n| n.as_wtf8()),
             class
                 .__module__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|m| m.as_str()),
+                .map(|m| m.as_wtf8()),
         ) {
             (None, _) => Err(vm.new_type_error("Unknown qualified name")),
             (Some(qualname), Some(module)) if module != "builtins" => Ok(PyStr::from(format!(
@@ -473,7 +476,7 @@ impl PyBaseObject {
                             a.len() == b.len()
                                 && a.iter()
                                     .zip(b.iter())
-                                    .all(|(x, y)| x.as_str() == y.as_str())
+                                    .all(|(x, y)| x.as_wtf8() == y.as_wtf8())
                         }
                         (None, None) => true,
                         _ => false,
diff --git a/crates/vm/src/builtins/range.rs b/crates/vm/src/builtins/range.rs
index 92de2463e2c..e42cf5f23f6 100644
--- a/crates/vm/src/builtins/range.rs
+++ b/crates/vm/src/builtins/range.rs
@@ -52,8 +52,9 @@ fn iter_search(
         SearchType::Index => Err(vm.new_value_error(format!(
             "{} not in range",
             item.repr(vm)
-                .map(|v| v.as_str().to_owned())
-                .unwrap_or_else(|_| "value".to_owned())
+                .as_ref()
+                .map_or("value".as_ref(), |s| s.as_wtf8())
+                .to_owned()
         ))),
     }
 }
diff --git a/crates/vm/src/builtins/set.rs b/crates/vm/src/builtins/set.rs
index 17f406bbcbe..7e2c10ac8f0 100644
--- a/crates/vm/src/builtins/set.rs
+++ b/crates/vm/src/builtins/set.rs
@@ -10,7 +10,7 @@ use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject,
     atomic_func,
     class::PyClassImpl,
-    common::{ascii, hash::PyHash, lock::PyMutex, rc::PyRc},
+    common::{ascii, hash::PyHash, lock::PyMutex, rc::PyRc, wtf8::Wtf8Buf},
     convert::ToPyResult,
     dict_inner::{self, DictSize},
     function::{ArgIterable, FuncArgs, OptionalArg, PosArgs, PyArithmeticValue, PyComparisonValue},
@@ -318,7 +318,7 @@ impl PySetInner {
         }
     }
 
-    fn repr(&self, class_name: Option<&str>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr(&self, class_name: Option<&str>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         collection_repr(class_name, "{", "}", self.elements().iter(), vm)
     }
 
@@ -942,23 +942,23 @@ impl AsNumber for PySet {
 
 impl Representable for PySet {
     #[inline]
-    fn repr_str(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let class = zelf.class();
         let borrowed_name = class.name();
         let class_name = borrowed_name.deref();
-        let s = if zelf.inner.len() == 0 {
-            format!("{class_name}()")
-        } else if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+        if zelf.inner.len() == 0 {
+            return Ok(Wtf8Buf::from(format!("{class_name}()")));
+        }
+        if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
             let name = if class_name != "set" {
                 Some(class_name)
             } else {
                 None
             };
-            zelf.inner.repr(name, vm)?
+            zelf.inner.repr(name, vm)
         } else {
-            format!("{class_name}(...)")
-        };
-        Ok(s)
+            Ok(Wtf8Buf::from(format!("{class_name}(...)")))
+        }
     }
 }
 
@@ -1289,18 +1289,18 @@ impl AsNumber for PyFrozenSet {
 
 impl Representable for PyFrozenSet {
     #[inline]
-    fn repr_str(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let inner = &zelf.inner;
         let class = zelf.class();
         let class_name = class.name();
-        let s = if inner.len() == 0 {
-            format!("{class_name}()")
-        } else if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-            inner.repr(Some(&class_name), vm)?
+        if inner.len() == 0 {
+            return Ok(Wtf8Buf::from(format!("{class_name}()")));
+        }
+        if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+            inner.repr(Some(&class_name), vm)
         } else {
-            format!("{class_name}(...)")
-        };
-        Ok(s)
+            Ok(Wtf8Buf::from(format!("{class_name}(...)")))
+        }
     }
 }
 
diff --git a/crates/vm/src/builtins/slice.rs b/crates/vm/src/builtins/slice.rs
index 3aa23b0746c..24c87aa1c11 100644
--- a/crates/vm/src/builtins/slice.rs
+++ b/crates/vm/src/builtins/slice.rs
@@ -1,5 +1,7 @@
 // sliceobject.{h,c} in CPython
 // spell-checker:ignore sliceobject
+use rustpython_common::wtf8::{Wtf8Buf, wtf8_concat};
+
 use super::{PyGenericAlias, PyStrRef, PyTupleRef, PyType, PyTypeRef};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
@@ -291,12 +293,20 @@ impl Comparable for PySlice {
 
 impl Representable for PySlice {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         let start_repr = zelf.start_ref(vm).repr(vm)?;
         let stop_repr = zelf.stop.repr(vm)?;
         let step_repr = zelf.step_ref(vm).repr(vm)?;
 
-        Ok(format!("slice({start_repr}, {stop_repr}, {step_repr})"))
+        Ok(wtf8_concat!(
+            "slice(",
+            start_repr.as_wtf8(),
+            ", ",
+            stop_repr.as_wtf8(),
+            ", ",
+            step_repr.as_wtf8(),
+            ")"
+        ))
     }
 }
 
diff --git a/crates/vm/src/builtins/staticmethod.rs b/crates/vm/src/builtins/staticmethod.rs
index ac363415a9f..fe0cc0a14e2 100644
--- a/crates/vm/src/builtins/staticmethod.rs
+++ b/crates/vm/src/builtins/staticmethod.rs
@@ -182,11 +182,11 @@ impl Representable for PyStaticMethod {
             class
                 .__qualname__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|n| n.as_str()),
+                .map(|n| n.as_wtf8()),
             class
                 .__module__(vm)
                 .downcast_ref::<PyStr>()
-                .map(|m| m.as_str()),
+                .map(|m| m.as_wtf8()),
         ) {
             (None, _) => Err(vm.new_type_error("Unknown qualified name")),
             (Some(qualname), Some(module)) if module != "builtins" => {
diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
index a1f0de1810f..576815e3b4f 100644
--- a/crates/vm/src/builtins/str.rs
+++ b/crates/vm/src/builtins/str.rs
@@ -41,7 +41,7 @@ use rustpython_common::{
     hash,
     lock::PyMutex,
     str::DeduceStrKind,
-    wtf8::{CodePoint, Wtf8, Wtf8Buf, Wtf8Chunk},
+    wtf8::{CodePoint, Wtf8, Wtf8Buf, Wtf8Chunk, Wtf8Concat},
 };
 use unic_ucd_bidi::BidiClass;
 use unic_ucd_category::GeneralCategory;
@@ -50,13 +50,13 @@ use unicode_casing::CharExt;
 
 impl<'a> TryFromBorrowedObject<'a> for String {
     fn try_from_borrowed_object(vm: &VirtualMachine, obj: &'a PyObject) -> PyResult<Self> {
-        obj.try_value_with(|pystr: &PyStr| Ok(pystr.as_str().to_owned()), vm)
+        obj.try_value_with(|pystr: &PyUtf8Str| Ok(pystr.as_str().to_owned()), vm)
     }
 }
 
 impl<'a> TryFromBorrowedObject<'a> for &'a str {
     fn try_from_borrowed_object(vm: &VirtualMachine, obj: &'a PyObject) -> PyResult<Self> {
-        let pystr: &Py<PyStr> = TryFromBorrowedObject::try_from_borrowed_object(vm, obj)?;
+        let pystr: &Py<PyUtf8Str> = TryFromBorrowedObject::try_from_borrowed_object(vm, obj)?;
         Ok(pystr.as_str())
     }
 }
@@ -90,21 +90,21 @@ impl fmt::Debug for PyStr {
 impl AsRef<str> for PyStr {
     #[track_caller] // <- can remove this once it doesn't panic
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str().expect("str has surrogates")
     }
 }
 
 impl AsRef<str> for Py<PyStr> {
     #[track_caller] // <- can remove this once it doesn't panic
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str().expect("str has surrogates")
     }
 }
 
 impl AsRef<str> for PyStrRef {
     #[track_caller] // <- can remove this once it doesn't panic
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str().expect("str has surrogates")
     }
 }
 
@@ -126,6 +126,20 @@ impl AsRef<Wtf8> for PyStrRef {
     }
 }
 
+impl Wtf8Concat for PyStr {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_wtf8(self.as_wtf8());
+    }
+}
+
+impl Wtf8Concat for Py<PyStr> {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_wtf8(self.as_wtf8());
+    }
+}
+
 impl<'a> From<&'a AsciiStr> for PyStr {
     fn from(s: &'a AsciiStr) -> Self {
         s.to_owned().into()
@@ -249,6 +263,13 @@ impl<'a> AsPyStr<'a> for &'a Py<PyStr> {
     }
 }
 
+impl<'a> AsPyStr<'a> for &'a Py<PyUtf8Str> {
+    #[inline]
+    fn as_pystr(self, _ctx: &Context) -> &'a Py<PyStr> {
+        Py::<PyUtf8Str>::as_pystr(self)
+    }
+}
+
 impl<'a> AsPyStr<'a> for &'a PyStrRef {
     #[inline]
     fn as_pystr(self, _ctx: &Context) -> &'a Py<PyStr> {
@@ -256,6 +277,13 @@ impl<'a> AsPyStr<'a> for &'a PyStrRef {
     }
 }
 
+impl<'a> AsPyStr<'a> for &'a PyUtf8StrRef {
+    #[inline]
+    fn as_pystr(self, _ctx: &Context) -> &'a Py<PyStr> {
+        Py::<PyUtf8Str>::as_pystr(self)
+    }
+}
+
 impl AsPyStr<'static> for &'static str {
     #[inline]
     fn as_pystr(self, ctx: &Context) -> &'static Py<PyStr> {
@@ -270,6 +298,13 @@ impl<'a> AsPyStr<'a> for &'a PyStrInterned {
     }
 }
 
+impl<'a> AsPyStr<'a> for &'a PyUtf8StrInterned {
+    #[inline]
+    fn as_pystr(self, _ctx: &Context) -> &'a Py<PyStr> {
+        Py::<PyUtf8Str>::as_pystr(self)
+    }
+}
+
 #[pyclass(module = false, name = "str_iterator", traverse = "manual")]
 #[derive(Debug)]
 pub struct PyStrIterator {
@@ -350,9 +385,9 @@ pub struct StrArgs {
     #[pyarg(any, optional)]
     object: OptionalArg<PyObjectRef>,
     #[pyarg(any, optional)]
-    encoding: OptionalArg<PyStrRef>,
+    encoding: OptionalArg<PyUtf8StrRef>,
     #[pyarg(any, optional)]
-    errors: OptionalArg<PyStrRef>,
+    errors: OptionalArg<PyUtf8StrRef>,
 }
 
 impl Constructor for PyStr {
@@ -440,17 +475,20 @@ impl PyStr {
         self.data.as_wtf8().as_bytes()
     }
 
-    // FIXME: make this return an Option
-    #[inline]
-    #[track_caller] // <- can remove this once it doesn't panic
-    pub fn as_str(&self) -> &str {
-        self.data.as_str().expect("str has surrogates")
-    }
-
     pub fn to_str(&self) -> Option<&str> {
         self.data.as_str()
     }
 
+    /// Returns `&str`
+    ///
+    /// # Panic
+    /// If the string contains surrogates.
+    #[inline]
+    #[track_caller]
+    pub fn expect_str(&self) -> &str {
+        self.to_str().expect("PyStr contains surrogates")
+    }
+
     pub(crate) fn ensure_valid_utf8(&self, vm: &VirtualMachine) -> PyResult<()> {
         if self.is_utf8() {
             Ok(())
@@ -660,14 +698,7 @@ impl PyStr {
         match self.as_str_kind() {
             PyKindStr::Ascii(s) => s.to_ascii_lowercase().into(),
             PyKindStr::Utf8(s) => s.to_lowercase().into(),
-            PyKindStr::Wtf8(w) => w
-                .chunks()
-                .map(|c| match c {
-                    Wtf8Chunk::Utf8(s) => s.to_lowercase().into(),
-                    Wtf8Chunk::Surrogate(c) => Wtf8Buf::from(c),
-                })
-                .collect::<Wtf8Buf>()
-                .into(),
+            PyKindStr::Wtf8(w) => w.to_lowercase().into(),
         }
     }
 
@@ -693,14 +724,7 @@ impl PyStr {
         match self.as_str_kind() {
             PyKindStr::Ascii(s) => s.to_ascii_uppercase().into(),
             PyKindStr::Utf8(s) => s.to_uppercase().into(),
-            PyKindStr::Wtf8(w) => w
-                .chunks()
-                .map(|c| match c {
-                    Wtf8Chunk::Utf8(s) => s.to_uppercase().into(),
-                    Wtf8Chunk::Surrogate(c) => Wtf8Buf::from(c),
-                })
-                .collect::<Wtf8Buf>()
-                .into(),
+            PyKindStr::Wtf8(w) => w.to_uppercase().into(),
         }
     }
 
@@ -732,12 +756,7 @@ impl PyStr {
                         Some(ch) => out.extend(ch.to_titlecase()),
                         None => out.push(ch),
                     }
-                    for chunk in chars.as_wtf8().chunks() {
-                        match chunk {
-                            Wtf8Chunk::Utf8(s) => out.push_str(&s.to_lowercase()),
-                            Wtf8Chunk::Surrogate(ch) => out.push(ch),
-                        }
-                    }
+                    out.push_wtf8(&chars.as_wtf8().to_lowercase());
                 }
                 out
             }
@@ -972,10 +991,9 @@ impl PyStr {
     #[pymethod]
     fn __format__(
         zelf: PyRef<PyStr>,
-        spec: PyStrRef,
+        spec: PyUtf8StrRef,
         vm: &VirtualMachine,
     ) -> PyResult<PyRef<PyStr>> {
-        let spec = spec.as_str();
         if spec.is_empty() {
             return if zelf.class().is(vm.ctx.types.str_type) {
                 Ok(zelf)
@@ -984,7 +1002,7 @@ impl PyStr {
             };
         }
         let zelf = zelf.try_into_utf8(vm)?;
-        let s = FormatSpec::parse(spec)
+        let s = FormatSpec::parse(spec.as_str())
             .and_then(|format_spec| {
                 format_spec.format_string(&CharLenStr(zelf.as_str(), zelf.char_len()))
             })
@@ -1345,33 +1363,34 @@ impl PyStr {
 
     // https://docs.python.org/3/library/stdtypes.html#str.translate
     #[pymethod]
-    fn translate(&self, table: PyObjectRef, vm: &VirtualMachine) -> PyResult<String> {
+    fn translate(&self, table: PyObjectRef, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
         vm.get_method_or_type_error(table.clone(), identifier!(vm, __getitem__), || {
             format!("'{}' object is not subscriptable", table.class().name())
         })?;
 
-        let mut translated = String::new();
-        for c in self.as_str().chars() {
-            match table.get_item(&*(c as u32).to_pyobject(vm), vm) {
+        let mut translated = Wtf8Buf::new();
+        for cp in self.as_wtf8().code_points() {
+            match table.get_item(&*cp.to_u32().to_pyobject(vm), vm) {
                 Ok(value) => {
                     if let Some(text) = value.downcast_ref::<Self>() {
-                        translated.push_str(text.as_str());
+                        translated.push_wtf8(text.as_wtf8());
                     } else if let Some(bigint) = value.downcast_ref::<PyInt>() {
-                        let ch = bigint
+                        let mapped = bigint
                             .as_bigint()
                             .to_u32()
-                            .and_then(core::char::from_u32)
+                            .and_then(CodePoint::from_u32)
                             .ok_or_else(|| {
                                 vm.new_value_error("character mapping must be in range(0x110000)")
                             })?;
-                        translated.push(ch);
+                        translated.push(mapped);
                     } else if !vm.is_none(&value) {
                         return Err(
                             vm.new_type_error("character mapping must return integer, None or str")
                         );
                     }
                 }
-                _ => translated.push(c),
+                Err(e) if e.fast_isinstance(vm.ctx.exceptions.key_error) => translated.push(cp),
+                Err(e) => return Err(e),
             }
         }
         Ok(translated)
@@ -1389,16 +1408,20 @@ impl PyStr {
             match dict_or_str.downcast::<Self>() {
                 Ok(from_str) => {
                     if to_str.len() == from_str.len() {
-                        for (c1, c2) in from_str.as_str().chars().zip(to_str.as_str().chars()) {
+                        for (c1, c2) in from_str
+                            .as_wtf8()
+                            .code_points()
+                            .zip(to_str.as_wtf8().code_points())
+                        {
                             new_dict.set_item(
-                                &*vm.new_pyobj(c1 as u32),
-                                vm.new_pyobj(c2 as u32),
+                                &*vm.new_pyobj(c1.to_u32()),
+                                vm.new_pyobj(c2.to_u32()),
                                 vm,
                             )?;
                         }
                         if let OptionalArg::Present(none_str) = none_str {
-                            for c in none_str.as_str().chars() {
-                                new_dict.set_item(&*vm.new_pyobj(c as u32), vm.ctx.none(), vm)?;
+                            for c in none_str.as_wtf8().code_points() {
+                                new_dict.set_item(&*vm.new_pyobj(c.to_u32()), vm.ctx.none(), vm)?;
                             }
                         }
                         Ok(new_dict.to_pyobject(vm))
@@ -1426,7 +1449,8 @@ impl PyStr {
                             )?;
                         } else if let Some(string) = key.downcast_ref::<Self>() {
                             if string.len() == 1 {
-                                let num_value = string.as_str().chars().next().unwrap() as u32;
+                                let num_value =
+                                    string.as_wtf8().code_points().next().unwrap().to_u32();
                                 new_dict.set_item(&*num_value.to_pyobject(vm), val, vm)?;
                             } else {
                                 return Err(vm.new_value_error(
@@ -1455,7 +1479,7 @@ impl PyStr {
 
     #[pymethod]
     fn __getnewargs__(zelf: PyRef<Self>, vm: &VirtualMachine) -> PyObjectRef {
-        (zelf.as_str(),).to_pyobject(vm)
+        (zelf.as_wtf8(),).to_pyobject(vm)
     }
 
     #[pymethod]
@@ -1601,20 +1625,21 @@ impl AsSequence for PyStr {
 #[derive(FromArgs)]
 struct EncodeArgs {
     #[pyarg(any, default)]
-    encoding: Option<PyStrRef>,
+    encoding: Option<PyUtf8StrRef>,
     #[pyarg(any, default)]
-    errors: Option<PyStrRef>,
+    errors: Option<PyUtf8StrRef>,
 }
 
 pub(crate) fn encode_string(
     s: PyStrRef,
-    encoding: Option<PyStrRef>,
-    errors: Option<PyStrRef>,
+    encoding: Option<PyUtf8StrRef>,
+    errors: Option<PyUtf8StrRef>,
     vm: &VirtualMachine,
 ) -> PyResult<PyBytesRef> {
-    let encoding = encoding
-        .as_ref()
-        .map_or(crate::codecs::DEFAULT_ENCODING, |s| s.as_str());
+    let encoding = match encoding.as_ref() {
+        None => crate::codecs::DEFAULT_ENCODING,
+        Some(s) => s.as_str(),
+    };
     vm.state.codec_registry.encode_text(s, encoding, errors, vm)
 }
 
@@ -1868,14 +1893,16 @@ impl SliceableSequenceOp for PyStr {
 }
 
 impl AsRef<str> for PyRefExact<PyStr> {
+    #[track_caller]
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str().expect("str has surrogates")
     }
 }
 
 impl AsRef<str> for PyExact<PyStr> {
+    #[track_caller]
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str().expect("str has surrogates")
     }
 }
 
@@ -2033,7 +2060,7 @@ impl AsRef<Wtf8> for PyUtf8Str {
 impl AsRef<str> for PyUtf8Str {
     #[inline]
     fn as_ref(&self) -> &str {
-        self.0.as_str()
+        self.as_str()
     }
 }
 
@@ -2045,6 +2072,12 @@ impl PyUtf8Str {
         Self(PyStr::from(data))
     }
 
+    /// Returns the underlying WTF-8 slice (always valid UTF-8 for this type).
+    #[inline]
+    pub fn as_wtf8(&self) -> &Wtf8 {
+        self.0.as_wtf8()
+    }
+
     /// Returns the underlying string slice.
     pub fn as_str(&self) -> &str {
         debug_assert!(
@@ -2055,6 +2088,11 @@ impl PyUtf8Str {
         unsafe { self.0.to_str().unwrap_unchecked() }
     }
 
+    #[inline]
+    pub fn as_bytes(&self) -> &[u8] {
+        self.as_str().as_bytes()
+    }
+
     #[inline]
     pub fn byte_len(&self) -> usize {
         self.0.byte_len()
@@ -2079,6 +2117,29 @@ impl Py<PyUtf8Str> {
             &*(self as *const Self as *const Py<PyStr>)
         }
     }
+
+    /// Returns the underlying `&str`.
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        self.as_pystr().to_str().unwrap_or_else(|| {
+            debug_assert!(false, "PyUtf8Str invariant violated");
+            // Safety: PyUtf8Str guarantees valid UTF-8
+            unsafe { core::hint::unreachable_unchecked() }
+        })
+    }
+}
+
+impl PyRef<PyUtf8Str> {
+    /// Convert to PyStrRef. Safe because PyUtf8Str is a subtype of PyStr.
+    pub fn into_wtf8(self) -> PyStrRef {
+        unsafe { mem::transmute::<Self, PyStrRef>(self) }
+    }
+}
+
+impl From<PyRef<PyUtf8Str>> for PyRef<PyStr> {
+    fn from(s: PyRef<PyUtf8Str>) -> Self {
+        s.into_wtf8()
+    }
 }
 
 impl PartialEq for PyUtf8Str {
@@ -2454,10 +2515,54 @@ impl AsRef<str> for PyStrInterned {
     }
 }
 
+/// Interned PyUtf8Str — guaranteed UTF-8 at type level.
+/// Same layout as `PyStrInterned` due to `#[repr(transparent)]` on both
+/// `PyInterned<T>` and `PyUtf8Str`.
+pub type PyUtf8StrInterned = PyInterned<PyUtf8Str>;
+
+impl PyUtf8StrInterned {
+    /// Returns the underlying `&str`.
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        Py::<PyUtf8Str>::as_str(self)
+    }
+
+    /// View as `PyStrInterned` (widening: UTF-8 → WTF-8).
+    #[inline]
+    pub fn as_interned_str(&self) -> &PyStrInterned {
+        // Safety: PyUtf8Str is #[repr(transparent)] over PyStr,
+        // so PyInterned<PyUtf8Str> has the same layout as PyInterned<PyStr>.
+        unsafe { &*(self as *const Self as *const PyStrInterned) }
+    }
+
+    /// Narrow a `PyStrInterned` to `PyUtf8StrInterned`.
+    ///
+    /// # Safety
+    /// The caller must ensure that the interned string is valid UTF-8.
+    #[inline]
+    pub unsafe fn from_str_interned_unchecked(s: &PyStrInterned) -> &Self {
+        unsafe { &*(s as *const PyStrInterned as *const Self) }
+    }
+}
+
+impl core::fmt::Display for PyUtf8StrInterned {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl AsRef<str> for PyUtf8StrInterned {
+    #[inline(always)]
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
     use crate::Interpreter;
+    use rustpython_common::wtf8::Wtf8Buf;
 
     #[test]
     fn str_title() {
@@ -2525,7 +2630,7 @@ mod tests {
                     .unwrap();
             let text = PyStr::from("abc");
             let translated = text.translate(translated, vm).unwrap();
-            assert_eq!(translated, "🎅xda".to_owned());
+            assert_eq!(translated, Wtf8Buf::from("🎅xda"));
             let translated = text.translate(vm.ctx.new_int(3).into(), vm);
             assert_eq!("TypeError", &*translated.unwrap_err().class().name(),);
         })
diff --git a/crates/vm/src/builtins/template.rs b/crates/vm/src/builtins/template.rs
index 0496504cbef..e9dd1e7adfb 100644
--- a/crates/vm/src/builtins/template.rs
+++ b/crates/vm/src/builtins/template.rs
@@ -1,4 +1,7 @@
-use super::{PyStr, PyTupleRef, PyType, PyTypeRef, genericalias::PyGenericAlias};
+use super::{
+    PyStr, PyTupleRef, PyType, PyTypeRef, genericalias::PyGenericAlias,
+    interpolation::PyInterpolation,
+};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
     atomic_func,
@@ -11,8 +14,7 @@ use crate::{
         SelfIter,
     },
 };
-
-use super::interpolation::PyInterpolation;
+use rustpython_common::wtf8::{Wtf8Buf, wtf8_concat};
 
 /// Template object for t-strings (PEP 750).
 ///
@@ -58,8 +60,9 @@ impl Constructor for PyTemplate {
                     // Concatenate adjacent strings
                     if let Some(last) = strings.last_mut() {
                         let last_str = last.downcast_ref::<PyStr>().unwrap();
-                        let concatenated = format!("{}{}", last_str.as_str(), s.as_str());
-                        *last = vm.ctx.new_str(concatenated).into();
+                        let mut buf = last_str.as_wtf8().to_owned();
+                        buf.push_wtf8(s.as_wtf8());
+                        *last = vm.ctx.new_str(buf).into();
                     }
                 } else {
                     strings.push(s.into());
@@ -143,18 +146,22 @@ impl PyTemplate {
         }
 
         // Concatenate last string of self with first string of other
-        let last_self = self
+        let mut buf = Wtf8Buf::new();
+        if let Some(s) = self
             .strings
             .get(self_strings_len.saturating_sub(1))
-            .and_then(|s| s.downcast_ref::<PyStr>().map(|s| s.as_str().to_owned()))
-            .unwrap_or_default();
-        let first_other = other
+            .and_then(|s| s.downcast_ref::<PyStr>())
+        {
+            buf.push_wtf8(s.as_wtf8());
+        }
+        if let Some(s) = other
             .strings
             .first()
-            .and_then(|s| s.downcast_ref::<PyStr>().map(|s| s.as_str().to_owned()))
-            .unwrap_or_default();
-        let concatenated = format!("{}{}", last_self, first_other);
-        new_strings.push(vm.ctx.new_str(concatenated).into());
+            .and_then(|s| s.downcast_ref::<PyStr>())
+        {
+            buf.push_wtf8(s.as_wtf8());
+        }
+        new_strings.push(vm.ctx.new_str(buf).into());
 
         // Add remaining strings from other (skip first)
         for i in 1..other.strings.len() {
@@ -243,31 +250,16 @@ impl Iterable for PyTemplate {
 
 impl Representable for PyTemplate {
     #[inline]
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-        let mut parts = Vec::new();
-
-        let strings_len = zelf.strings.len();
-        let interps_len = zelf.interpolations.len();
-
-        for i in 0..strings_len.max(interps_len * 2 + 1) {
-            if i % 2 == 0 {
-                // String position
-                let idx = i / 2;
-                if idx < strings_len {
-                    let s = zelf.strings.get(idx).unwrap();
-                    parts.push(s.repr(vm)?.as_str().to_owned());
-                }
-            } else {
-                // Interpolation position
-                let idx = i / 2;
-                if idx < interps_len {
-                    let interp = zelf.interpolations.get(idx).unwrap();
-                    parts.push(interp.repr(vm)?.as_str().to_owned());
-                }
-            }
-        }
-
-        Ok(format!("Template({})", parts.join(", ")))
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+        let strings_repr = zelf.strings.as_object().repr(vm)?;
+        let interp_repr = zelf.interpolations.as_object().repr(vm)?;
+        Ok(wtf8_concat!(
+            "Template(strings=",
+            strings_repr.as_wtf8(),
+            ", interpolations=",
+            interp_repr.as_wtf8(),
+            ')',
+        ))
     }
 }
 
@@ -317,7 +309,7 @@ impl IterNext for PyTemplateIter {
 
                     // Skip empty strings
                     if let Some(s) = item.downcast_ref::<PyStr>()
-                        && s.as_str().is_empty()
+                        && s.as_wtf8().is_empty()
                     {
                         continue;
                     }
diff --git a/crates/vm/src/builtins/tuple.rs b/crates/vm/src/builtins/tuple.rs
index 67c51127cb4..e0f88f08d22 100644
--- a/crates/vm/src/builtins/tuple.rs
+++ b/crates/vm/src/builtins/tuple.rs
@@ -5,6 +5,7 @@ use crate::common::lock::LazyLock;
 use crate::common::{
     hash::{PyHash, PyUHash},
     lock::PyMutex,
+    wtf8::wtf8_concat,
 };
 use crate::object::{Traverse, TraverseFn};
 use crate::{
@@ -481,7 +482,7 @@ impl Representable for PyTuple {
             vm.ctx.intern_str("()").to_owned()
         } else if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
             let s = if zelf.len() == 1 {
-                format!("({},)", zelf.elements[0].repr(vm)?)
+                wtf8_concat!("(", zelf.elements[0].repr(vm)?.as_wtf8(), ",)")
             } else {
                 collection_repr(None, "(", ")", zelf.elements.iter(), vm)?
             };
diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
index 9fcc27fae11..86865e9e083 100644
--- a/crates/vm/src/builtins/type.rs
+++ b/crates/vm/src/builtins/type.rs
@@ -1,6 +1,6 @@
 use super::{
     PyClassMethod, PyDictRef, PyList, PyStaticMethod, PyStr, PyStrInterned, PyStrRef, PyTupleRef,
-    PyWeak, mappingproxy::PyMappingProxy, object, union_,
+    PyUtf8StrRef, PyWeak, mappingproxy::PyMappingProxy, object, union_,
 };
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject,
@@ -32,6 +32,7 @@ use core::{any::Any, borrow::Borrow, ops::Deref, pin::Pin, ptr::NonNull};
 use indexmap::{IndexMap, map::Entry};
 use itertools::Itertools;
 use num_traits::ToPrimitive;
+use rustpython_common::wtf8::Wtf8;
 use std::collections::HashSet;
 
 #[pyclass(module = false, name = "type", traverse = "manual")]
@@ -88,7 +89,7 @@ unsafe impl crate::object::Traverse for PyType {
 
 // PyHeapTypeObject in CPython
 pub struct HeapTypeExt {
-    pub name: PyRwLock<PyStrRef>,
+    pub name: PyRwLock<PyUtf8StrRef>,
     pub qualname: PyRwLock<PyStrRef>,
     pub slots: Option<PyRef<PyTuple<PyStrRef>>>,
     pub type_data: PyRwLock<Option<TypeDataSlot>>,
@@ -215,9 +216,10 @@ impl PyType {
         // Set HEAPTYPE flag for heap-allocated types
         slots.flags |= PyTypeFlags::HEAPTYPE;
 
-        let name = ctx.new_str(name);
+        let name_utf8 = ctx.new_utf8_str(name);
+        let name = name_utf8.clone().into_wtf8();
         let heaptype_ext = HeapTypeExt {
-            name: PyRwLock::new(name.clone()),
+            name: PyRwLock::new(name_utf8),
             qualname: PyRwLock::new(name),
             slots: None,
             type_data: PyRwLock::new(None),
@@ -619,14 +621,24 @@ impl PyType {
     pub fn slot_name(&self) -> BorrowedValue<'_, str> {
         self.name_inner(
             |name| name.into(),
-            |ext| PyRwLockReadGuard::map(ext.name.read(), |name| name.as_str()).into(),
+            |ext| {
+                PyRwLockReadGuard::map(ext.name.read(), |name: &PyUtf8StrRef| -> &str {
+                    name.as_str()
+                })
+                .into()
+            },
         )
     }
 
     pub fn name(&self) -> BorrowedValue<'_, str> {
         self.name_inner(
             |name| name.rsplit_once('.').map_or(name, |(_, name)| name).into(),
-            |ext| PyRwLockReadGuard::map(ext.name.read(), |name| name.as_str()).into(),
+            |ext| {
+                PyRwLockReadGuard::map(ext.name.read(), |name: &PyUtf8StrRef| -> &str {
+                    name.as_str()
+                })
+                .into()
+            },
         )
     }
 
@@ -837,7 +849,7 @@ impl PyType {
                     })
                     .to_owned()
             },
-            |ext| ext.name.read().clone(),
+            |ext| ext.name.read().clone().into_wtf8(),
         )
     }
 
@@ -1158,7 +1170,7 @@ impl PyType {
         if name.as_bytes().contains(&0) {
             return Err(vm.new_value_error("type name must not contain null characters"));
         }
-        name.ensure_valid_utf8(vm)?;
+        let name = name.try_into_utf8(vm)?;
 
         let heap_type = self.heaptype_ext.as_ref().ok_or_else(|| {
             vm.new_type_error(format!(
@@ -1257,7 +1269,7 @@ impl Constructor for PyType {
         if name.as_bytes().contains(&0) {
             return Err(vm.new_value_error("type name must not contain null characters"));
         }
-        name.ensure_valid_utf8(vm)?;
+        let name = name.try_into_utf8(vm)?;
 
         let (metatype, base, bases, base_is_type) = if bases.is_empty() {
             let base = vm.ctx.types.object_type.to_owned();
@@ -1306,7 +1318,7 @@ impl Constructor for PyType {
             .transpose()?
             .unwrap_or_else(|| {
                 // If __qualname__ is not provided, we can use the name as default
-                name.clone()
+                name.clone().into_wtf8()
             });
         let mut attributes = dict.to_attributes(vm);
 
@@ -1355,144 +1367,141 @@ impl Constructor for PyType {
             attributes.insert(identifier!(vm, __hash__), vm.ctx.none.clone().into());
         }
 
-        let (heaptype_slots, add_dict): (Option<PyRef<PyTuple<PyStrRef>>>, bool) = if let Some(x) =
-            attributes.get(identifier!(vm, __slots__))
-        {
-            // Check if __slots__ is bytes - not allowed
-            if x.class().is(vm.ctx.types.bytes_type) {
-                return Err(
-                    vm.new_type_error("__slots__ items must be strings, not 'bytes'".to_owned())
-                );
-            }
+        let (heaptype_slots, add_dict): (Option<PyRef<PyTuple<PyStrRef>>>, bool) =
+            if let Some(x) = attributes.get(identifier!(vm, __slots__)) {
+                // Check if __slots__ is bytes - not allowed
+                if x.class().is(vm.ctx.types.bytes_type) {
+                    return Err(vm.new_type_error("__slots__ items must be strings, not 'bytes'"));
+                }
 
-            let slots = if x.class().is(vm.ctx.types.str_type) {
-                let x = unsafe { x.downcast_unchecked_ref::<PyStr>() };
-                PyTuple::new_ref_typed(vec![x.to_owned()], &vm.ctx)
-            } else {
-                let iter = x.get_iter(vm)?;
-                let elements = {
-                    let mut elements = Vec::new();
-                    while let PyIterReturn::Return(element) = iter.next(vm)? {
-                        // Check if any slot item is bytes
-                        if element.class().is(vm.ctx.types.bytes_type) {
-                            return Err(vm.new_type_error(
-                                "__slots__ items must be strings, not 'bytes'".to_owned(),
-                            ));
+                let slots = if x.class().is(vm.ctx.types.str_type) {
+                    let x = unsafe { x.downcast_unchecked_ref::<PyStr>() };
+                    PyTuple::new_ref_typed(vec![x.to_owned()], &vm.ctx)
+                } else {
+                    let iter = x.get_iter(vm)?;
+                    let elements = {
+                        let mut elements = Vec::new();
+                        while let PyIterReturn::Return(element) = iter.next(vm)? {
+                            // Check if any slot item is bytes
+                            if element.class().is(vm.ctx.types.bytes_type) {
+                                return Err(vm.new_type_error(
+                                    "__slots__ items must be strings, not 'bytes'",
+                                ));
+                            }
+                            elements.push(element);
                         }
-                        elements.push(element);
-                    }
-                    elements
+                        elements
+                    };
+                    let tuple = elements.into_pytuple(vm);
+                    tuple.try_into_typed(vm)?
                 };
-                let tuple = elements.into_pytuple(vm);
-                tuple.try_into_typed(vm)?
-            };
-
-            // Check if base has itemsize > 0 - can't add arbitrary slots to variable-size types
-            // Types like int, bytes, tuple have itemsize > 0 and don't allow custom slots
-            // But types like weakref.ref have itemsize = 0 and DO allow slots
-            let has_custom_slots = slots
-                .iter()
-                .any(|s| s.as_str() != "__dict__" && s.as_str() != "__weakref__");
-            if has_custom_slots && base.slots.itemsize > 0 {
-                return Err(vm.new_type_error(format!(
-                    "nonempty __slots__ not supported for subtype of '{}'",
-                    base.name()
-                )));
-            }
 
-            // Validate slot names and track duplicates
-            let mut seen_dict = false;
-            let mut seen_weakref = false;
-            for slot in slots.iter() {
-                // Use isidentifier for validation (handles Unicode properly)
-                if !slot.isidentifier() {
-                    return Err(vm.new_type_error("__slots__ must be identifiers".to_owned()));
+                // Check if base has itemsize > 0 - can't add arbitrary slots to variable-size types
+                // Types like int, bytes, tuple have itemsize > 0 and don't allow custom slots
+                // But types like weakref.ref have itemsize = 0 and DO allow slots
+                let has_custom_slots = slots
+                    .iter()
+                    .any(|s| !matches!(s.as_bytes(), b"__dict__" | b"__weakref__"));
+                if has_custom_slots && base.slots.itemsize > 0 {
+                    return Err(vm.new_type_error(format!(
+                        "nonempty __slots__ not supported for subtype of '{}'",
+                        base.name()
+                    )));
                 }
 
-                let slot_name = slot.as_str();
+                // Validate slot names and track duplicates
+                let mut seen_dict = false;
+                let mut seen_weakref = false;
+                for slot in slots.iter() {
+                    // Use isidentifier for validation (handles Unicode properly)
+                    if !slot.isidentifier() {
+                        return Err(vm.new_type_error("__slots__ must be identifiers"));
+                    }
+
+                    let slot_name = slot.as_bytes();
 
-                // Check for duplicate __dict__
-                if slot_name == "__dict__" {
-                    if seen_dict {
-                        return Err(vm.new_type_error(
-                            "__dict__ slot disallowed: we already got one".to_owned(),
-                        ));
+                    // Check for duplicate __dict__
+                    if slot_name == b"__dict__" {
+                        if seen_dict {
+                            return Err(
+                                vm.new_type_error("__dict__ slot disallowed: we already got one")
+                            );
+                        }
+                        seen_dict = true;
                     }
-                    seen_dict = true;
-                }
 
-                // Check for duplicate __weakref__
-                if slot_name == "__weakref__" {
-                    if seen_weakref {
-                        return Err(vm.new_type_error(
-                            "__weakref__ slot disallowed: we already got one".to_owned(),
-                        ));
+                    // Check for duplicate __weakref__
+                    if slot_name == b"__weakref__" {
+                        if seen_weakref {
+                            return Err(vm.new_type_error(
+                                "__weakref__ slot disallowed: we already got one",
+                            ));
+                        }
+                        seen_weakref = true;
                     }
-                    seen_weakref = true;
-                }
 
-                // Check if slot name conflicts with class attributes
-                if attributes.contains_key(vm.ctx.intern_str(slot_name)) {
-                    return Err(vm.new_value_error(format!(
-                        "'{}' in __slots__ conflicts with a class variable",
-                        slot_name
-                    )));
+                    // Check if slot name conflicts with class attributes
+                    if attributes.contains_key(vm.ctx.intern_str(slot.as_wtf8())) {
+                        return Err(vm.new_value_error(format!(
+                            "'{}' in __slots__ conflicts with a class variable",
+                            slot.as_wtf8()
+                        )));
+                    }
                 }
-            }
 
-            // Check if base class already has __dict__ - can't redefine it
-            if seen_dict && base.slots.flags.has_feature(PyTypeFlags::HAS_DICT) {
-                return Err(
-                    vm.new_type_error("__dict__ slot disallowed: we already got one".to_owned())
-                );
-            }
+                // Check if base class already has __dict__ - can't redefine it
+                if seen_dict && base.slots.flags.has_feature(PyTypeFlags::HAS_DICT) {
+                    return Err(vm.new_type_error("__dict__ slot disallowed: we already got one"));
+                }
 
-            // Check if base class already has __weakref__ - can't redefine it
-            // A base has weakref support if:
-            // 1. It's a heap type without explicit __slots__ (automatic weakref), OR
-            // 2. It's a heap type with __weakref__ in its __slots__
-            if seen_weakref {
-                let base_has_weakref = if let Some(ref ext) = base.heaptype_ext {
-                    match &ext.slots {
-                        // Heap type without __slots__ - has automatic weakref
-                        None => true,
-                        // Heap type with __slots__ - check if __weakref__ is in slots
-                        Some(base_slots) => base_slots.iter().any(|s| s.as_str() == "__weakref__"),
+                // Check if base class already has __weakref__ - can't redefine it
+                // A base has weakref support if:
+                // 1. It's a heap type without explicit __slots__ (automatic weakref), OR
+                // 2. It's a heap type with __weakref__ in its __slots__
+                if seen_weakref {
+                    let base_has_weakref = if let Some(ref ext) = base.heaptype_ext {
+                        match &ext.slots {
+                            // Heap type without __slots__ - has automatic weakref
+                            None => true,
+                            // Heap type with __slots__ - check if __weakref__ is in slots
+                            Some(base_slots) => {
+                                base_slots.iter().any(|s| s.as_bytes() == b"__weakref__")
+                            }
+                        }
+                    } else {
+                        // Builtin type - check if it has __weakref__ descriptor
+                        let weakref_name = vm.ctx.intern_str("__weakref__");
+                        base.attributes.read().contains_key(weakref_name)
+                    };
+
+                    if base_has_weakref {
+                        return Err(
+                            vm.new_type_error("__weakref__ slot disallowed: we already got one")
+                        );
                     }
-                } else {
-                    // Builtin type - check if it has __weakref__ descriptor
-                    let weakref_name = vm.ctx.intern_str("__weakref__");
-                    base.attributes.read().contains_key(weakref_name)
-                };
-
-                if base_has_weakref {
-                    return Err(vm.new_type_error(
-                        "__weakref__ slot disallowed: we already got one".to_owned(),
-                    ));
                 }
-            }
 
-            // Check if __dict__ is in slots
-            let dict_name = "__dict__";
-            let has_dict = slots.iter().any(|s| s.as_str() == dict_name);
+                // Check if __dict__ is in slots
+                let dict_name = "__dict__";
+                let has_dict = slots.iter().any(|s| s.as_wtf8() == dict_name);
+
+                // Filter out __dict__ from slots
+                let filtered_slots = if has_dict {
+                    let filtered: Vec<PyStrRef> = slots
+                        .iter()
+                        .filter(|s| s.as_wtf8() != dict_name)
+                        .cloned()
+                        .collect();
+                    PyTuple::new_ref_typed(filtered, &vm.ctx)
+                } else {
+                    slots
+                };
 
-            // Filter out __dict__ from slots
-            let filtered_slots = if has_dict {
-                let filtered: Vec<PyStrRef> = slots
-                    .iter()
-                    .filter(|s| s.as_str() != dict_name)
-                    .cloned()
-                    .collect();
-                PyTuple::new_ref_typed(filtered, &vm.ctx)
+                (Some(filtered_slots), has_dict)
             } else {
-                slots
+                (None, false)
             };
 
-            (Some(filtered_slots), has_dict)
-        } else {
-            (None, false)
-        };
-
         // FIXME: this is a temporary fix. multi bases with multiple slots will break object
         let base_member_count = bases
             .iter()
@@ -1548,7 +1557,10 @@ impl Constructor for PyType {
             let class_name = typ.name().to_string();
             for member in slots.as_slice() {
                 // Apply name mangling for private attributes (__x -> _ClassName__x)
-                let mangled_name = mangle_name(&class_name, member.as_str());
+                let member_str = member.to_str().ok_or_else(|| {
+                    vm.new_type_error("__slots__ must be valid UTF-8 strings".to_owned())
+                })?;
+                let mangled_name = mangle_name(&class_name, member_str);
                 let member_def = PyMemberDef {
                     name: mangled_name.clone(),
                     kind: MemberKind::ObjectEx,
@@ -1720,7 +1732,7 @@ impl GetAttr for PyType {
         #[cold]
         fn attribute_error(
             zelf: &Py<PyType>,
-            name: &str,
+            name: &Wtf8,
             vm: &VirtualMachine,
         ) -> PyBaseExceptionRef {
             vm.new_attribute_error(format!(
@@ -1731,7 +1743,7 @@ impl GetAttr for PyType {
         }
 
         let Some(name) = vm.ctx.interned_str(name_str) else {
-            return Err(attribute_error(zelf, name_str.as_str(), vm));
+            return Err(attribute_error(zelf, name_str.as_wtf8(), vm));
         };
         vm_trace!("type.__getattribute__({:?}, {:?})", zelf, name);
         let mcl = zelf.class();
@@ -1761,7 +1773,7 @@ impl GetAttr for PyType {
         } else if let Some(attr) = mcl_attr {
             vm.call_if_get_descriptor(&attr, zelf.to_owned().into())
         } else {
-            Err(attribute_error(zelf, name_str.as_str(), vm))
+            Err(attribute_error(zelf, name_str.as_wtf8(), vm))
         }
     }
 }
@@ -1863,8 +1875,7 @@ impl SetAttr for PyType {
         value: PySetterValue,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        // TODO: pass PyRefExact instead of &str
-        let attr_name = vm.ctx.intern_str(attr_name.as_str());
+        let attr_name = vm.ctx.intern_str(attr_name.as_wtf8());
         if zelf.slots.flags.has_feature(PyTypeFlags::IMMUTABLETYPE) {
             return Err(vm.new_type_error(format!(
                 "cannot set '{}' attribute of immutable type '{}'",
@@ -1888,11 +1899,11 @@ impl SetAttr for PyType {
                 return Err(vm.new_attribute_error(format!(
                     "type object '{}' has no attribute '{}'",
                     zelf.name(),
-                    attr_name.as_str(),
+                    attr_name,
                 )));
             }
         }
-        if attr_name.as_str().starts_with("__") && attr_name.as_str().ends_with("__") {
+        if attr_name.as_wtf8().starts_with("__") && attr_name.as_wtf8().ends_with("__") {
             if assign {
                 zelf.update_slot::<true>(attr_name, &vm.ctx);
             } else {
@@ -1949,19 +1960,15 @@ impl Representable for PyType {
     #[inline]
     fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
         let module = zelf.__module__(vm);
-        let module = module.downcast_ref::<PyStr>().map(|m| m.as_str());
+        let module = module.downcast_ref::<PyStr>().map(|m| m.as_wtf8());
 
         let repr = match module {
             Some(module) if module != "builtins" => {
+                let qualname = zelf.__qualname__(vm);
+                let qualname = qualname.downcast_ref::<PyStr>().map(|n| n.as_wtf8());
                 let name = zelf.name();
-                format!(
-                    "<class '{}.{}'>",
-                    module,
-                    zelf.__qualname__(vm)
-                        .downcast_ref::<PyStr>()
-                        .map(|n| n.as_str())
-                        .unwrap_or_else(|| &name)
-                )
+                let qualname = qualname.unwrap_or_else(|| name.as_ref());
+                format!("<class '{module}.{qualname}'>")
             }
             _ => format!("<class '{}'>", zelf.slot_name()),
         };
diff --git a/crates/vm/src/builtins/weakproxy.rs b/crates/vm/src/builtins/weakproxy.rs
index 437e0dc886e..a03eb1fe7e7 100644
--- a/crates/vm/src/builtins/weakproxy.rs
+++ b/crates/vm/src/builtins/weakproxy.rs
@@ -142,7 +142,7 @@ impl IterNext for PyWeakProxy {
 fn new_reference_error(vm: &VirtualMachine) -> PyRef<super::PyBaseException> {
     vm.new_exception_msg(
         vm.ctx.exceptions.reference_error.to_owned(),
-        "weakly-referenced object no longer exists".to_owned(),
+        "weakly-referenced object no longer exists".into(),
     )
 }
 
diff --git a/crates/vm/src/bytes_inner.rs b/crates/vm/src/bytes_inner.rs
index 7e1c1c2220c..d8e4a6c8eff 100644
--- a/crates/vm/src/bytes_inner.rs
+++ b/crates/vm/src/bytes_inner.rs
@@ -4,7 +4,7 @@ use crate::{
     anystr::{self, AnyStr, AnyStrContainer, AnyStrWrapper},
     builtins::{
         PyBaseExceptionRef, PyByteArray, PyBytes, PyBytesRef, PyInt, PyIntRef, PyStr, PyStrRef,
-        pystr,
+        pystr, pystr::PyUtf8StrRef,
     },
     byte::bytes_from_object,
     cformat::cformat_bytes,
@@ -45,16 +45,16 @@ pub struct ByteInnerNewOptions {
     #[pyarg(any, optional)]
     pub source: OptionalArg<PyObjectRef>,
     #[pyarg(any, optional)]
-    pub encoding: OptionalArg<PyStrRef>,
+    pub encoding: OptionalArg<PyUtf8StrRef>,
     #[pyarg(any, optional)]
-    pub errors: OptionalArg<PyStrRef>,
+    pub errors: OptionalArg<PyUtf8StrRef>,
 }
 
 impl ByteInnerNewOptions {
     fn get_value_from_string(
         s: PyStrRef,
-        encoding: PyStrRef,
-        errors: OptionalArg<PyStrRef>,
+        encoding: PyUtf8StrRef,
+        errors: OptionalArg<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult<PyBytesInner> {
         let bytes = pystr::encode_string(s, Some(encoding), errors.into_option(), vm)?;
@@ -1114,9 +1114,9 @@ impl AnyStr for [u8] {
 #[derive(FromArgs)]
 pub struct DecodeArgs {
     #[pyarg(any, default)]
-    encoding: Option<PyStrRef>,
+    encoding: Option<PyUtf8StrRef>,
     #[pyarg(any, default)]
-    errors: Option<PyStrRef>,
+    errors: Option<PyUtf8StrRef>,
 }
 
 pub fn bytes_decode(
@@ -1125,9 +1125,10 @@ pub fn bytes_decode(
     vm: &VirtualMachine,
 ) -> PyResult<PyStrRef> {
     let DecodeArgs { encoding, errors } = args;
-    let encoding = encoding
-        .as_ref()
-        .map_or(crate::codecs::DEFAULT_ENCODING, |s| s.as_str());
+    let encoding = match encoding.as_ref() {
+        None => crate::codecs::DEFAULT_ENCODING,
+        Some(e) => e.as_str(),
+    };
     vm.state
         .codec_registry
         .decode_text(zelf, encoding, errors, vm)
@@ -1205,7 +1206,7 @@ pub fn bytes_to_hex(
         let b_guard;
         let sep = match &sep {
             Either::A(s) => {
-                s_guard = s.as_str();
+                s_guard = s.as_wtf8();
                 s_guard.as_bytes()
             }
             Either::B(bytes) => {
diff --git a/crates/vm/src/codecs.rs b/crates/vm/src/codecs.rs
index a7eea0c9372..94b4b67e33c 100644
--- a/crates/vm/src/codecs.rs
+++ b/crates/vm/src/codecs.rs
@@ -10,9 +10,12 @@ use rustpython_common::{
 
 use crate::common::lock::OnceCell;
 use crate::{
-    AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyResult, TryFromBorrowedObject,
-    TryFromObject, VirtualMachine,
-    builtins::{PyBaseExceptionRef, PyBytes, PyBytesRef, PyStr, PyStrRef, PyTuple, PyTupleRef},
+    AsObject, Context, Py, PyObject, PyObjectRef, PyResult, TryFromBorrowedObject, TryFromObject,
+    VirtualMachine,
+    builtins::{
+        PyBaseExceptionRef, PyBytes, PyBytesRef, PyStr, PyStrRef, PyTuple, PyTupleRef, PyUtf8Str,
+        PyUtf8StrRef,
+    },
     common::{ascii, lock::PyRwLock},
     convert::ToPyObject,
     function::{ArgBytesLike, PyMethodDef},
@@ -71,11 +74,11 @@ impl PyCodec {
     pub fn encode(
         &self,
         obj: PyObjectRef,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult {
         let args = match errors {
-            Some(errors) => vec![obj, errors.into()],
+            Some(errors) => vec![obj, errors.into_wtf8().into()],
             None => vec![obj],
         };
         let res = self.get_encode_func().call(args, vm)?;
@@ -91,11 +94,11 @@ impl PyCodec {
     pub fn decode(
         &self,
         obj: PyObjectRef,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult {
         let args = match errors {
-            Some(errors) => vec![obj, errors.into()],
+            Some(errors) => vec![obj, errors.into_wtf8().into()],
             None => vec![obj],
         };
         let res = self.get_decode_func().call(args, vm)?;
@@ -263,7 +266,7 @@ impl CodecsRegistry {
             }
             inner.search_path.clone()
         };
-        let encoding = PyStr::from(encoding.into_owned()).into_ref(&vm.ctx);
+        let encoding: PyUtf8StrRef = vm.ctx.new_utf8_str(encoding.as_ref());
         for func in search_path {
             let res = func.call((encoding.clone(),), vm)?;
             let res: Option<PyCodec> = res.try_into_value(vm)?;
@@ -305,7 +308,7 @@ impl CodecsRegistry {
         &self,
         obj: PyObjectRef,
         encoding: &str,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult {
         let codec = self.lookup(encoding, vm)?;
@@ -318,7 +321,7 @@ impl CodecsRegistry {
         &self,
         obj: PyObjectRef,
         encoding: &str,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult {
         let codec = self.lookup(encoding, vm)?;
@@ -331,7 +334,7 @@ impl CodecsRegistry {
         &self,
         obj: PyStrRef,
         encoding: &str,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult<PyBytesRef> {
         let codec = self._lookup_text_encoding(encoding, "codecs.encode()", vm)?;
@@ -355,7 +358,7 @@ impl CodecsRegistry {
         &self,
         obj: PyObjectRef,
         encoding: &str,
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         vm: &VirtualMachine,
     ) -> PyResult<PyStrRef> {
         let codec = self._lookup_text_encoding(encoding, "codecs.decode()", vm)?;
@@ -868,7 +871,7 @@ impl<'a> DecodeErrorHandler<PyDecodeContext<'a>> for StandardError {
 }
 
 pub struct ErrorsHandler<'a> {
-    errors: &'a Py<PyStr>,
+    errors: &'a Py<PyUtf8Str>,
     resolved: OnceCell<ResolvedError>,
 }
 enum ResolvedError {
@@ -878,14 +881,14 @@ enum ResolvedError {
 
 impl<'a> ErrorsHandler<'a> {
     #[inline]
-    pub fn new(errors: Option<&'a Py<PyStr>>, vm: &VirtualMachine) -> Self {
+    pub fn new(errors: Option<&'a Py<PyUtf8Str>>, vm: &VirtualMachine) -> Self {
         match errors {
             Some(errors) => Self {
                 errors,
                 resolved: OnceCell::new(),
             },
             None => Self {
-                errors: identifier!(vm, strict).as_ref(),
+                errors: identifier_utf8!(vm, strict),
                 resolved: OnceCell::from(ResolvedError::Standard(StandardError::Strict)),
             },
         }
@@ -895,12 +898,13 @@ impl<'a> ErrorsHandler<'a> {
         if let Some(val) = self.resolved.get() {
             return Ok(val);
         }
-        let val = if let Ok(standard) = self.errors.as_str().parse() {
+        let errors_str = self.errors.as_str();
+        let val = if let Ok(standard) = errors_str.parse() {
             ResolvedError::Standard(standard)
         } else {
             vm.state
                 .codec_registry
-                .lookup_error(self.errors.as_str(), vm)
+                .lookup_error(errors_str, vm)
                 .map(ResolvedError::Handler)?
         };
         let _ = self.resolved.set(val);
@@ -1020,7 +1024,7 @@ where
     // let err = err.
     let range = extract_unicode_error_range(&err, vm)?;
     let s = PyStrRef::try_from_object(vm, err.get_attr("object", vm)?)?;
-    let s_encoding = PyStrRef::try_from_object(vm, err.get_attr("encoding", vm)?)?;
+    let s_encoding = PyUtf8StrRef::try_from_object(vm, err.get_attr("encoding", vm)?)?;
     let mut ctx = PyEncodeContext {
         vm,
         encoding: s_encoding.as_str(),
@@ -1059,7 +1063,7 @@ where
 {
     let range = extract_unicode_error_range(&err, vm)?;
     let s = ArgBytesLike::try_from_object(vm, err.get_attr("object", vm)?)?;
-    let s_encoding = PyStrRef::try_from_object(vm, err.get_attr("encoding", vm)?)?;
+    let s_encoding = PyUtf8StrRef::try_from_object(vm, err.get_attr("encoding", vm)?)?;
     let mut ctx = PyDecodeContext {
         vm,
         encoding: s_encoding.as_str(),
diff --git a/crates/vm/src/coroutine.rs b/crates/vm/src/coroutine.rs
index 857d7403d5c..ac7aeba5443 100644
--- a/crates/vm/src/coroutine.rs
+++ b/crates/vm/src/coroutine.rs
@@ -257,7 +257,7 @@ impl Coro {
         format!(
             "<{} object {} at {:#x}>",
             gen_name(jen, vm),
-            qualname.as_str(),
+            qualname.as_wtf8(),
             id
         )
     }
diff --git a/crates/vm/src/dict_inner.rs b/crates/vm/src/dict_inner.rs
index f2a379d99a5..fe5f40b8f33 100644
--- a/crates/vm/src/dict_inner.rs
+++ b/crates/vm/src/dict_inner.rs
@@ -5,7 +5,7 @@
 
 use crate::{
     AsObject, Py, PyExact, PyObject, PyObjectRef, PyRefExact, PyResult, VirtualMachine,
-    builtins::{PyBytes, PyInt, PyStr, PyStrInterned, PyStrRef},
+    builtins::{PyBytes, PyInt, PyStr, PyStrInterned, PyStrRef, PyUtf8Str, PyUtf8StrRef},
     convert::ToPyObject,
 };
 use crate::{
@@ -818,6 +818,33 @@ impl DictKey for Py<PyStr> {
     }
 }
 
+impl DictKey for Py<PyUtf8Str> {
+    type Owned = PyUtf8StrRef;
+    #[inline(always)]
+    fn _to_owned(&self, _vm: &VirtualMachine) -> Self::Owned {
+        self.to_owned()
+    }
+
+    #[inline]
+    fn key_hash(&self, vm: &VirtualMachine) -> PyResult<HashValue> {
+        self.as_pystr().key_hash(vm)
+    }
+
+    #[inline(always)]
+    fn key_is(&self, other: &PyObject) -> bool {
+        self.as_pystr().key_is(other)
+    }
+
+    fn key_eq(&self, vm: &VirtualMachine, other_key: &PyObject) -> PyResult<bool> {
+        self.as_pystr().key_eq(vm, other_key)
+    }
+
+    #[inline(always)]
+    fn key_as_isize(&self, vm: &VirtualMachine) -> PyResult<isize> {
+        self.as_pystr().key_as_isize(vm)
+    }
+}
+
 impl DictKey for PyStrInterned {
     type Owned = PyRefExact<PyStr>;
 
diff --git a/crates/vm/src/exception_group.rs b/crates/vm/src/exception_group.rs
index 7ad27c078af..f6abdee0fab 100644
--- a/crates/vm/src/exception_group.rs
+++ b/crates/vm/src/exception_group.rs
@@ -8,6 +8,8 @@ use crate::types::{PyTypeFlags, PyTypeSlots};
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyRef, PyResult, TryFromObject, VirtualMachine,
 };
+use core::fmt::Write;
+use rustpython_common::wtf8::{Wtf8, Wtf8Buf};
 
 use crate::exceptions::types::PyBaseException;
 
@@ -183,12 +185,7 @@ pub(super) mod types {
 
         #[pymethod]
         fn __str__(zelf: &Py<PyBaseException>, vm: &VirtualMachine) -> PyResult<PyStrRef> {
-            let message = zelf
-                .get_arg(0)
-                .map(|m| m.str(vm))
-                .transpose()?
-                .map(|s| s.as_str().to_owned())
-                .unwrap_or_default();
+            let message = zelf.get_arg(0).map(|m| m.str(vm)).transpose()?;
 
             let num_excs = zelf
                 .get_arg(1)
@@ -196,10 +193,12 @@ pub(super) mod types {
                 .unwrap_or(0);
 
             let suffix = if num_excs == 1 { "" } else { "s" };
-            Ok(vm.ctx.new_str(format!(
-                "{} ({} sub-exception{})",
-                message, num_excs, suffix
-            )))
+            let mut result = match message {
+                Some(s) => s.as_wtf8().to_owned(),
+                None => Wtf8Buf::new(),
+            };
+            write!(result, " ({num_excs} sub-exception{suffix})").unwrap();
+            Ok(vm.ctx.new_str(result))
         }
 
         #[pyslot]
@@ -208,31 +207,28 @@ pub(super) mod types {
                 .downcast_ref::<PyBaseException>()
                 .expect("exception group must be BaseException");
             let class_name = zelf.class().name().to_owned();
-            let message = zelf
-                .get_arg(0)
-                .map(|m| m.repr(vm))
-                .transpose()?
-                .map(|s| s.as_str().to_owned())
-                .unwrap_or_else(|| "''".to_owned());
-
-            // Format exceptions as list [exc1, exc2, ...] instead of tuple (exc1, exc2, ...)
-            // CPython displays exceptions in list format even though they're stored as tuple
-            let exceptions_str = if let Some(exceptions_obj) = zelf.get_arg(1) {
-                // Get exceptions using ArgIterable for robustness
+            let message = zelf.get_arg(0).map(|m| m.repr(vm)).transpose()?;
+
+            let mut result = Wtf8Buf::new();
+            write!(result, "{class_name}(").unwrap();
+            let message_wtf8: &Wtf8 = message.as_ref().map_or("''".as_ref(), |s| s.as_wtf8());
+            result.push_wtf8(message_wtf8);
+            result.push_str(", [");
+            if let Some(exceptions_obj) = zelf.get_arg(1) {
                 let iter: ArgIterable<PyObjectRef> =
                     ArgIterable::try_from_object(vm, exceptions_obj.clone())?;
-                let mut exc_repr_list = Vec::new();
+                let mut first = true;
                 for exc in iter.iter(vm)? {
-                    exc_repr_list.push(exc?.repr(vm)?.as_str().to_owned());
+                    if !first {
+                        result.push_str(", ");
+                    }
+                    first = false;
+                    result.push_wtf8(exc?.repr(vm)?.as_wtf8());
                 }
-                format!("[{}]", exc_repr_list.join(", "))
-            } else {
-                "[]".to_owned()
-            };
+            }
+            result.push_str("])");
 
-            Ok(vm
-                .ctx
-                .new_str(format!("{}({}, {})", class_name, message, exceptions_str)))
+            Ok(vm.ctx.new_str(result))
         }
     }
 
diff --git a/crates/vm/src/exceptions.rs b/crates/vm/src/exceptions.rs
index 027dbec6964..680a914889b 100644
--- a/crates/vm/src/exceptions.rs
+++ b/crates/vm/src/exceptions.rs
@@ -211,8 +211,16 @@ impl VirtualMachine {
 
         if let Some(text) = maybe_text {
             // if text ends with \n or \r\n, remove it
-            let r_text = text.as_str().trim_end_matches(['\n', '\r']);
-            let l_text = r_text.trim_start_matches([' ', '\n', '\x0c']); // \x0c is \f
+            use rustpython_common::wtf8::CodePoint;
+            let text_wtf8 = text.as_wtf8();
+            let r_text = text_wtf8.trim_end_matches(|cp: CodePoint| {
+                cp == CodePoint::from_char('\n') || cp == CodePoint::from_char('\r')
+            });
+            let l_text = r_text.trim_start_matches(|cp: CodePoint| {
+                cp == CodePoint::from_char(' ')
+                    || cp == CodePoint::from_char('\n')
+                    || cp == CodePoint::from_char('\x0c') // \f
+            });
             let spaces = (r_text.len() - l_text.len()) as isize;
 
             writeln!(output, "    {l_text}")?;
@@ -249,9 +257,9 @@ impl VirtualMachine {
                     let end_colno = end_offset - 1 - spaces;
                     if colno >= 0 {
                         let caret_space = l_text
-                            .chars()
+                            .code_points()
                             .take(colno as usize)
-                            .map(|c| if c.is_whitespace() { c } else { ' ' })
+                            .map(|cp| cp.to_char().filter(|c| c.is_whitespace()).unwrap_or(' '))
                             .collect::<String>();
 
                         let mut error_width = end_colno - colno;
@@ -716,7 +724,7 @@ impl PyRef<PyBaseException> {
 
             for (key, value) in &dict {
                 let key_str = key.str(vm)?;
-                if key_str.as_str().starts_with("__") {
+                if key_str.as_bytes().starts_with(b"__") {
                     continue;
                 }
                 self.as_object().set_attr(&key_str, value.clone(), vm)?;
@@ -1459,7 +1467,10 @@ pub(super) mod types {
     };
     use crossbeam_utils::atomic::AtomicCell;
     use itertools::Itertools;
-    use rustpython_common::str::UnicodeEscapeCodepoint;
+    use rustpython_common::{
+        str::UnicodeEscapeCodepoint,
+        wtf8::{Wtf8, Wtf8Buf, wtf8_concat},
+    };
 
     // Re-export exception group types from dedicated module
     pub use crate::exception_group::types::PyBaseExceptionGroup;
@@ -2317,19 +2328,25 @@ pub(super) mod types {
     impl PySyntaxError {
         #[pymethod]
         fn __str__(zelf: &Py<PyBaseException>, vm: &VirtualMachine) -> PyResult<PyStrRef> {
-            fn basename(filename: &str) -> &str {
-                let splitted = if cfg!(windows) {
-                    filename.rsplit(&['/', '\\']).next()
+            fn basename(filename: &Wtf8) -> &Wtf8 {
+                let bytes = filename.as_bytes();
+                let pos = if cfg!(windows) {
+                    bytes.iter().rposition(|&b| b == b'/' || b == b'\\')
                 } else {
-                    filename.rsplit('/').next()
+                    bytes.iter().rposition(|&b| b == b'/')
                 };
-                splitted.unwrap_or(filename)
+                match pos {
+                    // SAFETY: splitting at ASCII byte boundary preserves WTF-8 validity
+                    Some(pos) => unsafe { Wtf8::from_bytes_unchecked(&bytes[pos + 1..]) },
+                    None => filename,
+                }
             }
 
-            let maybe_lineno = zelf.as_object().get_attr("lineno", vm).ok().map(|obj| {
-                obj.str(vm)
-                    .unwrap_or_else(|_| vm.ctx.new_str("<lineno str() failed>"))
-            });
+            let maybe_lineno = zelf
+                .as_object()
+                .get_attr("lineno", vm)
+                .and_then(|obj| obj.str_utf8(vm))
+                .ok();
             let maybe_filename = zelf.as_object().get_attr("filename", vm).ok().map(|obj| {
                 obj.str(vm)
                     .unwrap_or_else(|_| vm.ctx.new_str("<filename str() failed>"))
@@ -2345,17 +2362,22 @@ pub(super) mod types {
                 }
             };
 
-            let msg_with_location_info: String = match (maybe_lineno, maybe_filename) {
-                (Some(lineno), Some(filename)) => {
-                    format!("{} ({}, line {})", msg, basename(filename.as_str()), lineno)
-                }
+            let msg_with_location_info: Wtf8Buf = match (maybe_lineno, maybe_filename) {
+                (Some(lineno), Some(filename)) => wtf8_concat!(
+                    msg.as_wtf8(),
+                    " (",
+                    basename(filename.as_wtf8()),
+                    ", line ",
+                    lineno.as_str(),
+                    ")"
+                ),
                 (Some(lineno), None) => {
-                    format!("{msg} (line {lineno})")
+                    wtf8_concat!(msg.as_wtf8(), " (line ", lineno.as_str(), ")")
                 }
                 (None, Some(filename)) => {
-                    format!("{} ({})", msg, basename(filename.as_str()))
+                    wtf8_concat!(msg.as_wtf8(), " (", basename(filename.as_wtf8()), ")")
                 }
-                (None, None) => msg.to_string(),
+                (None, None) => msg.as_wtf8().to_owned(),
             };
 
             Ok(vm.ctx.new_str(msg_with_location_info))
diff --git a/crates/vm/src/frame.rs b/crates/vm/src/frame.rs
index 2e3905f4f14..313266eef9d 100644
--- a/crates/vm/src/frame.rs
+++ b/crates/vm/src/frame.rs
@@ -5,7 +5,7 @@ use crate::{
     builtins::{
         PyBaseException, PyBaseExceptionRef, PyCode, PyCoroutine, PyDict, PyDictRef, PyGenerator,
         PyInterpolation, PyList, PySet, PySlice, PyStr, PyStrInterned, PyTemplate, PyTraceback,
-        PyType,
+        PyType, PyUtf8Str,
         asyncgenerator::PyAsyncGenWrappedValue,
         function::{PyCell, PyCellRef, PyFunction},
         tuple::{PyTuple, PyTupleRef},
@@ -24,13 +24,18 @@ use crate::{
     vm::{Context, PyMethod},
 };
 use alloc::fmt;
+use bstr::ByteSlice;
 use core::iter::zip;
 use core::sync::atomic;
 use core::sync::atomic::AtomicPtr;
 use indexmap::IndexMap;
 use itertools::Itertools;
 
-use rustpython_common::{boxvec::BoxVec, lock::PyMutex, wtf8::Wtf8Buf};
+use rustpython_common::{
+    boxvec::BoxVec,
+    lock::PyMutex,
+    wtf8::{Wtf8, Wtf8Buf, wtf8_concat},
+};
 use rustpython_compiler_core::SourceLocation;
 
 pub type FrameRef = PyRef<Frame>;
@@ -416,8 +421,8 @@ impl Py<Frame> {
     pub fn is_internal_frame(&self) -> bool {
         let code = self.f_code();
         let filename = code.co_filename();
-        let filename_s = filename.as_str();
-        filename_s.contains("importlib") && filename_s.contains("_bootstrap")
+        let filename = filename.as_bytes();
+        filename.find(b"importlib").is_some() && filename.find(b"_bootstrap").is_some()
     }
 
     pub fn next_external_frame(&self, vm: &VirtualMachine) -> Option<FrameRef> {
@@ -795,7 +800,7 @@ impl ExecutingFrame<'_> {
         if let Some(&name) = self.code.cellvars.get(i) {
             vm.new_exception_msg(
                 vm.ctx.exceptions.unbound_local_error.to_owned(),
-                format!("local variable '{name}' referenced before assignment"),
+                format!("local variable '{name}' referenced before assignment").into(),
             )
         } else {
             let name = self.code.freevars[i - self.code.cellvars.len()];
@@ -1052,7 +1057,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx]
-                        ),
+                        )
+                        .into(),
                     ));
                 }
                 fastlocals[idx] = None;
@@ -1157,7 +1163,7 @@ impl ExecutingFrame<'_> {
                         return Err(vm.new_type_error(format!(
                             "{} got multiple values for keyword argument '{}'",
                             func_str,
-                            key_str.as_str()
+                            key_str.as_wtf8()
                         )));
                     }
                     let value = vm.call_method(&source, "__getitem__", (key.clone(),))?;
@@ -1536,7 +1542,7 @@ impl ExecutingFrame<'_> {
                 ) -> PyBaseExceptionRef {
                     vm.new_exception_msg(
                         vm.ctx.exceptions.unbound_local_error.to_owned(),
-                        format!("local variable '{varname}' referenced before assignment",),
+                        format!("local variable '{varname}' referenced before assignment").into(),
                     )
                 }
                 let idx = idx.get(arg) as usize;
@@ -1566,7 +1572,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 self.push_value(x);
@@ -1585,7 +1592,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx1]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 let x2 = fastlocals[idx2].clone().ok_or_else(|| {
@@ -1594,7 +1602,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx2]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 drop(fastlocals);
@@ -1618,7 +1627,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 self.push_value(x);
@@ -1636,7 +1646,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx1]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 let x2 = fastlocals[idx2].clone().ok_or_else(|| {
@@ -1645,7 +1656,8 @@ impl ExecutingFrame<'_> {
                         format!(
                             "local variable '{}' referenced before assignment",
                             self.code.varnames[idx2]
-                        ),
+                        )
+                        .into(),
                     )
                 })?;
                 drop(fastlocals);
@@ -1744,8 +1756,9 @@ impl ExecutingFrame<'_> {
                                     // Get type names for error message
                                     let type_name = cls
                                         .downcast::<crate::builtins::PyType>()
-                                        .map(|t| t.__name__(vm).as_str().to_owned())
-                                        .unwrap_or_else(|_| String::from("?"));
+                                        .ok()
+                                        .and_then(|t| t.__name__(vm).to_str().map(str::to_owned))
+                                        .unwrap_or_else(|| String::from("?"));
                                     let match_args_type_name = match_args.class().__name__(vm);
                                     return Err(vm.new_type_error(format!(
                                         "{}.__match_args__ must be a tuple (got {})",
@@ -2413,7 +2426,7 @@ impl ExecutingFrame<'_> {
         let mod_name_obj = module.get_attr(identifier!(vm, __name__), vm).ok();
         let mod_name_str = mod_name_obj
             .as_ref()
-            .and_then(|n| n.downcast_ref::<PyStr>().map(|s| s.as_str().to_owned()));
+            .and_then(|n| n.downcast_ref::<PyUtf8Str>().map(|s| s.as_str().to_owned()));
         let module_name = mod_name_str.as_deref().unwrap_or("<unknown module name>");
 
         let spec = module
@@ -2472,7 +2485,7 @@ impl ExecutingFrame<'_> {
                 format!("cannot import name '{name}' from '{module_name}' (unknown location)")
             }
         };
-        let err = vm.new_import_error(msg, vm.ctx.new_str(module_name));
+        let err = vm.new_import_error(msg, vm.ctx.new_utf8_str(module_name));
 
         if let Some(ref path) = origin {
             let _ignore = err
@@ -2502,14 +2515,14 @@ impl ExecutingFrame<'_> {
         let require_str = |obj: PyObjectRef, attr: &str| -> PyResult<PyRef<PyStr>> {
             obj.downcast().map_err(|obj: PyObjectRef| {
                 let source = if let Some(ref mod_name) = mod_name {
-                    format!("{}.{attr}", mod_name.as_str())
+                    format!("{}.{attr}", mod_name.as_wtf8())
                 } else {
                     attr.to_owned()
                 };
                 let repr = obj.repr(vm).unwrap_or_else(|_| vm.ctx.new_str("?"));
                 vm.new_type_error(format!(
                     "{} in {} must be str, not {}",
-                    repr.as_str(),
+                    repr.as_wtf8(),
                     source,
                     obj.class().name()
                 ))
@@ -2528,7 +2541,7 @@ impl ExecutingFrame<'_> {
         } else {
             for (k, v) in dict {
                 let k = require_str(k, "__dict__")?;
-                if !k.as_str().starts_with('_') {
+                if !k.as_bytes().starts_with(b"_") {
                     self.locals.mapping().ass_subscript(&k, Some(v), vm)?;
                 }
             }
@@ -2650,10 +2663,13 @@ impl ExecutingFrame<'_> {
             .expect("kwarg names should be tuple of strings");
         let args = self.pop_multiple(nargs as usize);
 
-        let kwarg_names = kwarg_names
-            .as_slice()
-            .iter()
-            .map(|pyobj| pyobj.downcast_ref::<PyStr>().unwrap().as_str().to_owned());
+        let kwarg_names = kwarg_names.as_slice().iter().map(|pyobj| {
+            pyobj
+                .downcast_ref::<PyUtf8Str>()
+                .unwrap()
+                .as_str()
+                .to_owned()
+        });
         FuncArgs::with_kwargs_names(args, kwarg_names)
     }
 
@@ -2668,7 +2684,7 @@ impl ExecutingFrame<'_> {
 
             Self::iterate_mapping_keys(vm, &kw_obj, &func_str, |key| {
                 let key_str = key
-                    .downcast_ref::<PyStr>()
+                    .downcast_ref::<PyUtf8Str>()
                     .ok_or_else(|| vm.new_type_error("keywords must be strings"))?;
                 let value = kw_obj.get_item(&*key, vm)?;
                 kwargs.insert(key_str.as_str().to_owned(), value);
@@ -2708,26 +2724,26 @@ impl ExecutingFrame<'_> {
     /// Returns a display string for a callable object for use in error messages.
     /// For objects with `__qualname__`, returns "module.qualname()" or "qualname()".
     /// For other objects, returns repr(obj).
-    fn object_function_str(obj: &PyObject, vm: &VirtualMachine) -> String {
+    fn object_function_str(obj: &PyObject, vm: &VirtualMachine) -> Wtf8Buf {
+        let repr_fallback = || {
+            obj.repr(vm)
+                .as_ref()
+                .map_or("?".as_ref(), |s| s.as_wtf8())
+                .to_owned()
+        };
         let Ok(qualname) = obj.get_attr(vm.ctx.intern_str("__qualname__"), vm) else {
-            return obj
-                .repr(vm)
-                .map(|s| s.as_str().to_owned())
-                .unwrap_or_else(|_| "?".to_owned());
+            return repr_fallback();
         };
         let Some(qualname_str) = qualname.downcast_ref::<PyStr>() else {
-            return obj
-                .repr(vm)
-                .map(|s| s.as_str().to_owned())
-                .unwrap_or_else(|_| "?".to_owned());
+            return repr_fallback();
         };
         if let Ok(module) = obj.get_attr(vm.ctx.intern_str("__module__"), vm)
             && let Some(module_str) = module.downcast_ref::<PyStr>()
-            && module_str.as_str() != "builtins"
+            && module_str.as_bytes() != b"builtins"
         {
-            return format!("{}.{}()", module_str.as_str(), qualname_str.as_str());
+            return wtf8_concat!(module_str.as_wtf8(), ".", qualname_str.as_wtf8(), "()");
         }
-        format!("{}()", qualname_str.as_str())
+        wtf8_concat!(qualname_str.as_wtf8(), "()")
     }
 
     /// Helper function to iterate over mapping keys using the keys() method.
@@ -2735,7 +2751,7 @@ impl ExecutingFrame<'_> {
     fn iterate_mapping_keys<F>(
         vm: &VirtualMachine,
         mapping: &PyObject,
-        func_str: &str,
+        func_str: &Wtf8,
         mut key_handler: F,
     ) -> PyResult<()>
     where
diff --git a/crates/vm/src/function/buffer.rs b/crates/vm/src/function/buffer.rs
index e8f835e1dac..c0dd6473bdc 100644
--- a/crates/vm/src/function/buffer.rs
+++ b/crates/vm/src/function/buffer.rs
@@ -172,7 +172,7 @@ impl TryFromObject for ArgAsciiBuffer {
     fn try_from_object(vm: &VirtualMachine, obj: PyObjectRef) -> PyResult<Self> {
         match obj.downcast::<PyStr>() {
             Ok(string) => {
-                if string.as_str().is_ascii() {
+                if string.as_wtf8().is_ascii() {
                     Ok(Self::String(string))
                 } else {
                     Err(vm.new_value_error("string argument should contain only ASCII characters"))
@@ -186,7 +186,7 @@ impl TryFromObject for ArgAsciiBuffer {
 impl ArgAsciiBuffer {
     pub fn len(&self) -> usize {
         match self {
-            Self::String(s) => s.as_str().len(),
+            Self::String(s) => s.as_wtf8().len(),
             Self::Buffer(buffer) => buffer.len(),
         }
     }
diff --git a/crates/vm/src/function/fspath.rs b/crates/vm/src/function/fspath.rs
index 7d3a0dcbbd5..732fd0ca35a 100644
--- a/crates/vm/src/function/fspath.rs
+++ b/crates/vm/src/function/fspath.rs
@@ -106,7 +106,7 @@ impl FsPath {
 
     pub fn to_path_buf(&self, vm: &VirtualMachine) -> PyResult<PathBuf> {
         let path = match self {
-            Self::Str(s) => PathBuf::from(s.as_str()),
+            Self::Str(s) => PathBuf::from(vm.fsencode(s)?.as_ref() as &OsStr),
             Self::Bytes(b) => PathBuf::from(Self::bytes_as_os_str(b, vm)?),
         };
         Ok(path)
diff --git a/crates/vm/src/function/mod.rs b/crates/vm/src/function/mod.rs
index e86adf5f27b..15048919593 100644
--- a/crates/vm/src/function/mod.rs
+++ b/crates/vm/src/function/mod.rs
@@ -36,9 +36,9 @@ pub enum ArgByteOrder {
 impl<'a> TryFromBorrowedObject<'a> for ArgByteOrder {
     fn try_from_borrowed_object(vm: &VirtualMachine, obj: &'a PyObject) -> PyResult<Self> {
         obj.try_value_with(
-            |s: &PyStr| match s.as_str() {
-                "big" => Ok(Self::Big),
-                "little" => Ok(Self::Little),
+            |s: &PyStr| match s.as_bytes() {
+                b"big" => Ok(Self::Big),
+                b"little" => Ok(Self::Little),
                 _ => Err(vm.new_value_error("byteorder must be either 'little' or 'big'")),
             },
             vm,
diff --git a/crates/vm/src/import.rs b/crates/vm/src/import.rs
index 1957ccca663..4e89052e1a8 100644
--- a/crates/vm/src/import.rs
+++ b/crates/vm/src/import.rs
@@ -2,7 +2,7 @@
 
 use crate::{
     AsObject, Py, PyObjectRef, PyPayload, PyRef, PyResult,
-    builtins::{PyCode, PyStr, PyStrRef, traceback::PyTraceback},
+    builtins::{PyCode, PyStr, PyUtf8Str, PyUtf8StrRef, traceback::PyTraceback},
     exceptions::types::PyBaseException,
     scope::Scope,
     vm::{VirtualMachine, resolve_frozen_alias, thread},
@@ -69,7 +69,7 @@ pub fn make_frozen(vm: &VirtualMachine, name: &str) -> PyResult<PyRef<PyCode>> {
     let frozen = vm.state.frozen.get(name).ok_or_else(|| {
         vm.new_import_error(
             format!("No such frozen object named {name}"),
-            vm.ctx.new_str(name),
+            vm.ctx.new_utf8_str(name),
         )
     })?;
     Ok(vm.ctx.new_code(frozen.code))
@@ -79,13 +79,13 @@ pub fn import_frozen(vm: &VirtualMachine, module_name: &str) -> PyResult {
     let frozen = vm.state.frozen.get(module_name).ok_or_else(|| {
         vm.new_import_error(
             format!("No such frozen object named {module_name}"),
-            vm.ctx.new_str(module_name),
+            vm.ctx.new_utf8_str(module_name),
         )
     })?;
     let module = import_code_obj(vm, module_name, vm.ctx.new_code(frozen.code), false)?;
     debug_assert!(module.get_attr(identifier!(vm, __name__), vm).is_ok());
     let origname = resolve_frozen_alias(module_name);
-    module.set_attr("__origname__", vm.ctx.new_str(origname), vm)?;
+    module.set_attr("__origname__", vm.ctx.new_utf8_str(origname), vm)?;
     Ok(module)
 }
 
@@ -118,7 +118,7 @@ pub fn import_builtin(vm: &VirtualMachine, module_name: &str) -> PyResult {
     // Module not found in module_defs
     Err(vm.new_import_error(
         format!("Cannot import builtin module {module_name}"),
-        vm.ctx.new_str(module_name),
+        vm.ctx.new_utf8_str(module_name),
     ))
 }
 
@@ -169,7 +169,7 @@ fn import_ensure_initialized(
     };
     if initializing {
         let lock_unlock = vm.importlib.get_attr("_lock_unlock_module", vm)?;
-        lock_unlock.call((vm.ctx.new_str(name),), vm)?;
+        lock_unlock.call((vm.ctx.new_utf8_str(name),), vm)?;
     }
     Ok(())
 }
@@ -183,7 +183,7 @@ pub fn import_code_obj(
     let attrs = vm.ctx.new_dict();
     attrs.set_item(
         identifier!(vm, __name__),
-        vm.ctx.new_str(module_name).into(),
+        vm.ctx.new_utf8_str(module_name).into(),
         vm,
     )?;
     if set_file_attr {
@@ -312,7 +312,7 @@ pub(crate) fn is_possibly_shadowing_path(origin: &str, vm: &VirtualMachine) -> b
     let sys_path_0 = (|| -> Option<String> {
         let argv = vm.sys_module.get_attr("argv", vm).ok()?;
         let argv0 = argv.get_item(&0usize, vm).ok()?;
-        let argv0_str = argv0.downcast_ref::<PyStr>()?;
+        let argv0_str = argv0.downcast_ref::<PyUtf8Str>()?;
         let s = argv0_str.as_str();
 
         // For -c and REPL, original sys.path[0] is ""
@@ -412,7 +412,7 @@ pub(crate) fn import_module_level(
         if package.is_empty() {
             return Err(vm.new_import_error(
                 "attempted relative import with no known parent package".to_owned(),
-                vm.ctx.new_str(""),
+                vm.ctx.new_utf8_str(""),
             ));
         }
         resolve_name(name_str, &package, level as usize, vm)?
@@ -432,7 +432,7 @@ pub(crate) fn import_module_level(
         }
         _ => {
             let find_and_load = vm.importlib.get_attr("_find_and_load", vm)?;
-            let abs_name_obj = vm.ctx.new_str(&*abs_name);
+            let abs_name_obj = vm.ctx.new_utf8_str(&*abs_name);
             find_and_load.call((abs_name_obj, vm.import_func.clone()), vm)?
         }
     };
@@ -474,14 +474,14 @@ pub(crate) fn import_module_level(
                         // For absolute imports (level 0), try importing the
                         // parent. Matches _bootstrap.__import__ behavior.
                         let find_and_load = vm.importlib.get_attr("_find_and_load", vm)?;
-                        let to_return_obj = vm.ctx.new_str(&*to_return);
+                        let to_return_obj = vm.ctx.new_utf8_str(&*to_return);
                         find_and_load.call((to_return_obj, vm.import_func.clone()), vm)
                     }
                     Err(_) => {
                         // For relative imports (level > 0), raise KeyError
                         let to_return_obj: PyObjectRef = vm
                             .ctx
-                            .new_str(format!("'{to_return}' not in sys.modules as expected"))
+                            .new_utf8_str(format!("'{to_return}' not in sys.modules as expected"))
                             .into();
                         Err(vm.new_key_error(to_return_obj))
                     }
@@ -501,7 +501,7 @@ fn resolve_name(name: &str, package: &str, level: usize, vm: &VirtualMachine) ->
     if parts.len() < level {
         return Err(vm.new_import_error(
             "attempted relative import beyond top-level package".to_owned(),
-            vm.ctx.new_str(name),
+            vm.ctx.new_utf8_str(name),
         ));
     }
     // rsplitn returns parts right-to-left, so last() is the leftmost (base)
@@ -518,7 +518,7 @@ fn calc_package(globals: Option<&PyObjectRef>, vm: &VirtualMachine) -> PyResult<
     let globals = globals.ok_or_else(|| {
         vm.new_import_error(
             "attempted relative import with no known parent package".to_owned(),
-            vm.ctx.new_str(""),
+            vm.ctx.new_utf8_str(""),
         )
     })?;
 
@@ -528,7 +528,7 @@ fn calc_package(globals: Option<&PyObjectRef>, vm: &VirtualMachine) -> PyResult<
     if let Some(ref pkg) = package
         && !vm.is_none(pkg)
     {
-        let pkg_str: PyStrRef = pkg
+        let pkg_str: PyUtf8StrRef = pkg
             .clone()
             .downcast()
             .map_err(|_| vm.new_type_error("package must be a string".to_owned()))?;
@@ -543,7 +543,7 @@ fn calc_package(globals: Option<&PyObjectRef>, vm: &VirtualMachine) -> PyResult<
                 .unwrap_or(false)
         {
             let parent_repr = parent
-                .repr(vm)
+                .repr_utf8(vm)
                 .map(|s| s.as_str().to_owned())
                 .unwrap_or_default();
             let msg = format!(
@@ -570,7 +570,7 @@ fn calc_package(globals: Option<&PyObjectRef>, vm: &VirtualMachine) -> PyResult<
         && let Ok(parent) = spec.get_attr("parent", vm)
         && !vm.is_none(&parent)
     {
-        let parent_str: PyStrRef = parent
+        let parent_str: PyUtf8StrRef = parent
             .downcast()
             .map_err(|_| vm.new_type_error("package set to non-string".to_owned()))?;
         return Ok(parent_str.as_str().to_owned());
@@ -593,10 +593,10 @@ fn calc_package(globals: Option<&PyObjectRef>, vm: &VirtualMachine) -> PyResult<
     let mod_name = globals.get_item("__name__", vm).map_err(|_| {
         vm.new_import_error(
             "attempted relative import with no known parent package".to_owned(),
-            vm.ctx.new_str(""),
+            vm.ctx.new_utf8_str(""),
         )
     })?;
-    let mod_name_str: PyStrRef = mod_name
+    let mod_name_str: PyUtf8StrRef = mod_name
         .downcast()
         .map_err(|_| vm.new_type_error("__name__ must be a string".to_owned()))?;
     let mut package = mod_name_str.as_str().to_owned();
diff --git a/crates/vm/src/intern.rs b/crates/vm/src/intern.rs
index 8ad39c206fd..afc30e7ac04 100644
--- a/crates/vm/src/intern.rs
+++ b/crates/vm/src/intern.rs
@@ -152,10 +152,25 @@ impl CachedPyStrRef {
     }
 }
 
+#[repr(transparent)]
 pub struct PyInterned<T> {
     inner: Py<T>,
 }
 
+impl PyInterned<PyStr> {
+    /// Returns `&str` for interned strings.
+    ///
+    /// # Panics
+    /// Panics if the interned string contains unpaired surrogates (WTF-8 content).
+    /// Most interned strings are valid UTF-8, so this is an ergonomic default.
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        self.inner
+            .to_str()
+            .unwrap_or_else(|| panic!("interned str is always valid UTF-8"))
+    }
+}
+
 impl<T: PyPayload> PyInterned<T> {
     #[inline]
     pub fn leak(cache: PyRef<T>) -> &'static Self {
diff --git a/crates/vm/src/macros.rs b/crates/vm/src/macros.rs
index 32f3e4566ea..4fad50ac8f2 100644
--- a/crates/vm/src/macros.rs
+++ b/crates/vm/src/macros.rs
@@ -188,11 +188,11 @@ macro_rules! identifier(
 
 #[macro_export]
 macro_rules! identifier_utf8(
-    ($as_ctx:expr, $name:ident) => {
+    ($as_ctx:expr, $name:ident) => {{
         // Safety: All known identifiers are ascii strings.
-        #[allow(clippy::macro_metavars_in_unsafe, reason = "known identifiers are ASCII and downcast target is fixed")]
-        unsafe { $as_ctx.as_ref().names.$name.as_object().downcast_unchecked_ref::<$crate::builtins::PyUtf8Str>() }
-    };
+        let interned = $as_ctx.as_ref().names.$name;
+        unsafe { $crate::builtins::PyUtf8StrInterned::from_str_interned_unchecked(interned) }
+    }};
 );
 
 /// Super detailed logging. Might soon overflow your log buffers
diff --git a/crates/vm/src/protocol/object.rs b/crates/vm/src/protocol/object.rs
index d2e4b31aaea..78ac9903905 100644
--- a/crates/vm/src/protocol/object.rs
+++ b/crates/vm/src/protocol/object.rs
@@ -362,8 +362,8 @@ impl PyObject {
     }
 
     pub fn ascii(&self, vm: &VirtualMachine) -> PyResult<ascii::AsciiString> {
-        let repr = self.repr_utf8(vm)?;
-        let ascii = to_ascii(repr.as_str());
+        let repr = self.repr(vm)?;
+        let ascii = to_ascii(repr.as_wtf8());
         Ok(ascii)
     }
 
@@ -658,7 +658,7 @@ impl PyObject {
 
         Err(vm.new_exception_msg(
             vm.ctx.exceptions.type_error.to_owned(),
-            format!("unhashable type: '{}'", self.class().name()),
+            format!("unhashable type: '{}'", self.class().name()).into(),
         ))
     }
 
diff --git a/crates/vm/src/stdlib/_abc.rs b/crates/vm/src/stdlib/_abc.rs
index 72642c249f5..5657cda9865 100644
--- a/crates/vm/src/stdlib/_abc.rs
+++ b/crates/vm/src/stdlib/_abc.rs
@@ -365,7 +365,7 @@ mod _abc {
         if !ok.is(&vm.ctx.not_implemented) {
             return Err(vm.new_exception_msg(
                 vm.ctx.exceptions.assertion_error.to_owned(),
-                "__subclasshook__ must return either False, True, or NotImplemented".to_owned(),
+                "__subclasshook__ must return either False, True, or NotImplemented".into(),
             ));
         }
 
diff --git a/crates/vm/src/stdlib/_winapi.rs b/crates/vm/src/stdlib/_winapi.rs
index 6a78b36f869..1e52af5aaa4 100644
--- a/crates/vm/src/stdlib/_winapi.rs
+++ b/crates/vm/src/stdlib/_winapi.rs
@@ -15,6 +15,7 @@ mod _winapi {
         windows::{WinHandle, WindowsSysResult},
     };
     use core::ptr::{null, null_mut};
+    use rustpython_common::wtf8::Wtf8Buf;
     use windows_sys::Win32::Foundation::{HANDLE, MAX_PATH};
 
     #[pyattr]
@@ -296,19 +297,19 @@ mod _winapi {
             .map_or_else(null_mut, |l| l.attrlist.as_mut_ptr() as _);
 
         let wstr = |s: PyStrRef| {
-            let ws = widestring::WideCString::from_str(s.as_str())
+            let ws = widestring::WideCString::from_str(s.expect_str())
                 .map_err(|err| err.to_pyexception(vm))?;
             Ok(ws.into_vec_with_nul())
         };
 
         // Validate no embedded null bytes in command name and command line
         if let Some(ref name) = args.name
-            && name.as_str().contains('\0')
+            && name.as_bytes().contains(&0)
         {
             return Err(crate::exceptions::cstring_error(vm));
         }
         if let Some(ref cmd) = args.command_line
-            && cmd.as_str().contains('\0')
+            && cmd.as_bytes().contains(&0)
         {
             return Err(crate::exceptions::cstring_error(vm));
         }
@@ -396,8 +397,8 @@ mod _winapi {
         dest_path: PyStrRef,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        let src_path = std::path::Path::new(src_path.as_str());
-        let dest_path = std::path::Path::new(dest_path.as_str());
+        let src_path = std::path::Path::new(src_path.expect_str());
+        let dest_path = std::path::Path::new(dest_path.expect_str());
 
         junction::create(src_path, dest_path).map_err(|e| e.to_pyexception(vm))
     }
@@ -418,9 +419,9 @@ mod _winapi {
         let mut last_entry: HashMap<String, widestring::WideString> = HashMap::new();
         for (k, v) in keys.into_iter().zip(values.into_iter()) {
             let k = PyStrRef::try_from_object(vm, k)?;
-            let k = k.as_str();
+            let k = k.expect_str();
             let v = PyStrRef::try_from_object(vm, v)?;
-            let v = v.as_str();
+            let v = v.expect_str();
             if k.contains('\0') || v.contains('\0') {
                 return Err(crate::exceptions::cstring_error(vm));
             }
@@ -677,7 +678,6 @@ mod _winapi {
         src: PyStrRef,
         vm: &VirtualMachine,
     ) -> PyResult<PyStrRef> {
-        use rustpython_common::wtf8::Wtf8Buf;
         use windows_sys::Win32::Globalization::{
             LCMAP_BYTEREV, LCMAP_HASH, LCMAP_SORTHANDLE, LCMAP_SORTKEY,
             LCMapStringEx as WinLCMapStringEx,
@@ -1032,8 +1032,6 @@ mod _winapi {
         api_fn: unsafe extern "system" fn(*const u16, *mut u16, u32) -> u32,
         vm: &VirtualMachine,
     ) -> PyResult<PyStrRef> {
-        use rustpython_common::wtf8::Wtf8Buf;
-
         let path_wide = path.as_wtf8().to_wide_with_nul();
 
         // First call to get required buffer size
@@ -1808,7 +1806,7 @@ mod _winapi {
         use windows_sys::Win32::System::Memory::CreateFileMappingW;
 
         if let Some(ref n) = name
-            && n.as_str().contains('\0')
+            && n.as_bytes().contains(&0)
         {
             return Err(vm.new_value_error(
                 "CreateFileMapping: name must not contain null characters".to_owned(),
@@ -1844,7 +1842,7 @@ mod _winapi {
     ) -> PyResult<WinHandle> {
         use windows_sys::Win32::System::Memory::OpenFileMappingW;
 
-        if name.as_str().contains('\0') {
+        if name.as_bytes().contains(&0) {
             return Err(vm.new_value_error(
                 "OpenFileMapping: name must not contain null characters".to_owned(),
             ));
diff --git a/crates/vm/src/stdlib/_wmi.rs b/crates/vm/src/stdlib/_wmi.rs
index f25f4b23bfe..f2b088e96a3 100644
--- a/crates/vm/src/stdlib/_wmi.rs
+++ b/crates/vm/src/stdlib/_wmi.rs
@@ -561,7 +561,7 @@ mod _wmi {
     /// by null characters.
     #[pyfunction]
     fn exec_query(query: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
-        let query_str = query.as_str();
+        let query_str = query.expect_str();
 
         if !query_str
             .get(..7)
diff --git a/crates/vm/src/stdlib/ast.rs b/crates/vm/src/stdlib/ast.rs
index 92366b6e8e3..a04864634be 100644
--- a/crates/vm/src/stdlib/ast.rs
+++ b/crates/vm/src/stdlib/ast.rs
@@ -15,7 +15,7 @@ use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyRefExact, PyResult,
     TryFromObject, VirtualMachine,
     builtins::PyIntRef,
-    builtins::{PyDict, PyModule, PyStrRef, PyType},
+    builtins::{PyDict, PyModule, PyType, PyUtf8StrRef},
     class::{PyClassImpl, StaticType},
     compiler::{CompileError, ParseError},
     convert::ToPyObject,
diff --git a/crates/vm/src/stdlib/ast/basic.rs b/crates/vm/src/stdlib/ast/basic.rs
index ca518eaa520..28e4a6803ee 100644
--- a/crates/vm/src/stdlib/ast/basic.rs
+++ b/crates/vm/src/stdlib/ast/basic.rs
@@ -13,7 +13,7 @@ impl Node for ast::Identifier {
         _source_file: &SourceFile,
         object: PyObjectRef,
     ) -> PyResult<Self> {
-        let py_str = PyStrRef::try_from_object(vm, object)?;
+        let py_str = PyUtf8StrRef::try_from_object(vm, object)?;
         Ok(Self::new(py_str.as_str(), TextRange::default()))
     }
 }
diff --git a/crates/vm/src/stdlib/ast/python.rs b/crates/vm/src/stdlib/ast/python.rs
index 0de6f45b912..acc36202cd1 100644
--- a/crates/vm/src/stdlib/ast/python.rs
+++ b/crates/vm/src/stdlib/ast/python.rs
@@ -8,8 +8,9 @@ use super::{
 pub(crate) mod _ast {
     use crate::{
         AsObject, Context, Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef},
+        builtins::{PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef, PyUtf8Str, PyUtf8StrRef},
         class::{PyClassImpl, StaticType},
+        common::wtf8::Wtf8,
         function::{FuncArgs, KwArgs, PyMethodDef, PyMethodFlags},
         stdlib::ast::repr,
         types::{Constructor, Initializer},
@@ -99,7 +100,7 @@ pub(crate) mod _ast {
             let fields: Vec<PyStrRef> = fields.try_to_value(vm)?;
             let mut positional: Vec<PyObjectRef> = Vec::new();
             for field in fields {
-                if dict.get_item_opt::<str>(field.as_str(), vm)?.is_some() {
+                if dict.get_item_opt::<Wtf8>(field.as_wtf8(), vm)?.is_some() {
                     positional.push(vm.ctx.none());
                 } else {
                     break;
@@ -127,13 +128,13 @@ pub(crate) mod _ast {
 
         let mut expecting: std::collections::HashSet<String> = std::collections::HashSet::new();
         if let Some(fields) = fields.clone() {
-            let fields: Vec<PyStrRef> = fields.try_to_value(vm)?;
+            let fields: Vec<PyUtf8StrRef> = fields.try_to_value(vm)?;
             for field in fields {
                 expecting.insert(field.as_str().to_owned());
             }
         }
         if let Some(attributes) = attributes.clone() {
-            let attributes: Vec<PyStrRef> = attributes.try_to_value(vm)?;
+            let attributes: Vec<PyUtf8StrRef> = attributes.try_to_value(vm)?;
             for attr in attributes {
                 expecting.insert(attr.as_str().to_owned());
             }
@@ -151,12 +152,12 @@ pub(crate) mod _ast {
 
         if let Some(dict) = dict.as_ref() {
             for (key, _value) in dict.items_vec() {
-                if let Ok(key) = key.downcast::<PyStr>() {
+                if let Ok(key) = key.downcast::<PyUtf8Str>() {
                     expecting.remove(key.as_str());
                 }
             }
             if let Some(attributes) = attributes.clone() {
-                let attributes: Vec<PyStrRef> = attributes.try_to_value(vm)?;
+                let attributes: Vec<PyUtf8StrRef> = attributes.try_to_value(vm)?;
                 for attr in attributes {
                     expecting.remove(attr.as_str());
                 }
@@ -168,7 +169,7 @@ pub(crate) mod _ast {
             && let Ok(field_types) = field_types.downcast::<crate::builtins::PyDict>()
         {
             for (key, value) in field_types.items_vec() {
-                let Ok(key) = key.downcast::<PyStr>() else {
+                let Ok(key) = key.downcast::<PyUtf8Str>() else {
                     continue;
                 };
                 if value.fast_isinstance(vm.ctx.types.union_type) {
@@ -199,7 +200,7 @@ pub(crate) mod _ast {
             if let Some(fields) = fields.clone() {
                 let fields: Vec<PyStrRef> = fields.try_to_value(vm)?;
                 for field in fields {
-                    if let Some(value) = dict.get_item_opt::<str>(field.as_str(), vm)? {
+                    if let Some(value) = dict.get_item_opt::<Wtf8>(field.as_wtf8(), vm)? {
                         payload.set_item(field.as_object(), value, vm)?;
                     }
                 }
@@ -207,7 +208,7 @@ pub(crate) mod _ast {
             if let Some(attributes) = attributes.clone() {
                 let attributes: Vec<PyStrRef> = attributes.try_to_value(vm)?;
                 for attr in attributes {
-                    if let Some(value) = dict.get_item_opt::<str>(attr.as_str(), vm)? {
+                    if let Some(value) = dict.get_item_opt::<Wtf8>(attr.as_wtf8(), vm)? {
                         payload.set_item(attr.as_object(), value, vm)?;
                     }
                 }
@@ -223,7 +224,7 @@ pub(crate) mod _ast {
             .into_iter()
             .map(|(key, value)| {
                 let key = key
-                    .downcast::<PyStr>()
+                    .downcast::<PyUtf8Str>()
                     .map_err(|_| vm.new_type_error("keywords must be strings".to_owned()))?;
                 Ok((key.as_str().to_owned(), value))
             })
@@ -292,7 +293,7 @@ pub(crate) mod _ast {
                         zelf.class().name()
                     ))
                 })?;
-            let fields: Vec<PyStrRef> = fields.try_to_value(vm)?;
+            let fields: Vec<PyUtf8StrRef> = fields.try_to_value(vm)?;
             let n_args = args.args.len();
             if n_args > fields.len() {
                 return Err(vm.new_type_error(format!(
@@ -309,10 +310,10 @@ pub(crate) mod _ast {
 
             for (name, arg) in fields.iter().zip(args.args) {
                 zelf.set_attr(name, arg, vm)?;
-                set_fields.insert(name.as_str().to_string());
+                set_fields.insert(name.as_str().to_owned());
             }
             for (key, value) in args.kwargs {
-                if let Some(pos) = fields.iter().position(|f| f.as_str() == key)
+                if let Some(pos) = fields.iter().position(|f| f.as_bytes() == key.as_bytes())
                     && pos < n_args
                 {
                     return Err(vm.new_type_error(format!(
@@ -322,7 +323,10 @@ pub(crate) mod _ast {
                     )));
                 }
 
-                if fields.iter().all(|field| field.as_str() != key) {
+                if fields
+                    .iter()
+                    .all(|field| field.as_bytes() != key.as_bytes())
+                {
                     let attrs = if let Some(attrs) = &attributes {
                         attrs
                     } else {
@@ -334,7 +338,7 @@ pub(crate) mod _ast {
                         attributes = Some(attrs);
                         attributes.as_ref().unwrap()
                     };
-                    if attrs.iter().all(|attr| attr.as_str() != key) {
+                    if attrs.iter().all(|attr| attr.as_bytes() != key.as_bytes()) {
                         let message = vm.ctx.new_str(format!(
                             "{}.__init__ got an unexpected keyword argument '{}'. \
 Support for arbitrary keyword arguments is deprecated and will be removed in Python 3.15.",
@@ -368,13 +372,13 @@ Support for arbitrary keyword arguments is deprecated and will be removed in Pyt
                     if set_fields.contains(field.as_str()) {
                         continue;
                     }
-                    if let Some(ftype) = ft_dict.get_item_opt::<str>(field.as_str(), vm)? {
+                    if let Some(ftype) = ft_dict.get_item_opt::<Wtf8>(field.as_wtf8(), vm)? {
                         if ftype.fast_isinstance(vm.ctx.types.union_type) {
                             // Optional field (T | None) — no default
                         } else if ftype.fast_isinstance(vm.ctx.types.generic_alias_type) {
                             // List field (list[T]) — default to []
                             let empty_list: PyObjectRef = vm.ctx.new_list(vec![]).into();
-                            zelf.set_attr(vm.ctx.intern_str(field.as_str()), empty_list, vm)?;
+                            zelf.set_attr(vm.ctx.intern_str(field.as_wtf8()), empty_list, vm)?;
                         } else if ftype.is(&expr_ctx_type) {
                             // expr_context — default to Load()
                             let load_type =
@@ -384,13 +388,13 @@ Support for arbitrary keyword arguments is deprecated and will be removed in Pyt
                                 .unwrap_or_else(|| {
                                     vm.ctx.new_base_object(load_type, Some(vm.ctx.new_dict()))
                                 });
-                            zelf.set_attr(vm.ctx.intern_str(field.as_str()), load_instance, vm)?;
+                            zelf.set_attr(vm.ctx.intern_str(field.as_wtf8()), load_instance, vm)?;
                         } else {
-                            // Required field missing: emit DeprecationWarning (CPython behavior).
+                            // Required field missing: emit DeprecationWarning.
                             let message = vm.ctx.new_str(format!(
                                 "{}.__init__ missing 1 required positional argument: '{}'",
                                 zelf.class().name(),
-                                field.as_str()
+                                field.as_wtf8()
                             ));
                             warn::warn(
                                 message.into(),
diff --git a/crates/vm/src/stdlib/ast/repr.rs b/crates/vm/src/stdlib/ast/repr.rs
index 0810814cd06..0b7f903f807 100644
--- a/crates/vm/src/stdlib/ast/repr.rs
+++ b/crates/vm/src/stdlib/ast/repr.rs
@@ -4,14 +4,15 @@ use crate::{
     class::PyClassImpl,
     stdlib::ast::NodeAst,
 };
+use rustpython_common::wtf8::Wtf8Buf;
 
-fn repr_ast_list(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) -> PyResult<String> {
+fn repr_ast_list(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) -> PyResult<Wtf8Buf> {
     if items.is_empty() {
         let empty_list: PyObjectRef = vm.ctx.new_list(vec![]).into();
-        return Ok(empty_list.repr(vm)?.to_string());
+        return Ok(empty_list.repr(vm)?.as_wtf8().to_owned());
     }
 
-    let mut parts: Vec<String> = Vec::new();
+    let mut parts: Vec<Wtf8Buf> = Vec::new();
     let first = &items[0];
     let last = items.last().unwrap();
 
@@ -22,38 +23,38 @@ fn repr_ast_list(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) ->
         let repr = if item.fast_isinstance(&NodeAst::make_class(&vm.ctx)) {
             repr_ast_node(vm, item, depth.saturating_sub(1))?
         } else {
-            item.repr(vm)?.to_string()
+            item.repr(vm)?.as_wtf8().to_owned()
         };
         parts.push(repr);
     }
 
-    let mut rendered = String::from("[");
+    let mut rendered = Wtf8Buf::from("[");
     if !parts.is_empty() {
-        rendered.push_str(&parts[0]);
+        rendered.push_wtf8(&parts[0]);
     }
     if items.len() > 2 {
         if !parts[0].is_empty() {
-            rendered.push_str(", ...");
+            rendered.push_wtf8(", ...".as_ref());
         }
         if parts.len() > 1 {
-            rendered.push_str(", ");
-            rendered.push_str(&parts[1]);
+            rendered.push_wtf8(", ".as_ref());
+            rendered.push_wtf8(&parts[1]);
         }
     } else if parts.len() > 1 {
-        rendered.push_str(", ");
-        rendered.push_str(&parts[1]);
+        rendered.push_wtf8(", ".as_ref());
+        rendered.push_wtf8(&parts[1]);
     }
-    rendered.push(']');
+    rendered.push_wtf8("]".as_ref());
     Ok(rendered)
 }
 
-fn repr_ast_tuple(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) -> PyResult<String> {
+fn repr_ast_tuple(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) -> PyResult<Wtf8Buf> {
     if items.is_empty() {
         let empty_tuple: PyObjectRef = vm.ctx.empty_tuple.clone().into();
-        return Ok(empty_tuple.repr(vm)?.to_string());
+        return Ok(empty_tuple.repr(vm)?.as_wtf8().to_owned());
     }
 
-    let mut parts: Vec<String> = Vec::new();
+    let mut parts: Vec<Wtf8Buf> = Vec::new();
     let first = &items[0];
     let last = items.last().unwrap();
 
@@ -64,31 +65,31 @@ fn repr_ast_tuple(vm: &VirtualMachine, items: Vec<PyObjectRef>, depth: usize) ->
         let repr = if item.fast_isinstance(&NodeAst::make_class(&vm.ctx)) {
             repr_ast_node(vm, item, depth.saturating_sub(1))?
         } else {
-            item.repr(vm)?.to_string()
+            item.repr(vm)?.as_wtf8().to_owned()
         };
         parts.push(repr);
     }
 
-    let mut rendered = String::from("(");
+    let mut rendered = Wtf8Buf::from("(");
     if !parts.is_empty() {
-        rendered.push_str(&parts[0]);
+        rendered.push_wtf8(&parts[0]);
     }
     if items.len() > 2 {
         if !parts[0].is_empty() {
-            rendered.push_str(", ...");
+            rendered.push_wtf8(", ...".as_ref());
         }
         if parts.len() > 1 {
-            rendered.push_str(", ");
-            rendered.push_str(&parts[1]);
+            rendered.push_wtf8(", ".as_ref());
+            rendered.push_wtf8(&parts[1]);
         }
     } else if parts.len() > 1 {
-        rendered.push_str(", ");
-        rendered.push_str(&parts[1]);
+        rendered.push_wtf8(", ".as_ref());
+        rendered.push_wtf8(&parts[1]);
     }
     if items.len() == 1 {
-        rendered.push(',');
+        rendered.push_wtf8(",".as_ref());
     }
-    rendered.push(')');
+    rendered.push_wtf8(")".as_ref());
     Ok(rendered)
 }
 
@@ -96,25 +97,32 @@ pub(crate) fn repr_ast_node(
     vm: &VirtualMachine,
     obj: &PyObjectRef,
     depth: usize,
-) -> PyResult<String> {
+) -> PyResult<Wtf8Buf> {
     let cls = obj.class();
     if depth == 0 {
-        return Ok(format!("{}(...)", cls.name()));
+        let mut s = Wtf8Buf::from(&*cls.name());
+        s.push_wtf8("(...)".as_ref());
+        return Ok(s);
     }
 
     let fields = cls.get_attr(vm.ctx.intern_str("_fields"));
     let fields = match fields {
         Some(fields) => fields.try_to_value::<Vec<crate::builtins::PyStrRef>>(vm)?,
-        None => return Ok(format!("{}(...)", cls.name())),
+        None => {
+            let mut s = Wtf8Buf::from(&*cls.name());
+            s.push_wtf8("(...)".as_ref());
+            return Ok(s);
+        }
     };
 
     if fields.is_empty() {
-        return Ok(format!("{}()", cls.name()));
+        let mut s = Wtf8Buf::from(&*cls.name());
+        s.push_wtf8("()".as_ref());
+        return Ok(s);
     }
 
-    let mut rendered = String::new();
-    rendered.push_str(&cls.name());
-    rendered.push('(');
+    let mut rendered = Wtf8Buf::from(&*cls.name());
+    rendered.push_wtf8("(".as_ref());
 
     for (idx, field) in fields.iter().enumerate() {
         let value = obj.get_attr(field, vm)?;
@@ -131,17 +139,17 @@ pub(crate) fn repr_ast_node(
         } else if value.fast_isinstance(&NodeAst::make_class(&vm.ctx)) {
             repr_ast_node(vm, &value, depth.saturating_sub(1))?
         } else {
-            value.repr(vm)?.to_string()
+            value.repr(vm)?.as_wtf8().to_owned()
         };
 
         if idx > 0 {
-            rendered.push_str(", ");
+            rendered.push_wtf8(", ".as_ref());
         }
-        rendered.push_str(field.as_str());
-        rendered.push('=');
-        rendered.push_str(&value_repr);
+        rendered.push_wtf8(field.as_wtf8());
+        rendered.push_wtf8("=".as_ref());
+        rendered.push_wtf8(&value_repr);
     }
 
-    rendered.push(')');
+    rendered.push_wtf8(")".as_ref());
     Ok(rendered)
 }
diff --git a/crates/vm/src/stdlib/builtins.rs b/crates/vm/src/stdlib/builtins.rs
index 1b54a26e732..c09e025a44a 100644
--- a/crates/vm/src/stdlib/builtins.rs
+++ b/crates/vm/src/stdlib/builtins.rs
@@ -11,6 +11,7 @@ mod builtins {
         AsObject, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject, VirtualMachine,
         builtins::{
             PyByteArray, PyBytes, PyDictRef, PyStr, PyStrRef, PyTuple, PyTupleRef, PyType,
+            PyUtf8StrRef,
             enumerate::PyReverseSequenceIterator,
             function::{PyCellRef, PyFunction},
             int::PyIntRef,
@@ -30,7 +31,7 @@ mod builtins {
         types::PyComparisonOp,
     };
     use itertools::Itertools;
-    use num_traits::{Signed, ToPrimitive};
+    use num_traits::{Signed, ToPrimitive, Zero};
     use rustpython_common::wtf8::CodePoint;
 
     #[cfg(not(feature = "rustpython-compiler"))]
@@ -65,7 +66,7 @@ mod builtins {
     #[pyfunction]
     pub fn ascii(obj: PyObjectRef, vm: &VirtualMachine) -> PyResult<ascii::AsciiString> {
         let repr = obj.repr(vm)?;
-        let ascii = to_ascii(repr.as_str());
+        let ascii = to_ascii(repr.as_wtf8());
         Ok(ascii)
     }
 
@@ -99,7 +100,7 @@ mod builtins {
     struct CompileArgs {
         source: PyObjectRef,
         filename: FsPath,
-        mode: PyStrRef,
+        mode: PyUtf8StrRef,
         #[pyarg(any, optional)]
         flags: OptionalArg<PyIntRef>,
         #[pyarg(any, optional)]
@@ -139,7 +140,6 @@ mod builtins {
                 .source
                 .fast_isinstance(&ast::NodeAst::make_class(&vm.ctx))
             {
-                use num_traits::Zero;
                 let flags: i32 = args.flags.map_or(Ok(0), |v| v.try_to_primitive(vm))?;
                 let is_ast_only = !(flags & ast::PY_CF_ONLY_AST).is_zero();
 
@@ -197,7 +197,7 @@ mod builtins {
             #[cfg(feature = "parser")]
             {
                 use crate::convert::ToPyException;
-                use num_traits::Zero;
+
                 use ruff_python_parser as parser;
 
                 let source = ArgStrOrBytesLike::try_from_object(vm, args.source)?;
@@ -413,7 +413,7 @@ mod builtins {
                 if source.contains(&0) {
                     return Err(vm.new_exception_msg(
                         vm.ctx.exceptions.syntax_error.to_owned(),
-                        "source code string cannot contain null bytes".to_owned(),
+                        "source code string cannot contain null bytes".into(),
                     ));
                 }
 
@@ -424,9 +424,9 @@ mod builtins {
                         err.valid_up_to()
                     );
 
-                    vm.new_exception_msg(vm.ctx.exceptions.syntax_error.to_owned(), msg)
+                    vm.new_exception_msg(vm.ctx.exceptions.syntax_error.to_owned(), msg.into())
                 })?;
-                Ok(Either::A(vm.ctx.new_str(source.trim_start())))
+                Ok(Either::A(vm.ctx.new_utf8_str(source.trim_start())))
             }
             Either::B(code) => Ok(Either::B(code)),
         }?;
@@ -435,7 +435,7 @@ mod builtins {
 
     #[pyfunction]
     fn exec(
-        source: Either<PyStrRef, PyRef<crate::builtins::PyCode>>,
+        source: Either<PyUtf8StrRef, PyRef<crate::builtins::PyCode>>,
         scope: ScopeArgs,
         vm: &VirtualMachine,
     ) -> PyResult {
@@ -445,7 +445,7 @@ mod builtins {
 
     fn run_code(
         vm: &VirtualMachine,
-        source: Either<PyStrRef, PyRef<crate::builtins::PyCode>>,
+        source: Either<PyUtf8StrRef, PyRef<crate::builtins::PyCode>>,
         scope: crate::scope::Scope,
         #[allow(unused_variables)] mode: crate::compiler::Mode,
         func: &str,
@@ -453,9 +453,11 @@ mod builtins {
         // Determine code object:
         let code_obj = match source {
             #[cfg(feature = "rustpython-compiler")]
-            Either::A(string) => vm
-                .compile(string.as_str(), mode, "<string>".to_owned())
-                .map_err(|err| vm.new_syntax_error(&err, Some(string.as_str())))?,
+            Either::A(string) => {
+                let source = string.as_str();
+                vm.compile(source, mode, "<string>".to_owned())
+                    .map_err(|err| vm.new_syntax_error(&err, Some(source)))?
+            }
             #[cfg(not(feature = "rustpython-compiler"))]
             Either::A(_) => return Err(vm.new_type_error(CODEGEN_NOT_SUPPORTED.to_owned())),
             Either::B(code_obj) => code_obj,
@@ -567,8 +569,15 @@ mod builtins {
             && std::io::stdin().is_terminal()
             && !is_pty_child();
 
+        // Disable rustyline if prompt contains surrogates (not valid UTF-8 for terminal)
+        let prompt_str = match &prompt {
+            OptionalArg::Present(s) => s.to_str(),
+            OptionalArg::Missing => Some(""),
+        };
+        let use_rustyline = use_rustyline && prompt_str.is_some();
+
         if use_rustyline {
-            let prompt = prompt.as_ref().map_or("", |s| s.as_str());
+            let prompt = prompt_str.unwrap();
             let mut readline = Readline::new(());
             match readline.readline(prompt) {
                 ReadlineResult::Line(s) => Ok(vm.ctx.new_str(s).into()),
diff --git a/crates/vm/src/stdlib/codecs.rs b/crates/vm/src/stdlib/codecs.rs
index 85869e066cb..6c37ee4c9f9 100644
--- a/crates/vm/src/stdlib/codecs.rs
+++ b/crates/vm/src/stdlib/codecs.rs
@@ -42,40 +42,59 @@ mod _codecs {
     struct CodeArgs {
         obj: PyObjectRef,
         #[pyarg(any, optional)]
-        encoding: Option<PyStrRef>,
+        encoding: Option<PyUtf8StrRef>,
         #[pyarg(any, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
+    }
+
+    impl CodeArgs {
+        fn apply(
+            self,
+            vm: &VirtualMachine,
+            f: fn(
+                &codecs::CodecsRegistry,
+                PyObjectRef,
+                &str,
+                Option<PyUtf8StrRef>,
+                &VirtualMachine,
+            ) -> PyResult,
+        ) -> PyResult {
+            let encoding = self
+                .encoding
+                .as_deref()
+                .map(|s| s.as_str())
+                .unwrap_or(codecs::DEFAULT_ENCODING);
+            f(
+                &vm.state.codec_registry,
+                self.obj,
+                encoding,
+                self.errors,
+                vm,
+            )
+        }
     }
 
     #[pyfunction]
     fn encode(args: CodeArgs, vm: &VirtualMachine) -> PyResult {
-        let encoding = args
-            .encoding
-            .as_ref()
-            .map_or(codecs::DEFAULT_ENCODING, |s| s.as_str());
-        vm.state
-            .codec_registry
-            .encode(args.obj, encoding, args.errors, vm)
+        args.apply(vm, codecs::CodecsRegistry::encode)
     }
 
     #[pyfunction]
     fn decode(args: CodeArgs, vm: &VirtualMachine) -> PyResult {
-        let encoding = args
-            .encoding
-            .as_ref()
-            .map_or(codecs::DEFAULT_ENCODING, |s| s.as_str());
-        vm.state
-            .codec_registry
-            .decode(args.obj, encoding, args.errors, vm)
+        args.apply(vm, codecs::CodecsRegistry::decode)
     }
 
     #[pyfunction]
-    fn _forget_codec(encoding: PyStrRef, vm: &VirtualMachine) {
+    fn _forget_codec(encoding: PyUtf8StrRef, vm: &VirtualMachine) {
         vm.state.codec_registry.forget(encoding.as_str());
     }
 
     #[pyfunction]
-    fn register_error(name: PyStrRef, handler: PyObjectRef, vm: &VirtualMachine) -> PyResult<()> {
+    fn register_error(
+        name: PyUtf8StrRef,
+        handler: PyObjectRef,
+        vm: &VirtualMachine,
+    ) -> PyResult<()> {
         if !handler.is_callable() {
             return Err(vm.new_type_error("handler must be callable".to_owned()));
         }
@@ -86,28 +105,18 @@ mod _codecs {
     }
 
     #[pyfunction]
-    fn lookup_error(name: PyStrRef, vm: &VirtualMachine) -> PyResult {
-        if name.as_wtf8().as_bytes().contains(&0) {
+    fn lookup_error(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult {
+        if name.as_str().contains('\0') {
             return Err(cstring_error(vm));
         }
-        if !name.as_wtf8().is_utf8() {
-            return Err(vm.new_unicode_encode_error(
-                "'utf-8' codec can't encode character: surrogates not allowed".to_owned(),
-            ));
-        }
         vm.state.codec_registry.lookup_error(name.as_str(), vm)
     }
 
     #[pyfunction]
-    fn _unregister_error(errors: PyStrRef, vm: &VirtualMachine) -> PyResult<bool> {
-        if errors.as_wtf8().as_bytes().contains(&0) {
+    fn _unregister_error(errors: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<bool> {
+        if errors.as_str().contains('\0') {
             return Err(cstring_error(vm));
         }
-        if !errors.as_wtf8().is_utf8() {
-            return Err(vm.new_unicode_encode_error(
-                "'utf-8' codec can't encode character: surrogates not allowed".to_owned(),
-            ));
-        }
         vm.state
             .codec_registry
             .unregister_error(errors.as_str(), vm)
@@ -120,7 +129,7 @@ mod _codecs {
         #[pyarg(positional)]
         s: PyStrRef,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
     }
 
     impl EncodeArgs {
@@ -143,7 +152,7 @@ mod _codecs {
         #[pyarg(positional)]
         data: ArgBytesLike,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         #[pyarg(positional, default = false)]
         final_decode: bool,
     }
@@ -165,7 +174,7 @@ mod _codecs {
         #[pyarg(positional)]
         data: ArgBytesLike,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
     }
 
     impl DecodeArgsNoFinal {
@@ -366,14 +375,14 @@ fn delegate_pycodecs(
 #[pymodule(sub)]
 mod _codecs_windows {
     use crate::{PyResult, VirtualMachine};
-    use crate::{builtins::PyStrRef, function::ArgBytesLike};
+    use crate::{builtins::PyStrRef, builtins::PyUtf8StrRef, function::ArgBytesLike};
 
     #[derive(FromArgs)]
     struct MbcsEncodeArgs {
         #[pyarg(positional)]
         s: PyStrRef,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
     }
 
     #[pyfunction]
@@ -461,7 +470,7 @@ mod _codecs_windows {
         #[pyarg(positional)]
         data: ArgBytesLike,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         #[pyarg(positional, default = false)]
         #[allow(dead_code)]
         r#final: bool,
@@ -559,7 +568,7 @@ mod _codecs_windows {
         #[pyarg(positional)]
         s: PyStrRef,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
     }
 
     #[pyfunction]
@@ -647,7 +656,7 @@ mod _codecs_windows {
         #[pyarg(positional)]
         data: ArgBytesLike,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         #[pyarg(positional, default = false)]
         #[allow(dead_code)]
         r#final: bool,
@@ -747,7 +756,7 @@ mod _codecs_windows {
         #[pyarg(positional)]
         s: PyStrRef,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
     }
 
     fn code_page_encoding_name(code_page: u32) -> String {
@@ -1074,7 +1083,7 @@ mod _codecs_windows {
         #[pyarg(positional)]
         data: ArgBytesLike,
         #[pyarg(positional, optional)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         #[pyarg(positional, default = false)]
         r#final: bool,
     }
diff --git a/crates/vm/src/stdlib/collections.rs b/crates/vm/src/stdlib/collections.rs
index 0f84db80e74..80f80e2d28f 100644
--- a/crates/vm/src/stdlib/collections.rs
+++ b/crates/vm/src/stdlib/collections.rs
@@ -7,7 +7,7 @@ mod _collections {
         atomic_func,
         builtins::{
             IterStatus::{Active, Exhausted},
-            PositionIterInternal, PyGenericAlias, PyInt, PyType, PyTypeRef,
+            PositionIterInternal, PyGenericAlias, PyInt, PyStr, PyType, PyTypeRef,
         },
         common::lock::{PyMutex, PyRwLock, PyRwLockReadGuard, PyRwLockWriteGuard},
         function::{KwArgs, OptionalArg, PyComparisonValue},
@@ -559,7 +559,7 @@ mod _collections {
 
     impl Representable for PyDeque {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyRef<PyStr>> {
             let deque = zelf.borrow_deque().clone();
             let class = zelf.class();
             let class_name = class.name();
@@ -568,15 +568,24 @@ mod _collections {
                 .map(|maxlen| format!("], maxlen={maxlen}"))
                 .unwrap_or_else(|| "]".to_owned());
 
-            let s = if zelf.__len__() == 0 {
-                format!("{class_name}([{closing_part})")
-            } else if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-                collection_repr(Some(&class_name), "[", &closing_part, deque.iter(), vm)?
+            if zelf.__len__() == 0 {
+                return Ok(vm.ctx.new_str(format!("{class_name}([{closing_part})")));
+            }
+            if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+                Ok(vm.ctx.new_str(collection_repr(
+                    Some(&class_name),
+                    "[",
+                    &closing_part,
+                    deque.iter(),
+                    vm,
+                )?))
             } else {
-                "[...]".to_owned()
-            };
+                Ok(vm.ctx.intern_str("[...]").to_owned())
+            }
+        }
 
-            Ok(s)
+        fn repr_str(_zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
+            unreachable!("repr() is overridden directly")
         }
     }
 
diff --git a/crates/vm/src/stdlib/ctypes.rs b/crates/vm/src/stdlib/ctypes.rs
index 441a5ce37e4..b358d04a6e8 100644
--- a/crates/vm/src/stdlib/ctypes.rs
+++ b/crates/vm/src/stdlib/ctypes.rs
@@ -78,7 +78,7 @@ impl PyType {
         {
             return Err(vm.new_exception_msg(
                 vm.ctx.exceptions.system_error.to_owned(),
-                format!("class \"{}\" already initialized", self.name()),
+                format!("class \"{}\" already initialized", self.name()).into(),
             ));
         }
         Ok(())
@@ -627,7 +627,7 @@ pub(crate) mod _ctypes {
     #[pyfunction]
     fn dlsym(
         handle: usize,
-        name: crate::builtins::PyStrRef,
+        name: crate::builtins::PyUtf8StrRef,
         vm: &VirtualMachine,
     ) -> PyResult<usize> {
         let symbol_name = alloc::ffi::CString::new(name.as_str())
@@ -689,7 +689,7 @@ pub(crate) mod _ctypes {
         // PyUnicode_CheckExact(cls) - string creates incomplete pointer type
         if let Some(s) = cls.downcast_ref::<PyStr>() {
             // Incomplete pointer type: _type_ not set, cache key is id(result)
-            let name = format!("LP_{}", s.as_str());
+            let name = format!("LP_{}", s.as_wtf8());
 
             let new_type = metaclass
                 .as_object()
@@ -1117,7 +1117,7 @@ pub(crate) mod _ctypes {
                 arg_values.push(ptr);
                 arg_types.push(Type::pointer());
             } else if let Some(s) = arg.downcast_ref::<crate::builtins::PyStr>() {
-                let ptr = s.as_str().as_ptr() as isize;
+                let ptr = s.as_bytes().as_ptr() as isize;
                 arg_values.push(ptr);
                 arg_types.push(Type::pointer());
             } else {
diff --git a/crates/vm/src/stdlib/ctypes/array.rs b/crates/vm/src/stdlib/ctypes/array.rs
index 843dd7d5b8c..0672d0cbe80 100644
--- a/crates/vm/src/stdlib/ctypes/array.rs
+++ b/crates/vm/src/stdlib/ctypes/array.rs
@@ -1,6 +1,8 @@
 use super::StgInfo;
 use super::base::{CDATA_BUFFER_METHODS, PyCData};
 use super::type_info;
+use crate::common::lock::LazyLock;
+use crate::sliceable::SaturatedSliceIter;
 use crate::{
     AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject, VirtualMachine,
     atomic_func,
@@ -455,7 +457,6 @@ impl Initializer for PyCArray {
 
 impl AsSequence for PyCArray {
     fn as_sequence() -> &'static PySequenceMethods {
-        use crate::common::lock::LazyLock;
         static AS_SEQUENCE: LazyLock<PySequenceMethods> = LazyLock::new(|| PySequenceMethods {
             length: atomic_func!(|seq, _vm| {
                 let zelf = PyCArray::sequence_downcast(seq);
@@ -480,7 +481,6 @@ impl AsSequence for PyCArray {
 
 impl AsMapping for PyCArray {
     fn as_mapping() -> &'static PyMappingMethods {
-        use crate::common::lock::LazyLock;
         static AS_MAPPING: LazyLock<PyMappingMethods> = LazyLock::new(|| PyMappingMethods {
             length: atomic_func!(|mapping, _vm| {
                 let zelf = PyCArray::mapping_downcast(mapping);
@@ -775,7 +775,12 @@ impl PyCArray {
             }
             Some("u") => {
                 if let Some(s) = value.downcast_ref::<PyStr>() {
-                    let code = s.as_str().chars().next().map(|c| c as u32).unwrap_or(0);
+                    let code = s
+                        .as_wtf8()
+                        .code_points()
+                        .next()
+                        .map(|c| c.to_u32())
+                        .unwrap_or(0);
                     if offset + WCHAR_SIZE <= buffer.len() {
                         wchar_to_bytes(code, &mut buffer[offset..]);
                     }
@@ -808,7 +813,7 @@ impl PyCArray {
                 let (ptr_val, converted) = if value.is(&vm.ctx.none) {
                     (0usize, None)
                 } else if let Some(s) = value.downcast_ref::<PyStr>() {
-                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
                     (ptr, Some(holder))
                 } else if let Ok(int_val) = value.try_index(vm) {
                     (int_val.as_bigint().to_usize().unwrap_or(0), None)
@@ -965,8 +970,6 @@ impl PyCArray {
 
     // Array_subscript slice handling
     fn getitem_by_slice(zelf: &Py<Self>, slice: &PySlice, vm: &VirtualMachine) -> PyResult {
-        use crate::sliceable::SaturatedSliceIter;
-
         let stg = zelf.class().stg_info_opt();
         let length = stg.as_ref().map_or(0, |i| i.length);
 
@@ -1089,8 +1092,6 @@ impl PyCArray {
         value: PyObjectRef,
         vm: &VirtualMachine,
     ) -> PyResult<()> {
-        use crate::sliceable::SaturatedSliceIter;
-
         let length = zelf.class().stg_info_opt().map_or(0, |i| i.length);
 
         // PySlice_Unpack + PySlice_AdjustIndices
@@ -1249,15 +1250,15 @@ fn wchar_array_set_value(
         .ok_or_else(|| vm.new_type_error("unicode string expected"))?;
     let mut buffer = zelf.0.buffer.write();
     let wchar_count = buffer.len() / WCHAR_SIZE;
-    let char_count = s.as_str().chars().count();
+    let char_count = s.as_wtf8().code_points().count();
 
     if char_count > wchar_count {
         return Err(vm.new_value_error("string too long"));
     }
 
-    for (i, ch) in s.as_str().chars().enumerate() {
+    for (i, ch) in s.as_wtf8().code_points().enumerate() {
         let offset = i * WCHAR_SIZE;
-        wchar_to_bytes(ch as u32, &mut buffer.to_mut()[offset..]);
+        wchar_to_bytes(ch.to_u32(), &mut buffer.to_mut()[offset..]);
     }
 
     let terminator_offset = char_count * WCHAR_SIZE;
diff --git a/crates/vm/src/stdlib/ctypes/base.rs b/crates/vm/src/stdlib/ctypes/base.rs
index ba2b987330a..90137f2549d 100644
--- a/crates/vm/src/stdlib/ctypes/base.rs
+++ b/crates/vm/src/stdlib/ctypes/base.rs
@@ -1,6 +1,9 @@
 use super::array::{WCHAR_SIZE, wchar_from_bytes, wchar_to_bytes};
-use crate::builtins::{PyBytes, PyDict, PyMemoryView, PyStr, PyTuple, PyType, PyTypeRef};
+use crate::builtins::{
+    PyBytes, PyDict, PyList, PyMemoryView, PyStr, PyTuple, PyType, PyTypeRef, PyUtf8Str,
+};
 use crate::class::StaticType;
+use crate::convert::ToPyObject;
 use crate::function::{ArgBytesLike, OptionalArg, PySetterValue};
 use crate::protocol::{BufferMethods, PyBuffer};
 use crate::types::{Constructor, GetDescriptor, Representable};
@@ -16,6 +19,7 @@ use core::mem;
 use crossbeam_utils::atomic::AtomicCell;
 use num_traits::{Signed, ToPrimitive};
 use rustpython_common::lock::PyRwLock;
+use rustpython_common::wtf8::Wtf8;
 use widestring::WideChar;
 
 // StgInfo - Storage information for ctypes types
@@ -363,11 +367,10 @@ pub(super) fn get_field_format(
     if let Ok(type_attr) = field_type.get_attr("_type_", vm)
         && let Some(type_str) = type_attr.downcast_ref::<PyStr>()
     {
-        let s = type_str.as_str();
-        if !s.is_empty() {
-            return format!("{}{}", endian_prefix, s);
-        }
-        return s.to_string();
+        let s = type_str
+            .to_str()
+            .expect("_type_ is validated as ASCII at type creation");
+        return format!("{}{}", endian_prefix, s);
     }
 
     // Default: single byte
@@ -431,10 +434,10 @@ pub(super) fn ensure_z_null_terminated(
 }
 
 /// Convert str to null-terminated wchar_t buffer. Returns (PyBytes holder, pointer).
-pub(super) fn str_to_wchar_bytes(s: &str, vm: &VirtualMachine) -> (PyObjectRef, usize) {
+pub(super) fn str_to_wchar_bytes(s: &Wtf8, vm: &VirtualMachine) -> (PyObjectRef, usize) {
     let wchars: Vec<libc::wchar_t> = s
-        .chars()
-        .map(|c| c as libc::wchar_t)
+        .code_points()
+        .map(|cp| cp.to_u32() as libc::wchar_t)
         .chain(core::iter::once(0))
         .collect();
     let ptr = wchars.as_ptr() as usize;
@@ -964,9 +967,9 @@ impl PyCData {
             if let Some(str_val) = value.downcast_ref::<PyStr>() {
                 // Convert str to wchar_t bytes (platform-dependent size)
                 let mut wchar_bytes = Vec::with_capacity(size);
-                for ch in str_val.as_str().chars().take(size / WCHAR_SIZE) {
+                for cp in str_val.as_wtf8().code_points().take(size / WCHAR_SIZE) {
                     let mut bytes = [0u8; 4];
-                    wchar_to_bytes(ch as u32, &mut bytes);
+                    wchar_to_bytes(cp.to_u32(), &mut bytes);
                     wchar_bytes.extend_from_slice(&bytes[..WCHAR_SIZE]);
                 }
                 // Pad with nulls to fill the array
@@ -1299,13 +1302,13 @@ impl PyCData {
             .ok_or_else(|| vm.new_value_error("Library not found"))?;
         let inner_lib = library.lib.lock();
 
-        let symbol_name_with_nul = format!("{}\0", name.as_str());
+        let symbol_name_with_nul = format!("{}\0", name.as_wtf8());
         let ptr: *const u8 = if let Some(lib) = &*inner_lib {
             unsafe {
                 lib.get::<*const u8>(symbol_name_with_nul.as_bytes())
                     .map(|sym| *sym)
                     .map_err(|_| {
-                        vm.new_value_error(format!("symbol '{}' not found", name.as_str()))
+                        vm.new_value_error(format!("symbol '{}' not found", name.as_wtf8()))
                     })?
             }
         } else {
@@ -1315,7 +1318,7 @@ impl PyCData {
         // dlsym can return NULL for symbols that resolve to NULL (e.g., GNU IFUNC)
         // Treat NULL addresses as errors
         if ptr.is_null() {
-            return Err(vm.new_value_error(format!("symbol '{}' not found", name.as_str())));
+            return Err(vm.new_value_error(format!("symbol '{}' not found", name.as_wtf8())));
         }
 
         // PyCData_AtAddress
@@ -1706,7 +1709,7 @@ impl PyCField {
             "Z" => {
                 // c_wchar_p: store pointer to null-terminated wchar_t buffer
                 if let Some(s) = value.downcast_ref::<PyStr>() {
-                    let (holder, ptr) = str_to_wchar_bytes(s.as_str(), vm);
+                    let (holder, ptr) = str_to_wchar_bytes(s.as_wtf8(), vm);
                     let mut result = vec![0u8; size];
                     let addr_bytes = ptr.to_ne_bytes();
                     let len = core::cmp::min(addr_bytes.len(), size);
@@ -1759,7 +1762,7 @@ impl PyCField {
             if let Ok(type_code) = element_type.as_object().get_attr("_type_", vm)
                 && let Some(s) = type_code.downcast_ref::<PyStr>()
             {
-                return s.as_str() == "c";
+                return s.as_bytes() == b"c";
             }
         }
         false
@@ -1776,7 +1779,7 @@ impl PyCField {
             if let Ok(type_code) = element_type.as_object().get_attr("_type_", vm)
                 && let Some(s) = type_code.downcast_ref::<PyStr>()
             {
-                return s.as_str() == "u";
+                return s.as_bytes() == b"u";
             }
         }
         false
@@ -2508,9 +2511,6 @@ fn make_fields(
     offset: isize,
     vm: &VirtualMachine,
 ) -> PyResult<()> {
-    use crate::builtins::{PyList, PyTuple};
-    use crate::convert::ToPyObject;
-
     let fields = descr.proto.as_object().get_attr("_fields_", vm)?;
     let fieldlist: Vec<PyObjectRef> = if let Some(list) = fields.downcast_ref::<PyList>() {
         list.borrow_vec().to_vec()
@@ -2532,7 +2532,7 @@ fn make_fields(
         let fname = field_tuple
             .first()
             .expect("len checked")
-            .downcast_ref::<PyStr>()
+            .downcast_ref::<PyUtf8Str>()
             .ok_or_else(|| vm.new_type_error("field name must be a string"))?;
 
         let fdescr_obj = descr
@@ -2555,7 +2555,10 @@ fn make_fields(
         }
 
         let new_descr = super::PyCField::new_from_field(fdescr, index, offset);
-        cls.set_attr(vm.ctx.intern_str(fname.as_str()), new_descr.to_pyobject(vm));
+        cls.set_attr(
+            vm.ctx.intern_str(fname.as_wtf8()),
+            new_descr.to_pyobject(vm),
+        );
     }
 
     Ok(())
@@ -2563,9 +2566,6 @@ fn make_fields(
 
 /// Process _anonymous_ attribute for struct/union
 pub(super) fn make_anon_fields(cls: &Py<PyType>, vm: &VirtualMachine) -> PyResult<()> {
-    use crate::builtins::{PyList, PyTuple};
-    use crate::convert::ToPyObject;
-
     let anon = match cls.as_object().get_attr("_anonymous_", vm) {
         Ok(anon) => anon,
         Err(_) => return Ok(()),
@@ -2586,18 +2586,21 @@ pub(super) fn make_anon_fields(cls: &Py<PyType>, vm: &VirtualMachine) -> PyResul
 
         let descr_obj = cls
             .as_object()
-            .get_attr(vm.ctx.intern_str(fname.as_str()), vm)?;
+            .get_attr(vm.ctx.intern_str(fname.as_wtf8()), vm)?;
 
         let descr = descr_obj.downcast_ref::<super::PyCField>().ok_or_else(|| {
             vm.new_attribute_error(format!(
                 "'{}' is specified in _anonymous_ but not in _fields_",
-                fname.as_str()
+                fname.as_wtf8()
             ))
         })?;
 
         let mut new_descr = super::PyCField::new_from_field(descr, 0, 0);
         new_descr.set_anonymous(true);
-        cls.set_attr(vm.ctx.intern_str(fname.as_str()), new_descr.to_pyobject(vm));
+        cls.set_attr(
+            vm.ctx.intern_str(fname.as_wtf8()),
+            new_descr.to_pyobject(vm),
+        );
 
         make_fields(cls, descr, descr.index, descr.offset, vm)?;
     }
diff --git a/crates/vm/src/stdlib/ctypes/function.rs b/crates/vm/src/stdlib/ctypes/function.rs
index d24102ed635..bf6dcfad53d 100644
--- a/crates/vm/src/stdlib/ctypes/function.rs
+++ b/crates/vm/src/stdlib/ctypes/function.rs
@@ -280,10 +280,10 @@ fn conv_param(value: &PyObject, vm: &VirtualMachine) -> PyResult<Argument> {
 
     // 4. Python str -> wide string pointer (like PyUnicode_AsWideCharString)
     if let Some(s) = value.downcast_ref::<PyStr>() {
-        // Convert to null-terminated UTF-16 (wide string)
+        // Convert to null-terminated UTF-16, preserving lone surrogates
         let wide: Vec<u16> = s
-            .as_str()
-            .encode_utf16()
+            .as_wtf8()
+            .encode_wide()
             .chain(core::iter::once(0))
             .collect();
         let wide_bytes: Vec<u8> = wide.iter().flat_map(|&x| x.to_ne_bytes()).collect();
@@ -448,7 +448,7 @@ impl ReturnType for PyTypeRef {
         // Try to get _type_ attribute first (for ctypes types like c_void_p)
         if let Ok(type_attr) = self.as_object().get_attr(vm.ctx.intern_str("_type_"), vm)
             && let Some(s) = type_attr.downcast_ref::<PyStr>()
-            && let Some(ffi_type) = get_ffi_type(s.as_str())
+            && let Some(ffi_type) = s.to_str().and_then(get_ffi_type)
         {
             return Some(ffi_type);
         }
@@ -598,7 +598,7 @@ fn extract_ptr_from_arg(arg: &PyObject, vm: &VirtualMachine) -> PyResult<usize>
     }
     // PyStr: return internal buffer address
     if let Some(s) = arg.downcast_ref::<PyStr>() {
-        return Ok(s.as_str().as_ptr() as usize);
+        return Ok(s.as_bytes().as_ptr() as usize);
     }
     // PyBytes: return internal buffer address
     if let Some(bytes) = arg.downcast_ref::<PyBytes>() {
@@ -753,7 +753,9 @@ fn cast_check_pointertype(ctype: &PyObject, vm: &VirtualMachine) -> bool {
     if let Ok(type_attr) = ctype.get_attr("_type_", vm)
         && let Some(s) = type_attr.downcast_ref::<PyStr>()
     {
-        let c = s.as_str();
+        let c = s
+            .to_str()
+            .expect("_type_ is validated as ASCII at type creation");
         if c.len() == 1 && "sPzUZXO".contains(c) {
             return true;
         }
@@ -790,7 +792,7 @@ pub(super) fn cast_impl(
         bytes.as_bytes().as_ptr() as usize
     } else if let Some(s) = obj.downcast_ref::<PyStr>() {
         // unicode/str → buffer address (c_void_p_from_param: PyUnicode_Check)
-        s.as_str().as_ptr() as usize
+        s.as_bytes().as_ptr() as usize
     } else if let Some(ptr) = obj.downcast_ref::<PyCPointer>() {
         // Pointer instance → contained pointer value
         ptr.get_ptr_value()
diff --git a/crates/vm/src/stdlib/ctypes/pointer.rs b/crates/vm/src/stdlib/ctypes/pointer.rs
index 6000bb57a37..a127f76ead1 100644
--- a/crates/vm/src/stdlib/ctypes/pointer.rs
+++ b/crates/vm/src/stdlib/ctypes/pointer.rs
@@ -639,7 +639,7 @@ impl PyCPointer {
             } else if type_code.as_deref() == Some("Z")
                 && let Some(s) = value.downcast_ref::<PyStr>()
             {
-                let (holder, ptr_val) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+                let (holder, ptr_val) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
                 unsafe {
                     *(addr as *mut usize) = ptr_val;
                 }
diff --git a/crates/vm/src/stdlib/ctypes/simple.rs b/crates/vm/src/stdlib/ctypes/simple.rs
index 67c07dcb73b..107d603677c 100644
--- a/crates/vm/src/stdlib/ctypes/simple.rs
+++ b/crates/vm/src/stdlib/ctypes/simple.rs
@@ -117,8 +117,8 @@ fn set_primitive(_type_: &str, value: &PyObject, vm: &VirtualMachine) -> PyResul
             }
         }
         "u" => {
-            if let Ok(b) = value.str(vm).map(|v| v.to_string().chars().count() == 1) {
-                if b {
+            if let Some(s) = value.downcast_ref::<PyStr>() {
+                if s.as_wtf8().code_points().count() == 1 {
                     Ok(value.to_owned())
                 } else {
                     Err(vm.new_type_error("one character unicode string expected"))
@@ -331,7 +331,7 @@ impl PyCSimpleType {
             // c_wchar: 1 unicode character
             Some("u") => {
                 if let Some(s) = value.downcast_ref::<PyStr>()
-                    && s.as_str().chars().count() == 1
+                    && s.as_wtf8().code_points().count() == 1
                 {
                     return create_simple_with_value("u", &value);
                 }
@@ -366,7 +366,7 @@ impl PyCSimpleType {
             Some("Z") => {
                 // 1. str → create CArgObject with null-terminated wchar buffer
                 if let Some(s) = value.downcast_ref::<PyStr>() {
-                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
                     return Ok(CArgObject {
                         tag: b'Z',
                         value: FfiArgValue::OwnedPointer(ptr, holder),
@@ -407,7 +407,7 @@ impl PyCSimpleType {
                 }
                 // 3. str → create CArgObject with null-terminated wchar buffer
                 if let Some(s) = value.downcast_ref::<PyStr>() {
-                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+                    let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
                     return Ok(CArgObject {
                         tag: b'Z',
                         value: FfiArgValue::OwnedPointer(ptr, holder),
@@ -775,15 +775,16 @@ fn value_to_bytes_endian(
         }
         "u" => {
             // c_wchar - platform-dependent size (2 on Windows, 4 on Unix)
-            if let Ok(s) = value.str(vm)
-                && let Some(c) = s.as_str().chars().next()
-            {
-                let mut buffer = vec![0u8; WCHAR_SIZE];
-                wchar_to_bytes(c as u32, &mut buffer);
-                if swapped {
-                    buffer.reverse();
+            if let Some(s) = value.downcast_ref::<PyStr>() {
+                let mut cps = s.as_wtf8().code_points();
+                if let (Some(c), None) = (cps.next(), cps.next()) {
+                    let mut buffer = vec![0u8; WCHAR_SIZE];
+                    wchar_to_bytes(c.to_u32(), &mut buffer);
+                    if swapped {
+                        buffer.reverse();
+                    }
+                    return buffer;
                 }
-                return buffer;
             }
             vec![0; WCHAR_SIZE]
         }
@@ -1050,7 +1051,7 @@ impl Constructor for PyCSimple {
             } else if _type_ == "Z"
                 && let Some(s) = v.downcast_ref::<PyStr>()
             {
-                let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+                let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
                 let buffer = ptr.to_ne_bytes().to_vec();
                 let cdata = PyCData::from_bytes(buffer, Some(holder));
                 return PyCSimple(cdata).into_ref_with_type(vm, cls).map(Into::into);
@@ -1282,7 +1283,7 @@ impl PyCSimple {
         } else if type_code == "Z"
             && let Some(s) = value.downcast_ref::<PyStr>()
         {
-            let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_str(), vm);
+            let (holder, ptr) = super::base::str_to_wchar_bytes(s.as_wtf8(), vm);
             *zelf.0.buffer.write() = alloc::borrow::Cow::Owned(ptr.to_ne_bytes().to_vec());
             *zelf.0.objects.write() = Some(holder);
             return Ok(());
diff --git a/crates/vm/src/stdlib/ctypes/structure.rs b/crates/vm/src/stdlib/ctypes/structure.rs
index c0116d9d76c..69d267f287e 100644
--- a/crates/vm/src/stdlib/ctypes/structure.rs
+++ b/crates/vm/src/stdlib/ctypes/structure.rs
@@ -1,5 +1,5 @@
 use super::base::{CDATA_BUFFER_METHODS, PyCData, PyCField, StgInfo, StgInfoFlags};
-use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef};
+use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef, PyUtf8Str};
 use crate::convert::ToPyObject;
 use crate::function::{FuncArgs, OptionalArg, PySetterValue};
 use crate::protocol::{BufferDescriptor, PyBuffer, PyNumberMethods};
@@ -318,9 +318,10 @@ impl PyCStructType {
             let name = field_tuple
                 .first()
                 .expect("len checked")
-                .downcast_ref::<PyStr>()
+                .downcast_ref::<PyUtf8Str>()
                 .ok_or_else(|| vm.new_type_error("field name must be a string"))?
-                .to_string();
+                .as_str()
+                .to_owned();
 
             let field_type = field_tuple.get(1).expect("len checked").clone();
 
@@ -604,7 +605,7 @@ impl SetAttr for PyCStructType {
         vm: &VirtualMachine,
     ) -> PyResult<()> {
         // Check if _fields_ is being set
-        if attr_name.as_str() == "_fields_" {
+        if attr_name.as_bytes() == b"_fields_" {
             let pytype: &Py<PyType> = zelf.to_base();
 
             // Check finalization in separate scope to release read lock before process_fields
@@ -634,7 +635,7 @@ impl SetAttr for PyCStructType {
             return Ok(());
         }
         // Delegate to PyType's setattro logic for type attributes
-        let attr_name_interned = vm.ctx.intern_str(attr_name.as_str());
+        let attr_name_interned = vm.ctx.intern_str(attr_name.as_wtf8());
         let pytype: &Py<PyType> = zelf.to_base();
 
         // Check for data descriptor first
@@ -654,7 +655,7 @@ impl SetAttr for PyCStructType {
                 return Err(vm.new_attribute_error(format!(
                     "type object '{}' has no attribute '{}'",
                     pytype.name(),
-                    attr_name.as_str(),
+                    attr_name.as_wtf8(),
                 )));
             }
         }
@@ -747,7 +748,7 @@ impl PyCStructure {
                 }
                 if let Some(tuple) = field.downcast_ref::<PyTuple>()
                     && let Some(name) = tuple.first()
-                    && let Some(name_str) = name.downcast_ref::<PyStr>()
+                    && let Some(name_str) = name.downcast_ref::<PyUtf8Str>()
                 {
                     let field_name = name_str.as_str().to_owned();
                     // Check for duplicate in kwargs
diff --git a/crates/vm/src/stdlib/ctypes/union.rs b/crates/vm/src/stdlib/ctypes/union.rs
index fba9a75e955..7526fa92eff 100644
--- a/crates/vm/src/stdlib/ctypes/union.rs
+++ b/crates/vm/src/stdlib/ctypes/union.rs
@@ -1,6 +1,6 @@
 use super::base::{CDATA_BUFFER_METHODS, StgInfoFlags};
 use super::{PyCData, PyCField, StgInfo};
-use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef};
+use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef, PyUtf8Str};
 use crate::convert::ToPyObject;
 use crate::function::{ArgBytesLike, FuncArgs, OptionalArg, PySetterValue};
 use crate::protocol::{BufferDescriptor, PyBuffer};
@@ -217,9 +217,10 @@ impl PyCUnionType {
             let name = field_tuple
                 .first()
                 .expect("len checked")
-                .downcast_ref::<PyStr>()
+                .downcast_ref::<PyUtf8Str>()
                 .ok_or_else(|| vm.new_type_error("field name must be a string"))?
-                .to_string();
+                .as_str()
+                .to_owned();
 
             let field_type = field_tuple.get(1).expect("len checked").clone();
 
@@ -486,7 +487,7 @@ impl SetAttr for PyCUnionType {
         vm: &VirtualMachine,
     ) -> PyResult<()> {
         let pytype: &Py<PyType> = zelf.to_base();
-        let attr_name_interned = vm.ctx.intern_str(attr_name.as_str());
+        let attr_name_interned = vm.ctx.intern_str(attr_name.as_wtf8());
 
         // 1. First, do PyType's setattro (PyType_Type.tp_setattro first)
         // Check for data descriptor first
@@ -495,7 +496,7 @@ impl SetAttr for PyCUnionType {
             if let Some(descriptor) = descr_set {
                 descriptor(&attr, pytype.to_owned().into(), value.clone(), vm)?;
                 // After successful setattro, check if _fields_ and call process_fields
-                if attr_name.as_str() == "_fields_"
+                if attr_name.as_bytes() == b"_fields_"
                     && let PySetterValue::Assign(fields_value) = value
                 {
                     PyCUnionType::process_fields(pytype, fields_value, vm)?;
@@ -506,7 +507,7 @@ impl SetAttr for PyCUnionType {
 
         // 2. If _fields_, call process_fields (which checks FINAL internally)
         // Check BEFORE writing to dict to avoid storing _fields_ when FINAL
-        if attr_name.as_str() == "_fields_"
+        if attr_name.as_bytes() == b"_fields_"
             && let PySetterValue::Assign(ref fields_value) = value
         {
             PyCUnionType::process_fields(pytype, fields_value.clone(), vm)?;
@@ -526,7 +527,7 @@ impl SetAttr for PyCUnionType {
                     return Err(vm.new_attribute_error(format!(
                         "type object '{}' has no attribute '{}'",
                         pytype.name(),
-                        attr_name.as_str(),
+                        attr_name.as_wtf8(),
                     )));
                 }
             }
@@ -618,7 +619,7 @@ impl PyCUnion {
                 }
                 if let Some(tuple) = field.downcast_ref::<PyTuple>()
                     && let Some(name) = tuple.first()
-                    && let Some(name_str) = name.downcast_ref::<PyStr>()
+                    && let Some(name_str) = name.downcast_ref::<PyUtf8Str>()
                 {
                     let field_name = name_str.as_str().to_owned();
                     // Check for duplicate in kwargs
diff --git a/crates/vm/src/stdlib/functools.rs b/crates/vm/src/stdlib/functools.rs
index 6c5c8f2e4c5..2c3f70ab52a 100644
--- a/crates/vm/src/stdlib/functools.rs
+++ b/crates/vm/src/stdlib/functools.rs
@@ -14,6 +14,7 @@ mod _functools {
         types::{Callable, Constructor, GetDescriptor, Representable},
     };
     use indexmap::IndexMap;
+    use rustpython_common::wtf8::Wtf8Buf;
 
     #[derive(FromArgs)]
     struct ReduceArgs {
@@ -44,7 +45,7 @@ mod _functools {
                 let exc_type = vm.ctx.exceptions.type_error.to_owned();
                 vm.new_exception_msg(
                     exc_type,
-                    "reduce() of empty sequence with no initial value".to_owned(),
+                    "reduce() of empty sequence with no initial value".into(),
                 )
             })?
         };
@@ -424,9 +425,9 @@ mod _functools {
             // Add keywords from self.keywords
             for (key, value) in &*keywords {
                 let key_str = key
-                    .downcast::<crate::builtins::PyStr>()
-                    .map_err(|_| vm.new_type_error("keywords must be strings"))?;
-                final_kwargs.insert(key_str.as_str().to_owned(), value);
+                    .downcast_ref::<crate::builtins::PyStr>()
+                    .ok_or_else(|| vm.new_type_error("keywords must be strings"))?;
+                final_kwargs.insert(key_str.expect_str().to_owned(), value);
             }
 
             // Add keywords from args.kwargs (these override self.keywords)
@@ -455,7 +456,7 @@ mod _functools {
 
     impl Representable for PyPartial {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             // Check for recursive repr
             let obj = zelf.as_object();
             if let Some(_guard) = ReprGuard::enter(vm, obj) {
@@ -469,52 +470,46 @@ mod _functools {
                     )
                 };
 
-                let func_repr = func.repr(vm)?;
-                let mut parts = vec![func_repr.as_str().to_owned()];
+                let qualname = zelf.class().__qualname__(vm);
+                let qualname_wtf8 = qualname
+                    .downcast_ref::<crate::builtins::PyStr>()
+                    .map(|s| s.as_wtf8().to_owned())
+                    .unwrap_or_else(|| Wtf8Buf::from(zelf.class().name().to_owned()));
+                let module = zelf.class().__module__(vm);
+
+                let mut result = Wtf8Buf::new();
+                if let Ok(module_str) = module.downcast::<crate::builtins::PyStr>() {
+                    let module_name = module_str.as_wtf8();
+                    if module_name != "builtins" && !module_name.is_empty() {
+                        result.push_wtf8(module_name);
+                        result.push_char('.');
+                    }
+                }
+                result.push_wtf8(&qualname_wtf8);
+                result.push_char('(');
+                result.push_wtf8(func.repr(vm)?.as_wtf8());
 
                 for arg in args.as_slice() {
-                    parts.push(arg.repr(vm)?.as_str().to_owned());
+                    result.push_str(", ");
+                    result.push_wtf8(arg.repr(vm)?.as_wtf8());
                 }
 
                 for (key, value) in &*keywords {
-                    // For string keys, use them directly without quotes
-                    let key_part = if let Ok(s) = key.clone().downcast::<crate::builtins::PyStr>() {
-                        s.as_str().to_owned()
+                    result.push_str(", ");
+                    let key_str = if let Ok(s) = key.clone().downcast::<crate::builtins::PyStr>() {
+                        s
                     } else {
-                        // For non-string keys, convert to string using __str__
-                        key.str(vm)?.as_str().to_owned()
+                        key.str(vm)?
                     };
-                    let value_str = value.repr(vm)?;
-                    parts.push(format!(
-                        "{key_part}={value_str}",
-                        value_str = value_str.as_str()
-                    ));
+                    result.push_wtf8(key_str.as_wtf8());
+                    result.push_char('=');
+                    result.push_wtf8(value.repr(vm)?.as_wtf8());
                 }
 
-                let qualname = zelf.class().__qualname__(vm);
-                let qualname_str = qualname
-                    .downcast::<crate::builtins::PyStr>()
-                    .map(|s| s.as_str().to_owned())
-                    .unwrap_or_else(|_| zelf.class().name().to_owned());
-                let module = zelf.class().__module__(vm);
-
-                let qualified_name = match module.downcast::<crate::builtins::PyStr>() {
-                    Ok(module_str) => {
-                        let module_name = module_str.as_str();
-                        match module_name {
-                            "builtins" | "" => qualname_str,
-                            _ => format!("{module_name}.{qualname_str}"),
-                        }
-                    }
-                    Err(_) => qualname_str,
-                };
-
-                Ok(format!(
-                    "{qualified_name}({parts})",
-                    parts = parts.join(", ")
-                ))
+                result.push_char(')');
+                Ok(result)
             } else {
-                Ok("...".to_owned())
+                Ok(Wtf8Buf::from("..."))
             }
         }
     }
diff --git a/crates/vm/src/stdlib/imp.rs b/crates/vm/src/stdlib/imp.rs
index cf9aba02265..fefcd383f58 100644
--- a/crates/vm/src/stdlib/imp.rs
+++ b/crates/vm/src/stdlib/imp.rs
@@ -71,7 +71,7 @@ impl FrozenError {
             Excluded => format!("Excluded frozen object named {mod_name}"),
             Invalid => format!("Frozen object named {mod_name} is invalid"),
         };
-        vm.new_import_error(msg, vm.ctx.new_str(mod_name))
+        vm.new_import_error(msg, vm.ctx.new_utf8_str(mod_name))
     }
 }
 
@@ -106,7 +106,7 @@ fn find_frozen(name: &str, vm: &VirtualMachine) -> Result<FrozenModule, FrozenEr
 mod _imp {
     use crate::{
         PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyBytesRef, PyCode, PyMemoryView, PyModule, PyStrRef},
+        builtins::{PyBytesRef, PyCode, PyMemoryView, PyModule, PyStrRef, PyUtf8StrRef},
         convert::TryFromBorrowedObject,
         function::OptionalArg,
         import, version,
@@ -127,27 +127,27 @@ mod _imp {
     }
 
     #[pyfunction]
-    fn is_builtin(name: PyStrRef, vm: &VirtualMachine) -> bool {
+    fn is_builtin(name: PyUtf8StrRef, vm: &VirtualMachine) -> bool {
         vm.state.module_defs.contains_key(name.as_str())
     }
 
     #[pyfunction]
-    fn is_frozen(name: PyStrRef, vm: &VirtualMachine) -> bool {
+    fn is_frozen(name: PyUtf8StrRef, vm: &VirtualMachine) -> bool {
         super::find_frozen(name.as_str(), vm).is_ok()
     }
 
     #[pyfunction]
     fn create_builtin(spec: PyObjectRef, vm: &VirtualMachine) -> PyResult {
         let sys_modules = vm.sys_module.get_attr("modules", vm).unwrap();
-        let name: PyStrRef = spec.get_attr("name", vm)?.try_into_value(vm)?;
+        let name: PyUtf8StrRef = spec.get_attr("name", vm)?.try_into_value(vm)?;
 
         // Check sys.modules first
         if let Ok(module) = sys_modules.get_item(&*name, vm) {
             return Ok(module);
         }
 
-        // Try multi-phase init modules first (they need special handling)
-        if let Some(&def) = vm.state.module_defs.get(name.as_str()) {
+        let name_str = name.as_str();
+        if let Some(&def) = vm.state.module_defs.get(name_str) {
             // Phase 1: Create module (use create slot if provided, else default creation)
             let module = if let Some(create) = def.slots.create {
                 // Custom module creation
@@ -163,7 +163,7 @@ mod _imp {
             module.__init_methods(vm)?;
 
             // Add to sys.modules BEFORE exec (critical for circular import handling)
-            sys_modules.set_item(&*name, module.clone().into(), vm)?;
+            sys_modules.set_item(name.as_pystr(), module.clone().into(), vm)?;
 
             // Phase 2: Call exec slot (can safely import other modules now)
             if let Some(exec) = def.slots.exec {
@@ -184,7 +184,7 @@ mod _imp {
 
     #[pyfunction]
     fn get_frozen_object(
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         data: OptionalArg<PyObjectRef>,
         vm: &VirtualMachine,
     ) -> PyResult<PyRef<PyCode>> {
@@ -198,7 +198,7 @@ mod _imp {
             let invalid_err = || {
                 vm.new_import_error(
                     format!("Frozen object named '{}' is invalid", name.as_str()),
-                    name.clone(),
+                    name.clone().into_wtf8(),
                 )
             };
             let bag = crate::builtins::code::PyObjBag(&vm.ctx);
@@ -211,15 +211,16 @@ mod _imp {
     }
 
     #[pyfunction]
-    fn init_frozen(name: PyStrRef, vm: &VirtualMachine) -> PyResult {
+    fn init_frozen(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult {
         import::import_frozen(vm, name.as_str())
     }
 
     #[pyfunction]
-    fn is_frozen_package(name: PyStrRef, vm: &VirtualMachine) -> PyResult<bool> {
-        super::find_frozen(name.as_str(), vm)
+    fn is_frozen_package(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<bool> {
+        let name_str = name.as_str();
+        super::find_frozen(name_str, vm)
             .map(|frozen| frozen.package)
-            .map_err(|e| e.to_pyexception(name.as_str(), vm))
+            .map_err(|e| e.to_pyexception(name_str, vm))
     }
 
     #[pyfunction]
@@ -230,7 +231,7 @@ mod _imp {
     #[pyfunction]
     fn _fix_co_filename(code: PyRef<PyCode>, path: PyStrRef, vm: &VirtualMachine) {
         let old_name = code.source_path();
-        let new_name = vm.ctx.intern_str(path.as_str());
+        let new_name = vm.ctx.intern_str(path.as_wtf8());
         super::update_code_filenames(&code, old_name, new_name);
     }
 
@@ -240,7 +241,7 @@ mod _imp {
             .state
             .frozen
             .keys()
-            .map(|&name| vm.ctx.new_str(name).into())
+            .map(|&name| vm.ctx.new_utf8_str(name).into())
             .collect();
         Ok(names)
     }
@@ -248,7 +249,7 @@ mod _imp {
     #[allow(clippy::type_complexity)]
     #[pyfunction]
     fn find_frozen(
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         withdata: OptionalArg<bool>,
         vm: &VirtualMachine,
     ) -> PyResult<Option<(Option<PyRef<PyMemoryView>>, bool, Option<PyStrRef>)>> {
@@ -259,19 +260,20 @@ mod _imp {
             unimplemented!();
         }
 
-        let info = match super::find_frozen(name.as_str(), vm) {
+        let name_str = name.as_str();
+        let info = match super::find_frozen(name_str, vm) {
             Ok(info) => info,
             Err(NotFound | Disabled | BadName) => return Ok(None),
-            Err(e) => return Err(e.to_pyexception(name.as_str(), vm)),
+            Err(e) => return Err(e.to_pyexception(name_str, vm)),
         };
 
         // When origname is empty (e.g. __hello_only__), return None.
         // Otherwise return the resolved alias name.
-        let origname_str = super::resolve_frozen_alias(name.as_str());
+        let origname_str = super::resolve_frozen_alias(name_str);
         let origname = if origname_str.is_empty() {
             None
         } else {
-            Some(vm.ctx.new_str(origname_str))
+            Some(vm.ctx.new_utf8_str(origname_str).into())
         };
         Ok(Some((None, info.package, origname)))
     }
diff --git a/crates/vm/src/stdlib/io.rs b/crates/vm/src/stdlib/io.rs
index 64c02ee6ae4..93fe6339034 100644
--- a/crates/vm/src/stdlib/io.rs
+++ b/crates/vm/src/stdlib/io.rs
@@ -1575,10 +1575,11 @@ mod _io {
                 args.bind(vm).map_err(|e| {
                     let str_repr = e
                         .__str__(vm)
-                        .map(|s| s.as_str().to_owned())
-                        .unwrap_or_else(|_| "<error getting exception str>".to_owned());
+                        .as_ref()
+                        .map_or("<error getting exception str>".as_ref(), |s| s.as_wtf8())
+                        .to_owned();
                     let msg = format!("{}() {}", Self::CLASS_NAME, str_repr);
-                    vm.new_exception_msg(e.class().to_owned(), msg)
+                    vm.new_exception_msg(e.class().to_owned(), msg.into())
                 })?;
             zelf.init(raw, BufferSize { buffer_size }, vm)
         }
@@ -2273,7 +2274,7 @@ mod _io {
         #[pyarg(any, default)]
         encoding: Option<PyUtf8StrRef>,
         #[pyarg(any, default)]
-        errors: Option<PyStrRef>,
+        errors: Option<PyUtf8StrRef>,
         #[pyarg(any, default)]
         newline: OptionalOption<Newlines>,
         #[pyarg(any, default)]
@@ -2457,7 +2458,7 @@ mod _io {
         encoder: Option<(PyObjectRef, Option<EncodeFunc>)>,
         decoder: Option<PyObjectRef>,
         encoding: PyUtf8StrRef,
-        errors: PyStrRef,
+        errors: PyUtf8StrRef,
         newline: Newlines,
         line_buffering: bool,
         write_through: bool,
@@ -2727,17 +2728,14 @@ mod _io {
 
             let encoding = Self::resolve_encoding(args.encoding, vm)?;
 
-            let errors = args
-                .errors
-                .unwrap_or_else(|| identifier!(vm, strict).to_owned());
+            let errors = args.errors.unwrap_or_else(|| vm.ctx.new_utf8_str("strict"));
             Self::validate_errors(&errors, vm)?;
 
             let has_read1 = vm.get_attribute_opt(buffer.clone(), "read1")?.is_some();
             let seekable = vm.call_method(&buffer, "seekable", ())?.try_to_bool(vm)?;
 
             let newline = match args.newline {
-                OptionalArg::Missing => Newlines::default(),
-                OptionalArg::Present(None) => Newlines::default(),
+                OptionalArg::Missing | OptionalArg::Present(None) => Newlines::default(),
                 OptionalArg::Present(Some(newline)) => newline,
             };
             let (encoder, decoder) =
@@ -2808,15 +2806,10 @@ mod _io {
                 .map_err(|_| vm.new_value_error("I/O operation on uninitialized object"))
         }
 
-        fn validate_errors(errors: &PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
-            if errors.as_wtf8().as_bytes().contains(&0) {
+        fn validate_errors(errors: &PyRef<PyUtf8Str>, vm: &VirtualMachine) -> PyResult<()> {
+            if errors.as_str().contains('\0') {
                 return Err(cstring_error(vm));
             }
-            if !errors.as_wtf8().is_utf8() {
-                return Err(vm.new_unicode_encode_error(
-                    "'utf-8' codec can't encode character: surrogates not allowed".to_owned(),
-                ));
-            }
             vm.state
                 .codec_registry
                 .lookup_error(errors.as_str(), vm)
@@ -2910,7 +2903,7 @@ mod _io {
         fn find_coder(
             buffer: &PyObject,
             encoding: &str,
-            errors: &Py<PyStr>,
+            errors: &Py<PyUtf8Str>,
             newline: Newlines,
             vm: &VirtualMachine,
         ) -> PyResult<(
@@ -2923,10 +2916,11 @@ mod _io {
                     "'{encoding}' is not a text encoding; use codecs.open() to handle arbitrary codecs"
                 )));
             }
+            let errors = errors.to_owned().into_wtf8();
 
             let encoder = if vm.call_method(buffer, "writable", ())?.try_to_bool(vm)? {
                 let incremental_encoder =
-                    match codec.get_incremental_encoder(Some(errors.to_owned()), vm) {
+                    match codec.get_incremental_encoder(Some(errors.clone()), vm) {
                         Ok(encoder) => encoder,
                         Err(err)
                             if err.fast_isinstance(vm.ctx.exceptions.type_error)
@@ -2937,7 +2931,7 @@ mod _io {
                                 .and_then(|obj| obj.downcast::<PyStr>().ok());
                             StatelessIncrementalEncoder {
                                 encode: codec.get_encode_func().to_owned(),
-                                errors: Some(errors.to_owned()),
+                                errors: Some(errors.clone()),
                                 name,
                             }
                             .into_ref(&vm.ctx)
@@ -2948,7 +2942,7 @@ mod _io {
                 let encoding_name = vm.get_attribute_opt(incremental_encoder.clone(), "name")?;
                 let encode_func = encoding_name.and_then(|name| {
                     let name = name.downcast_ref::<PyStr>()?;
-                    match name.as_str() {
+                    match name.to_str()? {
                         "utf-8" => Some(textio_encode_utf8 as EncodeFunc),
                         _ => None,
                     }
@@ -2959,7 +2953,7 @@ mod _io {
             };
 
             let decoder = if vm.call_method(buffer, "readable", ())?.try_to_bool(vm)? {
-                let decoder = match codec.get_incremental_decoder(Some(errors.to_owned()), vm) {
+                let decoder = match codec.get_incremental_decoder(Some(errors.clone()), vm) {
                     Ok(decoder) => decoder,
                     Err(err)
                         if err.fast_isinstance(vm.ctx.exceptions.type_error)
@@ -2967,7 +2961,7 @@ mod _io {
                     {
                         StatelessIncrementalDecoder {
                             decode: codec.get_decode_func().to_owned(),
-                            errors: Some(errors.to_owned()),
+                            errors: Some(errors),
                         }
                         .into_ref(&vm.ctx)
                         .into()
@@ -3040,7 +3034,7 @@ mod _io {
                 errors_changed = errs.as_str() != errors.as_str();
                 errors = errs;
             } else if encoding_changed {
-                errors = identifier!(vm, strict).to_owned();
+                errors = identifier_utf8!(vm, strict).to_owned();
                 errors_changed = true;
             }
 
@@ -3433,7 +3427,7 @@ mod _io {
         }
 
         #[pygetset]
-        fn errors(&self, vm: &VirtualMachine) -> PyResult<PyStrRef> {
+        fn errors(&self, vm: &VirtualMachine) -> PyResult<PyUtf8StrRef> {
             Ok(self.lock(vm)?.errors.clone())
         }
 
@@ -3995,7 +3989,7 @@ mod _io {
 
     impl Representable for TextIOWrapper {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyRef<PyStr>> {
             let type_name = zelf.class().slot_name();
             let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) else {
                 return Err(
@@ -4004,19 +3998,19 @@ mod _io {
             };
             let Some(data) = zelf.data.lock() else {
                 // Reentrant call
-                return Ok(format!("<{type_name}>"));
+                return Ok(vm.ctx.new_str(Wtf8Buf::from(format!("<{type_name}>"))));
             };
             let Some(data) = data.as_ref() else {
                 return Err(vm.new_value_error("I/O operation on uninitialized object".to_owned()));
             };
 
-            let mut result = format!("<{type_name}");
+            let mut result = Wtf8Buf::from(format!("<{type_name}"));
 
             // Add name if present
             if let Ok(Some(name)) = vm.get_attribute_opt(data.buffer.clone(), "name") {
                 let name_repr = name.repr(vm)?;
-                result.push_str(" name=");
-                result.push_str(name_repr.as_str());
+                result.push_wtf8(" name=".as_ref());
+                result.push_wtf8(name_repr.as_wtf8());
             }
 
             // Add mode if present (prefer the wrapper's attribute)
@@ -4029,17 +4023,20 @@ mod _io {
             };
             if let Some(mode) = mode_obj {
                 let mode_repr = mode.repr(vm)?;
-                result.push_str(" mode=");
-                result.push_str(mode_repr.as_str());
+                result.push_wtf8(" mode=".as_ref());
+                result.push_wtf8(mode_repr.as_wtf8());
             }
 
-            // Add encoding
-            result.push_str(" encoding='");
-            result.push_str(data.encoding.as_str());
-            result.push('\'');
+            // Add encoding (always valid UTF-8)
+            result.push_wtf8(" encoding='".as_ref());
+            result.push_wtf8(data.encoding.as_str().as_ref());
+            result.push_wtf8("'>".as_ref());
 
-            result.push('>');
-            Ok(result)
+            Ok(vm.ctx.new_str(result))
+        }
+
+        fn repr_str(_zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
+            unreachable!("repr() is overridden directly")
         }
     }
 
@@ -4240,7 +4237,7 @@ mod _io {
                 output.to_mut().insert(0, '\r'.into());
                 self.pendingcr = false;
             }
-            if !final_ && let Some(s) = output.strip_suffix("\r".as_ref()) {
+            if !final_ && let Some(s) = output.strip_suffix("\r") {
                 output = Cow::Owned(s.to_owned());
                 self.pendingcr = true;
             }
@@ -4935,9 +4932,9 @@ mod _io {
         #[pyarg(any, default)]
         pub encoding: Option<PyUtf8StrRef>,
         #[pyarg(any, default)]
-        pub errors: Option<PyStrRef>,
+        pub errors: Option<PyUtf8StrRef>,
         #[pyarg(any, default)]
-        pub newline: Option<PyStrRef>,
+        pub newline: Option<PyUtf8StrRef>,
         #[pyarg(any, default = true)]
         pub closefd: bool,
         #[pyarg(any, default)]
@@ -5237,7 +5234,7 @@ mod fileio {
         AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject,
         VirtualMachine,
         builtins::{PyBaseExceptionRef, PyUtf8Str, PyUtf8StrRef},
-        common::crt_fd,
+        common::{crt_fd, wtf8::Wtf8Buf},
         convert::{IntoPyException, ToPyException},
         exceptions::OSErrorBuilder,
         function::{ArgBytesLike, ArgMemoryBuffer, OptionalArg, OptionalOption},
@@ -5864,8 +5861,8 @@ mod fileio {
             if zelf.fd.load() >= 0 && zelf.closefd.load() {
                 let repr = source
                     .repr(vm)
-                    .map(|s| s.as_str().to_owned())
-                    .unwrap_or_else(|_| "<file>".to_owned());
+                    .map(|s| s.as_wtf8().to_owned())
+                    .unwrap_or_else(|_| Wtf8Buf::from("<file>"));
                 if let Err(e) = crate::stdlib::warnings::warn(
                     vm.ctx.exceptions.resource_warning,
                     format!("unclosed file {repr}"),
@@ -5902,7 +5899,7 @@ mod winconsoleio {
     use crate::{
         AsObject, Py, PyObject, PyObjectRef, PyRef, PyResult, TryFromObject, VirtualMachine,
         builtins::{PyBaseExceptionRef, PyUtf8StrRef},
-        common::lock::PyMutex,
+        common::{lock::PyMutex, wtf8::Wtf8Buf},
         convert::{IntoPyException, ToPyException},
         function::{ArgBytesLike, ArgMemoryBuffer, OptionalArg},
         types::{Constructor, DefaultConstructor, Destructor, Initializer, Representable},
@@ -6187,8 +6184,8 @@ mod winconsoleio {
 
                 let name_str = nameobj.str(vm)?;
                 let wide: Vec<u16> = name_str
-                    .as_str()
-                    .encode_utf16()
+                    .as_wtf8()
+                    .encode_wide()
                     .chain(core::iter::once(0))
                     .collect();
 
@@ -6417,8 +6414,8 @@ mod winconsoleio {
             if zelf.fd.load() >= 0 && zelf.closefd.load() {
                 let repr = source
                     .repr(vm)
-                    .map(|s| s.as_str().to_owned())
-                    .unwrap_or_else(|_| "<file>".to_owned());
+                    .map(|s| s.as_wtf8().to_owned())
+                    .unwrap_or_else(|_| Wtf8Buf::from("<file>"));
                 if let Err(e) = crate::stdlib::warnings::warn(
                     vm.ctx.exceptions.resource_warning,
                     format!("unclosed file {repr}"),
diff --git a/crates/vm/src/stdlib/itertools.rs b/crates/vm/src/stdlib/itertools.rs
index d1c188b8892..2140f44e56e 100644
--- a/crates/vm/src/stdlib/itertools.rs
+++ b/crates/vm/src/stdlib/itertools.rs
@@ -20,6 +20,7 @@ mod decl {
     use crossbeam_utils::atomic::AtomicCell;
     use malachite_bigint::BigInt;
     use num_traits::One;
+    use rustpython_common::wtf8::Wtf8Buf;
 
     use alloc::fmt;
     use num_traits::{Signed, ToPrimitive};
@@ -220,13 +221,17 @@ mod decl {
 
     impl Representable for PyItertoolsCount {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let cur = format!("{}", zelf.cur.read().clone().repr(vm)?);
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            let cur_repr = zelf.cur.read().clone().repr(vm)?;
             let step = &zelf.step;
-            if vm.bool_eq(step, vm.ctx.new_int(1).as_object())? {
-                return Ok(format!("count({cur})"));
+            let mut result = Wtf8Buf::from("count(");
+            result.push_wtf8(cur_repr.as_wtf8());
+            if !vm.bool_eq(step, vm.ctx.new_int(1).as_object())? {
+                result.push_str(", ");
+                result.push_wtf8(step.repr(vm)?.as_wtf8());
             }
-            Ok(format!("count({}, {})", cur, step.repr(vm)?))
+            result.push_char(')');
+            Ok(result)
         }
     }
 
@@ -345,13 +350,15 @@ mod decl {
 
     impl Representable for PyItertoolsRepeat {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let mut fmt = format!("{}", &zelf.object.repr(vm)?);
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            let mut result = Wtf8Buf::from("repeat(");
+            result.push_wtf8(zelf.object.repr(vm)?.as_wtf8());
             if let Some(ref times) = zelf.times {
-                fmt.push_str(", ");
-                fmt.push_str(&times.read().to_string());
+                result.push_str(", ");
+                result.push_str(&times.read().to_string());
             }
-            Ok(format!("repeat({fmt})"))
+            result.push_char(')');
+            Ok(result)
         }
     }
 
diff --git a/crates/vm/src/stdlib/marshal.rs b/crates/vm/src/stdlib/marshal.rs
index cf7abe65194..412d71f49e2 100644
--- a/crates/vm/src/stdlib/marshal.rs
+++ b/crates/vm/src/stdlib/marshal.rs
@@ -226,7 +226,7 @@ mod decl {
         marshal::deserialize_value(&mut &buf[..], PyMarshalBag(vm)).map_err(|e| match e {
             marshal::MarshalError::Eof => vm.new_exception_msg(
                 vm.ctx.exceptions.eof_error.to_owned(),
-                "marshal data too short".to_owned(),
+                "marshal data too short".into(),
             ),
             marshal::MarshalError::InvalidBytecode => {
                 vm.new_value_error("Couldn't deserialize python bytecode")
diff --git a/crates/vm/src/stdlib/msvcrt.rs b/crates/vm/src/stdlib/msvcrt.rs
index cf194a3f7ba..93364ea3596 100644
--- a/crates/vm/src/stdlib/msvcrt.rs
+++ b/crates/vm/src/stdlib/msvcrt.rs
@@ -82,7 +82,7 @@ mod msvcrt {
     #[pyfunction]
     fn putwch(s: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
         let c = s
-            .as_str()
+            .expect_str()
             .chars()
             .exactly_one()
             .map_err(|_| vm.new_type_error("putch() argument must be a string of length 1"))?;
@@ -107,7 +107,7 @@ mod msvcrt {
     #[pyfunction]
     fn ungetwch(s: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
         let c =
-            s.as_str().chars().exactly_one().map_err(|_| {
+            s.expect_str().chars().exactly_one().map_err(|_| {
                 vm.new_type_error("ungetwch() argument must be a string of length 1")
             })?;
         let ret = unsafe { suppress_iph!(_ungetwch(c as u32)) };
diff --git a/crates/vm/src/stdlib/nt.rs b/crates/vm/src/stdlib/nt.rs
index 5b2cf3b92f5..f9fad0a2033 100644
--- a/crates/vm/src/stdlib/nt.rs
+++ b/crates/vm/src/stdlib/nt.rs
@@ -7,7 +7,9 @@ pub use module::raw_set_handle_inheritable;
 pub(crate) mod module {
     use crate::{
         Py, PyResult, TryFromObject, VirtualMachine,
-        builtins::{PyBaseExceptionRef, PyDictRef, PyListRef, PyStrRef, PyTupleRef},
+        builtins::{
+            PyBaseExceptionRef, PyBytes, PyDictRef, PyListRef, PyStr, PyStrRef, PyTupleRef,
+        },
         common::{crt_fd, suppress_iph, windows::ToWideString},
         convert::ToPyException,
         exceptions::OSErrorBuilder,
@@ -15,9 +17,9 @@ pub(crate) mod module {
         ospath::{OsPath, OsPathOrFd},
         stdlib::os::{_os, DirFd, SupportFunc, TargetIsDirectory},
     };
-
     use core::mem::MaybeUninit;
     use libc::intptr_t;
+    use rustpython_common::wtf8::Wtf8Buf;
     use std::os::windows::io::AsRawHandle;
     use std::{env, io, os::windows::ffi::OsStringExt};
     use windows_sys::Win32::{
@@ -1188,7 +1190,7 @@ pub(crate) mod module {
 
         let argv = vm.extract_elements_with(argv.as_ref(), |obj| {
             let arg = PyStrRef::try_from_object(vm, obj)?;
-            make_widestring(arg.as_str())
+            make_widestring(arg.expect_str())
         })?;
 
         let first = argv
@@ -1229,7 +1231,7 @@ pub(crate) mod module {
 
         let argv = vm.extract_elements_with(argv.as_ref(), |obj| {
             let arg = PyStrRef::try_from_object(vm, obj)?;
-            make_widestring(arg.as_str())
+            make_widestring(arg.expect_str())
         })?;
 
         let first = argv
@@ -1252,8 +1254,8 @@ pub(crate) mod module {
         for (key, value) in env.into_iter() {
             let key = PyStrRef::try_from_object(vm, key)?;
             let value = PyStrRef::try_from_object(vm, value)?;
-            let key_str = key.as_str();
-            let value_str = value.as_str();
+            let key_str = key.expect_str();
+            let value_str = value.expect_str();
 
             // Validate: no null characters in key or value
             if key_str.contains('\0') || value_str.contains('\0') {
@@ -1463,9 +1465,6 @@ pub(crate) mod module {
 
     #[pyfunction]
     fn _path_splitroot_ex(path: crate::PyObjectRef, vm: &VirtualMachine) -> PyResult<PyTupleRef> {
-        use crate::builtins::{PyBytes, PyStr};
-        use rustpython_common::wtf8::Wtf8Buf;
-
         // Handle path-like objects via os.fspath, but without null check (non_strict=True)
         let path = if let Some(fspath) = vm.get_method(path.clone(), identifier!(vm, __fspath__)) {
             fspath?.call((), vm)?
@@ -1487,7 +1486,8 @@ pub(crate) mod module {
                         "'utf-8' codec can't decode byte {:#x} in position {}: invalid start byte",
                         b.as_bytes().get(e.valid_up_to()).copied().unwrap_or(0),
                         e.valid_up_to()
-                    ),
+                    )
+                    .into(),
                 )
             })?;
             let wide: Vec<u16> = s.encode_utf16().collect();
@@ -1535,15 +1535,7 @@ pub(crate) mod module {
     }
 
     #[pyfunction]
-    fn _path_splitroot(
-        path: OsPath,
-        _vm: &VirtualMachine,
-    ) -> (
-        rustpython_common::wtf8::Wtf8Buf,
-        rustpython_common::wtf8::Wtf8Buf,
-    ) {
-        use rustpython_common::wtf8::Wtf8Buf;
-
+    fn _path_splitroot(path: OsPath, _vm: &VirtualMachine) -> (Wtf8Buf, Wtf8Buf) {
         let orig: Vec<_> = path.path.to_wide();
         if orig.is_empty() {
             return (Wtf8Buf::new(), Wtf8Buf::new());
@@ -1706,9 +1698,6 @@ pub(crate) mod module {
 
     #[pyfunction]
     fn _path_normpath(path: crate::PyObjectRef, vm: &VirtualMachine) -> PyResult {
-        use crate::builtins::{PyBytes, PyStr};
-        use rustpython_common::wtf8::Wtf8Buf;
-
         // Handle path-like objects via os.fspath
         let path = if let Some(fspath) = vm.get_method(path.clone(), identifier!(vm, __fspath__)) {
             fspath?.call((), vm)?
@@ -1727,7 +1716,8 @@ pub(crate) mod module {
                         "'utf-8' codec can't decode byte {:#x} in position {}: invalid start byte",
                         b.as_bytes().get(e.valid_up_to()).copied().unwrap_or(0),
                         e.valid_up_to()
-                    ),
+                    )
+                    .into(),
                 )
             })?;
             let wide: Vec<u16> = s.encode_utf16().collect();
diff --git a/crates/vm/src/stdlib/operator.rs b/crates/vm/src/stdlib/operator.rs
index fb0d652361e..38b0f715d31 100644
--- a/crates/vm/src/stdlib/operator.rs
+++ b/crates/vm/src/stdlib/operator.rs
@@ -4,8 +4,8 @@ pub(crate) use _operator::module_def;
 mod _operator {
     use crate::{
         AsObject, Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyInt, PyIntRef, PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef},
-        common::wtf8::Wtf8,
+        builtins::{PyInt, PyIntRef, PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef, PyUtf8StrRef},
+        common::wtf8::{Wtf8, Wtf8Buf},
         function::{ArgBytesLike, Either, FuncArgs, KwArgs, OptionalArg},
         protocol::PyIter,
         recursion::ReprGuard,
@@ -432,17 +432,22 @@ mod _operator {
 
     impl Representable for PyAttrGetter {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let fmt = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-                let mut parts = Vec::with_capacity(zelf.attrs.len());
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            let mut result = Wtf8Buf::from("operator.attrgetter(");
+            if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+                let mut first = true;
                 for part in &zelf.attrs {
-                    parts.push(part.as_object().repr(vm)?.as_str().to_owned());
+                    if !first {
+                        result.push_str(", ");
+                    }
+                    first = false;
+                    result.push_wtf8(part.as_object().repr(vm)?.as_wtf8());
                 }
-                parts.join(", ")
             } else {
-                "...".to_owned()
-            };
-            Ok(format!("operator.attrgetter({fmt})"))
+                result.push_str("...");
+            }
+            result.push_char(')');
+            Ok(result)
         }
     }
 
@@ -505,17 +510,22 @@ mod _operator {
 
     impl Representable for PyItemGetter {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let fmt = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
-                let mut items = Vec::with_capacity(zelf.items.len());
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            let mut result = Wtf8Buf::from("operator.itemgetter(");
+            if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+                let mut first = true;
                 for item in &zelf.items {
-                    items.push(item.repr(vm)?.as_str().to_owned());
+                    if !first {
+                        result.push_str(", ");
+                    }
+                    first = false;
+                    result.push_wtf8(item.repr(vm)?.as_wtf8());
                 }
-                items.join(", ")
             } else {
-                "...".to_owned()
-            };
-            Ok(format!("operator.itemgetter({fmt})"))
+                result.push_str("...");
+            }
+            result.push_char(')');
+            Ok(result)
         }
     }
 
@@ -530,7 +540,7 @@ mod _operator {
     #[pyclass(name = "methodcaller")]
     #[derive(Debug, PyPayload)]
     struct PyMethodCaller {
-        name: PyStrRef,
+        name: PyUtf8StrRef,
         args: FuncArgs,
     }
 
@@ -570,11 +580,10 @@ mod _operator {
             (name, args): Self::Args,
             vm: &VirtualMachine,
         ) -> PyResult<Self> {
-            if let Ok(name) = name.try_into_value(vm) {
-                Ok(Self { name, args })
-            } else {
-                Err(vm.new_type_error("method name must be a string"))
-            }
+            let name = name
+                .try_into_value(vm)
+                .map_err(|_| vm.new_type_error("method name must be a string"))?;
+            Ok(Self { name, args })
         }
     }
 
@@ -589,32 +598,27 @@ mod _operator {
 
     impl Representable for PyMethodCaller {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let fmt = if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            let mut result = Wtf8Buf::from("operator.methodcaller(");
+            if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
                 let args = &zelf.args.args;
                 let kwargs = &zelf.args.kwargs;
-                let mut fmt = vec![zelf.name.as_object().repr(vm)?.as_str().to_owned()];
-                if !args.is_empty() {
-                    let mut parts = Vec::with_capacity(args.len());
-                    for v in args {
-                        parts.push(v.repr(vm)?.as_str().to_owned());
-                    }
-                    fmt.push(parts.join(", "));
+                result.push_wtf8(zelf.name.as_object().repr(vm)?.as_wtf8());
+                for v in args {
+                    result.push_str(", ");
+                    result.push_wtf8(v.repr(vm)?.as_wtf8());
                 }
-                // build name=value pairs from KwArgs.
-                if !kwargs.is_empty() {
-                    let mut parts = Vec::with_capacity(kwargs.len());
-                    for (key, value) in kwargs {
-                        let value_repr = value.repr(vm)?;
-                        parts.push(format!("{key}={value_repr}"));
-                    }
-                    fmt.push(parts.join(", "));
+                for (key, value) in kwargs {
+                    result.push_str(", ");
+                    result.push_str(key);
+                    result.push_char('=');
+                    result.push_wtf8(value.repr(vm)?.as_wtf8());
                 }
-                fmt.join(", ")
             } else {
-                "...".to_owned()
-            };
-            Ok(format!("operator.methodcaller({fmt})"))
+                result.push_str("...");
+            }
+            result.push_char(')');
+            Ok(result)
         }
     }
 }
diff --git a/crates/vm/src/stdlib/os.rs b/crates/vm/src/stdlib/os.rs
index d67e97874f4..fd03be9175e 100644
--- a/crates/vm/src/stdlib/os.rs
+++ b/crates/vm/src/stdlib/os.rs
@@ -179,6 +179,7 @@ pub(super) mod _os {
     use core::time::Duration;
     use crossbeam_utils::atomic::AtomicCell;
     use itertools::Itertools;
+    use rustpython_common::wtf8::Wtf8Buf;
     use std::{env, fs, fs::OpenOptions, io, path::PathBuf, time::SystemTime};
 
     const OPEN_DIR_FD: bool = cfg!(not(any(windows, target_os = "redox")));
@@ -357,7 +358,8 @@ pub(super) mod _os {
 
     #[pyfunction]
     fn mkdirs(path: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
-        fs::create_dir_all(path.as_str()).map_err(|err| err.into_pyexception(vm))
+        let os_path = vm.fsencode(&path)?;
+        fs::create_dir_all(&*os_path).map_err(|err| err.into_pyexception(vm))
     }
 
     #[cfg(not(windows))]
@@ -491,8 +493,8 @@ pub(super) mod _os {
     #[cfg(windows)]
     #[pyfunction]
     fn putenv(key: PyStrRef, value: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
-        let key_str = key.as_str();
-        let value_str = value.as_str();
+        let key_str = key.expect_str();
+        let value_str = value.expect_str();
         // Search from index 1 because on Windows starting '=' is allowed for
         // defining hidden environment variables.
         if key_str.is_empty()
@@ -539,7 +541,7 @@ pub(super) mod _os {
     #[cfg(windows)]
     #[pyfunction]
     fn unsetenv(key: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
-        let key_str = key.as_str();
+        let key_str = key.expect_str();
         // Search from index 1 because on Windows starting '=' is allowed for
         // defining hidden environment variables.
         if key_str.is_empty()
@@ -868,7 +870,7 @@ pub(super) mod _os {
 
     impl Representable for DirEntry {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             let name = match zelf.as_object().get_attr("name", vm) {
                 Ok(name) => Some(name),
                 Err(e)
@@ -882,7 +884,10 @@ pub(super) mod _os {
             if let Some(name) = name {
                 if let Some(_guard) = ReprGuard::enter(vm, zelf.as_object()) {
                     let repr = name.repr(vm)?;
-                    Ok(format!("<{} {}>", zelf.class(), repr))
+                    let mut result = Wtf8Buf::from(format!("<{} ", zelf.class()));
+                    result.push_wtf8(repr.as_wtf8());
+                    result.push_char('>');
+                    Ok(result)
                 } else {
                     Err(vm.new_runtime_error(format!(
                         "reentrant call inside {}.__repr__",
@@ -890,7 +895,7 @@ pub(super) mod _os {
                     )))
                 }
             } else {
-                Ok(format!("<{}>", zelf.class()))
+                Ok(Wtf8Buf::from(format!("<{}>", zelf.class())))
             }
         }
     }
diff --git a/crates/vm/src/stdlib/posix.rs b/crates/vm/src/stdlib/posix.rs
index 35ab3d48f97..961f0c6fe0d 100644
--- a/crates/vm/src/stdlib/posix.rs
+++ b/crates/vm/src/stdlib/posix.rs
@@ -28,7 +28,7 @@ pub fn set_inheritable(fd: BorrowedFd<'_>, inheritable: bool) -> nix::Result<()>
 pub mod module {
     use crate::{
         AsObject, Py, PyObjectRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PyInt, PyListRef, PyStr, PyTupleRef},
+        builtins::{PyDictRef, PyInt, PyListRef, PyTupleRef, PyUtf8Str},
         convert::{IntoPyException, ToPyObject, TryFromObject},
         exceptions::OSErrorBuilder,
         function::{ArgMapping, Either, KwArgs, OptionalArg},
@@ -44,18 +44,22 @@ pub mod module {
         target_os = "linux",
         target_os = "openbsd"
     ))]
-    use crate::{builtins::PyStrRef, utils::ToCString};
+    use crate::{builtins::PyUtf8StrRef, utils::ToCString};
     use alloc::ffi::CString;
     use bitflags::bitflags;
     use core::ffi::CStr;
     use nix::{
+        errno::Errno,
         fcntl,
+        sys::signal,
         unistd::{self, Gid, Pid, Uid},
     };
+    use rustpython_common::os::ffi::OsStringExt;
     use std::{
         env, fs, io,
         os::fd::{AsFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd},
     };
+    use strum::IntoEnumIterator;
     use strum_macros::{EnumIter, EnumString};
 
     #[cfg(any(target_os = "android", target_os = "linux"))]
@@ -379,7 +383,7 @@ pub mod module {
     fn getgroups_impl() -> nix::Result<Vec<Gid>> {
         use core::ptr;
         use libc::{c_int, gid_t};
-        use nix::errno::Errno;
+
         let ret = unsafe { libc::getgroups(0, ptr::null_mut()) };
         let mut groups = Vec::<Gid>::with_capacity(Errno::result(ret)? as usize);
         let ret = unsafe {
@@ -449,8 +453,6 @@ pub mod module {
 
     #[pyattr]
     fn environ(vm: &VirtualMachine) -> PyDictRef {
-        use rustpython_common::os::ffi::OsStringExt;
-
         let environ = vm.ctx.new_dict();
         for (key, value) in env::vars_os() {
             let key: PyObjectRef = vm.ctx.new_bytes(key.into_vec()).into();
@@ -463,8 +465,6 @@ pub mod module {
 
     #[pyfunction]
     fn _create_environ(vm: &VirtualMachine) -> PyDictRef {
-        use rustpython_common::os::ffi::OsStringExt;
-
         let environ = vm.ctx.new_dict();
         for (key, value) in env::vars_os() {
             let key: PyObjectRef = vm.ctx.new_bytes(key.into_vec()).into();
@@ -909,7 +909,6 @@ pub mod module {
     #[cfg(not(target_os = "redox"))]
     #[pyfunction]
     fn nice(increment: i32, vm: &VirtualMachine) -> PyResult<i32> {
-        use nix::errno::Errno;
         Errno::clear();
         let res = unsafe { libc::nice(increment) };
         if res == -1 && Errno::last_raw() != 0 {
@@ -1428,7 +1427,7 @@ pub mod module {
         target_os = "openbsd"
     ))]
     #[pyfunction]
-    fn initgroups(user_name: PyStrRef, gid: Gid, vm: &VirtualMachine) -> PyResult<()> {
+    fn initgroups(user_name: PyUtf8StrRef, gid: Gid, vm: &VirtualMachine) -> PyResult<()> {
         let user = user_name.to_cstring(vm)?;
         unistd::initgroups(&user, gid).map_err(|err| err.into_pyexception(vm))
     }
@@ -1584,7 +1583,6 @@ pub mod module {
             let mut flags = nix::spawn::PosixSpawnFlags::empty();
 
             if let Some(sigs) = self.setsigdef {
-                use nix::sys::signal;
                 let mut set = signal::SigSet::empty();
                 for sig in sigs.iter(vm)? {
                     let sig = sig?;
@@ -1630,7 +1628,6 @@ pub mod module {
             }
 
             if let Some(sigs) = self.setsigmask {
-                use nix::sys::signal;
                 let mut set = signal::SigSet::empty();
                 for sig in sigs.iter(vm)? {
                     let sig = sig?;
@@ -1669,7 +1666,7 @@ pub mod module {
                 envp_from_dict(env_dict, vm)?
             } else {
                 // env=None means use the current environment
-                use rustpython_common::os::ffi::OsStringExt;
+
                 env::vars_os()
                     .map(|(k, v)| {
                         let mut entry = k.into_vec();
@@ -1920,8 +1917,12 @@ pub mod module {
         target_os = "openbsd"
     ))]
     #[pyfunction]
-    fn getgrouplist(user: PyStrRef, group: u32, vm: &VirtualMachine) -> PyResult<Vec<PyObjectRef>> {
-        let user = CString::new(user.as_str()).unwrap();
+    fn getgrouplist(
+        user: PyUtf8StrRef,
+        group: u32,
+        vm: &VirtualMachine,
+    ) -> PyResult<Vec<PyObjectRef>> {
+        let user = user.to_cstring(vm)?;
         let gid = Gid::from_raw(group);
         let group_ids = unistd::getgrouplist(&user, gid).map_err(|err| err.into_pyexception(vm))?;
         Ok(group_ids
@@ -1954,7 +1955,6 @@ pub mod module {
         who: PriorityWhoType,
         vm: &VirtualMachine,
     ) -> PyResult {
-        use nix::errno::Errno;
         Errno::clear();
         let retval = unsafe { libc::getpriority(which, who) };
         if Errno::last_raw() != 0 {
@@ -1987,7 +1987,7 @@ pub mod module {
             let i = match obj.downcast::<PyInt>() {
                 Ok(int) => int.try_to_primitive(vm)?,
                 Err(obj) => {
-                    let s = obj.downcast::<PyStr>().map_err(|_| {
+                    let s = obj.downcast::<PyUtf8Str>().map_err(|_| {
                         vm.new_type_error(
                             "configuration names must be strings or integers".to_owned(),
                         )
@@ -2176,8 +2176,6 @@ pub mod module {
         PathconfName(name): PathconfName,
         vm: &VirtualMachine,
     ) -> PyResult<Option<libc::c_long>> {
-        use nix::errno::Errno;
-
         Errno::clear();
         debug_assert_eq!(Errno::last_raw(), 0);
         let raw = match &path {
@@ -2214,7 +2212,6 @@ pub mod module {
 
     #[pyattr]
     fn pathconf_names(vm: &VirtualMachine) -> PyDictRef {
-        use strum::IntoEnumIterator;
         let pathname = vm.ctx.new_dict();
         for variant in PathconfVar::iter() {
             // get the name of variant as a string to use as the dictionary key
@@ -2385,18 +2382,21 @@ pub mod module {
             let i = match obj.downcast::<PyInt>() {
                 Ok(int) => int.try_to_primitive(vm)?,
                 Err(obj) => {
-                    let s = obj.downcast::<PyStr>().map_err(|_| {
+                    let s = obj.downcast::<PyUtf8Str>().map_err(|_| {
                         vm.new_type_error(
                             "configuration names must be strings or integers".to_owned(),
                         )
                     })?;
-                    s.as_str().parse::<SysconfVar>().or_else(|_| {
-                        if s.as_str() == "SC_PAGESIZE" {
-                            Ok(SysconfVar::SC_PAGESIZE)
-                        } else {
-                            Err(vm.new_value_error("unrecognized configuration name"))
-                        }
-                    })? as i32
+                    {
+                        let name = s.as_str();
+                        name.parse::<SysconfVar>().or_else(|_| {
+                            if name == "SC_PAGESIZE" {
+                                Ok(SysconfVar::SC_PAGESIZE)
+                            } else {
+                                Err(vm.new_value_error("unrecognized configuration name"))
+                            }
+                        })? as i32
+                    }
                 }
             };
             Ok(Self(i))
@@ -2415,7 +2415,6 @@ pub mod module {
 
     #[pyattr]
     fn sysconf_names(vm: &VirtualMachine) -> PyDictRef {
-        use strum::IntoEnumIterator;
         let names = vm.ctx.new_dict();
         for variant in SysconfVar::iter() {
             // get the name of variant as a string to use as the dictionary key
diff --git a/crates/vm/src/stdlib/signal.rs b/crates/vm/src/stdlib/signal.rs
index 9e51cd3b425..e6ad7b53348 100644
--- a/crates/vm/src/stdlib/signal.rs
+++ b/crates/vm/src/stdlib/signal.rs
@@ -323,7 +323,7 @@ pub(crate) mod _signal {
         if ret != 0 {
             let err = std::io::Error::last_os_error();
             let itimer_error = itimer_error(vm);
-            return Err(vm.new_exception_msg(itimer_error, err.to_string()));
+            return Err(vm.new_exception_msg(itimer_error, err.to_string().into()));
         }
         let old = unsafe { old.assume_init() };
         Ok(itimerval_to_tuple(&old))
@@ -340,7 +340,7 @@ pub(crate) mod _signal {
         if ret != 0 {
             let err = std::io::Error::last_os_error();
             let itimer_error = itimer_error(vm);
-            return Err(vm.new_exception_msg(itimer_error, err.to_string()));
+            return Err(vm.new_exception_msg(itimer_error, err.to_string().into()));
         }
         let old = unsafe { old.assume_init() };
         Ok(itimerval_to_tuple(&old))
diff --git a/crates/vm/src/stdlib/sre.rs b/crates/vm/src/stdlib/sre.rs
index eb0cb05eb7d..2c18bab4ba1 100644
--- a/crates/vm/src/stdlib/sre.rs
+++ b/crates/vm/src/stdlib/sre.rs
@@ -9,7 +9,7 @@ mod _sre {
             PyCallableIterator, PyDictRef, PyGenericAlias, PyInt, PyList, PyListRef, PyStr,
             PyStrRef, PyTuple, PyTupleRef, PyTypeRef,
         },
-        common::wtf8::{Wtf8, Wtf8Buf},
+        common::wtf8::{Wtf8, Wtf8Buf, wtf8_concat},
         common::{ascii, hash::PyHash},
         convert::ToPyObject,
         function::{ArgCallable, OptionalArg, PosArgs, PyComparisonValue},
@@ -552,7 +552,7 @@ mod _sre {
 
     impl Representable for Pattern {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             let flag_names = [
                 ("re.IGNORECASE", SreFlag::IGNORECASE),
                 ("re.LOCALE", SreFlag::LOCALE),
@@ -580,19 +580,19 @@ mod _sre {
                 .join("|");
 
             let pattern = zelf.pattern.repr(vm)?;
-            let truncated: String;
-            let s = if pattern.char_len() > 200 {
-                truncated = pattern.as_str().chars().take(200).collect();
-                &truncated
+            let mut result = Wtf8Buf::from("re.compile(");
+            let pat = if pattern.char_len() > 200 {
+                pattern.as_wtf8().code_points().take(200).collect()
             } else {
-                pattern.as_str()
+                pattern.as_wtf8().to_owned()
             };
-
-            if flags.is_empty() {
-                Ok(format!("re.compile({s})"))
-            } else {
-                Ok(format!("re.compile({s}, {flags})"))
+            result.push_wtf8(&pat);
+            if !flags.is_empty() {
+                result.push_str(", ");
+                result.push_str(&flags);
             }
+            result.push_char(')');
+            Ok(result)
         }
     }
 
@@ -851,13 +851,17 @@ mod _sre {
 
     impl Representable for Match {
         #[inline]
-        fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
+        fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
             with_sre_str!(zelf.pattern, &zelf.string, vm, |str_drive| {
-                Ok(format!(
-                    "<re.Match object; span=({}, {}), match={}>",
+                let match_repr = zelf.get_slice(0, str_drive, vm).unwrap().repr(vm)?;
+                Ok(wtf8_concat!(
+                    "<re.Match object; span=(",
                     zelf.regs[0].0,
+                    ", ",
                     zelf.regs[0].1,
-                    zelf.get_slice(0, str_drive, vm).unwrap().repr(vm)?
+                    "), match=",
+                    match_repr.as_wtf8(),
+                    '>',
                 ))
             })
         }
diff --git a/crates/vm/src/stdlib/symtable.rs b/crates/vm/src/stdlib/symtable.rs
index 570a48a8e24..eede3d102c2 100644
--- a/crates/vm/src/stdlib/symtable.rs
+++ b/crates/vm/src/stdlib/symtable.rs
@@ -4,7 +4,7 @@ pub(crate) use _symtable::module_def;
 mod _symtable {
     use crate::{
         Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PyStrRef},
+        builtins::{PyDictRef, PyUtf8StrRef},
         compiler,
         types::Representable,
     };
@@ -111,9 +111,9 @@ mod _symtable {
 
     #[pyfunction]
     fn symtable(
-        source: PyStrRef,
-        filename: PyStrRef,
-        mode: PyStrRef,
+        source: PyUtf8StrRef,
+        filename: PyUtf8StrRef,
+        mode: PyUtf8StrRef,
         vm: &VirtualMachine,
     ) -> PyResult<PyRef<PySymbolTable>> {
         let mode = mode
diff --git a/crates/vm/src/stdlib/sys.rs b/crates/vm/src/stdlib/sys.rs
index 53b05f477f5..ec21cc7f46e 100644
--- a/crates/vm/src/stdlib/sys.rs
+++ b/crates/vm/src/stdlib/sys.rs
@@ -34,7 +34,7 @@ mod sys {
         AsObject, PyObject, PyObjectRef, PyPayload, PyRef, PyRefExact, PyResult,
         builtins::{
             PyBaseExceptionRef, PyDictRef, PyFrozenSet, PyNamespace, PyStr, PyStrRef, PyTuple,
-            PyTupleRef, PyTypeRef,
+            PyTupleRef, PyTypeRef, PyUtf8StrRef,
         },
         common::{
             ascii,
@@ -974,7 +974,7 @@ mod sys {
     }
 
     #[pyfunction]
-    fn getfilesystemencodeerrors(vm: &VirtualMachine) -> PyStrRef {
+    fn getfilesystemencodeerrors(vm: &VirtualMachine) -> PyUtf8StrRef {
         vm.fs_encode_errors().to_owned()
     }
 
@@ -1250,7 +1250,7 @@ mod sys {
         // Print module name (if not builtins or __main__)
         let module_name = unraisable.exc_type.__module__(vm);
         if let Ok(module_str) = module_name.downcast::<PyStr>() {
-            let module = module_str.as_str();
+            let module = module_str.as_wtf8();
             if module != "builtins" && module != "__main__" {
                 write!(stderr, "{}.", module);
             }
@@ -1261,7 +1261,7 @@ mod sys {
         // Print qualname
         let qualname = unraisable.exc_type.__qualname__(vm);
         if let Ok(qualname_str) = qualname.downcast::<PyStr>() {
-            write!(stderr, "{}", qualname_str.as_str());
+            write!(stderr, "{}", qualname_str.as_wtf8());
         } else {
             write!(stderr, "{}", unraisable.exc_type.name());
         }
@@ -1270,7 +1270,7 @@ mod sys {
         if !vm.is_none(&unraisable.exc_value) {
             write!(stderr, ": ");
             if let Ok(str) = unraisable.exc_value.str(vm) {
-                write!(stderr, "{}", str.to_str().unwrap_or("<str with surrogate>"));
+                write!(stderr, "{}", str.as_wtf8());
             } else {
                 write!(stderr, "<exception str() failed>");
             }
diff --git a/crates/vm/src/stdlib/thread.rs b/crates/vm/src/stdlib/thread.rs
index ff6f0fd8b8f..21b19fb7560 100644
--- a/crates/vm/src/stdlib/thread.rs
+++ b/crates/vm/src/stdlib/thread.rs
@@ -12,7 +12,8 @@ pub(crate) use _thread::{
 pub(crate) mod _thread {
     use crate::{
         AsObject, Py, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef},
+        builtins::{PyDictRef, PyStr, PyTupleRef, PyType, PyTypeRef, PyUtf8StrRef},
+        common::wtf8::Wtf8Buf,
         frame::FrameRef,
         function::{ArgCallable, Either, FuncArgs, KwArgs, OptionalArg, PySetterValue},
         types::{Constructor, GetAttr, Representable, SetAttr},
@@ -318,7 +319,7 @@ pub(crate) mod _thread {
 
     /// Set the name of the current thread
     #[pyfunction]
-    fn set_name(name: PyStrRef) {
+    fn set_name(name: PyUtf8StrRef) {
         #[cfg(target_os = "linux")]
         {
             use alloc::ffi::CString;
@@ -710,11 +711,11 @@ pub(crate) mod _thread {
                 .get_attr("name", vm)
                 .ok()
                 .and_then(|n| n.str(vm).ok())
-                .map(|s| s.as_str().to_owned())
+                .map(|s| s.as_wtf8().to_owned())
         } else {
             None
         };
-        let name = thread_name.unwrap_or_else(|| format!("{}", get_ident()));
+        let name = thread_name.unwrap_or_else(|| Wtf8Buf::from(format!("{}", get_ident())));
 
         let _ = vm.call_method(
             &file,
@@ -836,7 +837,7 @@ pub(crate) mod _thread {
     impl GetAttr for Local {
         fn getattro(zelf: &Py<Self>, attr: &Py<PyStr>, vm: &VirtualMachine) -> PyResult {
             let l_dict = zelf.l_dict(vm);
-            if attr.as_str() == "__dict__" {
+            if attr.as_bytes() == b"__dict__" {
                 Ok(l_dict.into())
             } else {
                 zelf.as_object()
@@ -859,7 +860,7 @@ pub(crate) mod _thread {
             value: PySetterValue,
             vm: &VirtualMachine,
         ) -> PyResult<()> {
-            if attr.as_str() == "__dict__" {
+            if attr.as_bytes() == b"__dict__" {
                 Err(vm.new_attribute_error(format!(
                     "{} attribute '__dict__' is read-only",
                     zelf.class().name()
diff --git a/crates/vm/src/stdlib/time.rs b/crates/vm/src/stdlib/time.rs
index 05790fe332b..ee19fce7dac 100644
--- a/crates/vm/src/stdlib/time.rs
+++ b/crates/vm/src/stdlib/time.rs
@@ -101,7 +101,7 @@ mod decl {
         let dur = seconds.try_into_value::<Duration>(vm).map_err(|e| {
             if e.class().is(vm.ctx.exceptions.value_error)
                 && let Some(s) = e.args().first().and_then(|arg| arg.str(vm).ok())
-                && s.as_str() == "negative duration"
+                && s.as_bytes() == b"negative duration"
             {
                 return vm.new_value_error("sleep length must be non-negative");
             }
@@ -434,10 +434,11 @@ mod decl {
 
         #[cfg(unix)]
         {
+            use crate::builtins::PyUtf8StrRef;
             let zone = if t.tm_zone.is(&vm.ctx.none) {
                 None
             } else {
-                let zone: PyStrRef = t
+                let zone: PyUtf8StrRef = t
                     .tm_zone
                     .clone()
                     .try_into_value(vm)
@@ -1047,7 +1048,7 @@ mod platform {
     #[cfg_attr(target_os = "macos", allow(unused_imports))]
     use crate::{
         PyObject, PyRef, PyResult, TryFromBorrowedObject, VirtualMachine,
-        builtins::{PyNamespace, PyStrRef},
+        builtins::{PyNamespace, PyUtf8StrRef},
         convert::IntoPyException,
     };
     use core::time::Duration;
@@ -1216,8 +1217,8 @@ mod platform {
         target_os = "linux",
     ))]
     #[pyfunction]
-    fn get_clock_info(name: PyStrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
-        let (adj, imp, mono, res) = match name.as_ref() {
+    fn get_clock_info(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
+        let (adj, imp, mono, res) = match name.as_str() {
             "monotonic" | "perf_counter" => (
                 false,
                 "time.clock_gettime(CLOCK_MONOTONIC)",
@@ -1263,7 +1264,7 @@ mod platform {
         target_os = "linux",
     )))]
     #[pyfunction]
-    fn get_clock_info(_name: PyStrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
+    fn get_clock_info(_name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
         Err(vm.new_not_implemented_error("get_clock_info unsupported on this system"))
     }
 
@@ -1330,7 +1331,7 @@ mod platform {
     use super::decl::{MS_TO_NS, SEC_TO_NS, StructTimeData, get_tz_info, time_muldiv};
     use crate::{
         PyRef, PyResult, VirtualMachine,
-        builtins::{PyNamespace, PyStrRef},
+        builtins::{PyNamespace, PyUtf8StrRef},
     };
     use core::time::Duration;
     use windows_sys::Win32::{
@@ -1500,8 +1501,8 @@ mod platform {
     }
 
     #[pyfunction]
-    fn get_clock_info(name: PyStrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
-        let (adj, imp, mono, res) = match name.as_ref() {
+    fn get_clock_info(name: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<PyRef<PyNamespace>> {
+        let (adj, imp, mono, res) = match name.as_str() {
             "monotonic" => (
                 false,
                 "GetTickCount64()",
diff --git a/crates/vm/src/stdlib/typevar.rs b/crates/vm/src/stdlib/typevar.rs
index 1b3b5fecdd4..d0bd3f5666d 100644
--- a/crates/vm/src/stdlib/typevar.rs
+++ b/crates/vm/src/stdlib/typevar.rs
@@ -30,7 +30,7 @@ pub(crate) mod typevar {
         contravariant: bool,
     ) -> String {
         if infer_variance {
-            return name.to_string();
+            return name.to_owned();
         }
         let prefix = if covariant {
             '+'
@@ -62,23 +62,19 @@ pub(crate) mod typevar {
     fn set_module_from_caller(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {
         // Note: CPython gets module from frame->f_funcobj, but RustPython's Frame
         // architecture is different - we use globals['__name__'] instead
-        if let Some(module_name) = caller(vm) {
+        let module_value: PyObjectRef = if let Some(module_name) = caller(vm) {
             // Special handling for certain module names
-            if let Ok(name_str) = module_name.str(vm) {
-                let name = name_str.as_str();
-                // CPython sets __module__ to None for builtins and <...> modules
-                // Also set to None for exec contexts (no __name__ in globals means exec)
-                if name == "builtins" || name.starts_with('<') {
-                    // Don't set __module__ attribute at all (CPython behavior)
-                    // This allows the typing module to handle it
-                    return Ok(());
-                }
+            if let Ok(name_str) = module_name.str(vm)
+                && let Some(name) = name_str.to_str()
+                && (name == "builtins" || name.starts_with('<'))
+            {
+                return Ok(());
             }
-            obj.set_attr("__module__", module_name, vm)?;
+            module_name
         } else {
-            // If no module name is found (e.g., in exec context), set __module__ to None
-            obj.set_attr("__module__", vm.ctx.none(), vm)?;
-        }
+            vm.ctx.none()
+        };
+        obj.set_attr("__module__", module_value, vm)?;
         Ok(())
     }
 
@@ -288,7 +284,7 @@ pub(crate) mod typevar {
     impl Representable for TypeVar {
         #[inline(always)]
         fn repr_str(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let name = zelf.name.str(vm)?;
+            let name = zelf.name.str_utf8(vm)?;
             Ok(variance_repr(
                 name.as_str(),
                 zelf.infer_variance,
@@ -684,7 +680,7 @@ pub(crate) mod typevar {
     impl Representable for ParamSpec {
         #[inline(always)]
         fn repr_str(zelf: &crate::Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
-            let name = zelf.__name__().str(vm)?;
+            let name = zelf.__name__().str_utf8(vm)?;
             Ok(variance_repr(
                 name.as_str(),
                 zelf.infer_variance,
diff --git a/crates/vm/src/stdlib/typing.rs b/crates/vm/src/stdlib/typing.rs
index bafc02e5764..2a1ed8e91a2 100644
--- a/crates/vm/src/stdlib/typing.rs
+++ b/crates/vm/src/stdlib/typing.rs
@@ -32,6 +32,7 @@ pub(crate) mod decl {
     use crate::{
         AsObject, Py, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine, atomic_func,
         builtins::{PyGenericAlias, PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef, type_},
+        common::wtf8::Wtf8Buf,
         function::FuncArgs,
         protocol::{PyMappingMethods, PyNumberMethods},
         types::{AsMapping, AsNumber, Callable, Constructor, Iterable, Representable},
@@ -145,10 +146,14 @@ pub(crate) mod decl {
             && !vm.is_none(&module)
             && let Some(module_str) = module.downcast_ref::<crate::builtins::PyStr>()
         {
-            if module_str.as_str() == "builtins" {
-                return Ok(qualname.str(vm)?.to_string());
+            if module_str.as_bytes() == b"builtins" {
+                return Ok(qualname.str_utf8(vm)?.as_str().to_owned());
             }
-            return Ok(format!("{}.{}", module_str.as_str(), qualname.str(vm)?));
+            return Ok(format!(
+                "{}.{}",
+                module_str.as_wtf8(),
+                qualname.str_utf8(vm)?.as_str()
+            ));
         }
         // Fallback to repr
         Ok(obj.repr(vm)?.to_string())
@@ -426,8 +431,8 @@ pub(crate) mod decl {
     }
 
     impl Representable for TypeAliasType {
-        fn repr_str(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
-            Ok(zelf.name.as_str().to_owned())
+        fn repr_wtf8(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+            Ok(zelf.name.as_wtf8().to_owned())
         }
     }
 
diff --git a/crates/vm/src/stdlib/winreg.rs b/crates/vm/src/stdlib/winreg.rs
index 1b165f911d0..026d8d38c63 100644
--- a/crates/vm/src/stdlib/winreg.rs
+++ b/crates/vm/src/stdlib/winreg.rs
@@ -994,7 +994,7 @@ mod winreg {
                 let s = value
                     .downcast::<PyStr>()
                     .map_err(|_| vm.new_type_error("value must be a string".to_string()))?;
-                let wide = s.as_str().to_wide_with_nul();
+                let wide = s.as_wtf8().to_wide_with_nul();
                 // Convert Vec<u16> to Vec<u8>
                 let bytes: Vec<u8> = wide.iter().flat_map(|&c| c.to_le_bytes()).collect();
                 Ok(Some(bytes))
@@ -1013,7 +1013,7 @@ mod winreg {
                     let s = item.downcast_ref::<PyStr>().ok_or_else(|| {
                         vm.new_type_error("list items must be strings".to_string())
                     })?;
-                    let wide = s.as_str().to_wide_with_nul();
+                    let wide = s.as_wtf8().to_wide_with_nul();
                     bytes.extend(wide.iter().flat_map(|&c| c.to_le_bytes()));
                 }
                 // Add final null terminator (double null at end)
diff --git a/crates/vm/src/stdlib/winsound.rs b/crates/vm/src/stdlib/winsound.rs
index 3f65abbb890..729305f879d 100644
--- a/crates/vm/src/stdlib/winsound.rs
+++ b/crates/vm/src/stdlib/winsound.rs
@@ -118,7 +118,7 @@ mod winsound {
 
         // os.fspath(sound)
         let path = match sound.downcast_ref::<PyStr>() {
-            Some(s) => s.as_str().to_owned(),
+            Some(s) => s.as_wtf8().to_owned(),
             None => {
                 let fspath = vm.get_method_or_type_error(
                     sound.clone(),
@@ -151,12 +151,12 @@ mod winsound {
                     ))
                 })?;
 
-                s.as_str().to_owned()
+                s.as_wtf8().to_owned()
             }
         };
 
         // Check for embedded null characters
-        if path.contains('\0') {
+        if path.as_bytes().contains(&0) {
             return Err(vm.new_value_error("embedded null character".to_owned()));
         }
 
diff --git a/crates/vm/src/suggestion.rs b/crates/vm/src/suggestion.rs
index 2d732160f07..b48b78af755 100644
--- a/crates/vm/src/suggestion.rs
+++ b/crates/vm/src/suggestion.rs
@@ -89,7 +89,7 @@ pub fn offer_suggestions(exc: &Py<PyBaseException>, vm: &VirtualMachine) -> Opti
 
         // Look up the module in sys.modules
         let sys_modules = vm.sys_module.get_attr("modules", vm).ok()?;
-        let module = sys_modules.get_item(mod_name_str.as_str(), vm).ok()?;
+        let module = sys_modules.get_item(mod_name_str, vm).ok()?;
 
         calculate_suggestions(vm.dir(Some(module)).ok()?.borrow_vec().iter(), &wrong_name)
     } else {
diff --git a/crates/vm/src/types/slot.rs b/crates/vm/src/types/slot.rs
index ae00158aeb4..84eaa817ac4 100644
--- a/crates/vm/src/types/slot.rs
+++ b/crates/vm/src/types/slot.rs
@@ -20,6 +20,7 @@ use crate::{
 use core::{any::Any, any::TypeId, borrow::Borrow, cmp::Ordering, ops::Deref};
 use crossbeam_utils::atomic::AtomicCell;
 use num_traits::{Signed, ToPrimitive};
+use rustpython_common::wtf8::Wtf8Buf;
 
 /// Type-erased storage for extension module data attached to heap types.
 pub struct TypeDataSlot {
@@ -1548,8 +1549,11 @@ pub trait Initializer: PyPayload {
                 #[cfg(debug_assertions)]
                 {
                     if let Ok(msg) = err.as_object().repr(vm) {
-                        let double_appearance =
-                            msg.as_str().matches(&class_name_for_debug as &str).count() == 2;
+                        let double_appearance = msg
+                            .to_string_lossy()
+                            .matches(&class_name_for_debug as &str)
+                            .count()
+                            == 2;
                         if double_appearance {
                             panic!(
                                 "This type `{}` doesn't seem to support `init`. Override `slot_init` instead: {}",
@@ -1596,12 +1600,14 @@ pub trait Callable: PyPayload {
     fn slot_call(zelf: &PyObject, args: FuncArgs, vm: &VirtualMachine) -> PyResult {
         let zelf = zelf.downcast_ref().ok_or_else(|| {
             let repr = zelf.repr(vm);
-            let help = if let Ok(repr) = repr.as_ref() {
-                repr.as_str().to_owned()
+            let help: Wtf8Buf = if let Ok(repr) = repr.as_ref() {
+                repr.as_wtf8().to_owned()
             } else {
-                zelf.class().name().to_owned()
+                zelf.class().name().to_owned().into()
             };
-            vm.new_type_error(format!("unexpected payload for __call__ of {help}"))
+            let mut msg = Wtf8Buf::from("unexpected payload for __call__ of ");
+            msg.push_wtf8(&help);
+            vm.new_type_error(msg)
         })?;
         let args = args.bind(vm)?;
         Self::call(zelf, args, vm)
@@ -1706,11 +1712,16 @@ pub trait Representable: PyPayload {
 
     #[inline]
     fn repr(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyRef<PyStr>> {
-        let repr = Self::repr_str(zelf, vm)?;
+        let repr = Self::repr_wtf8(zelf, vm)?;
         Ok(vm.ctx.new_str(repr))
     }
 
-    fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String>;
+    fn repr_wtf8(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<Wtf8Buf> {
+        Self::repr_str(zelf, vm).map(|utf8| utf8.into())
+    }
+    fn repr_str(_zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
+        unreachable!("Representable requires overriding either repr_str or repr_wtf8")
+    }
 }
 
 #[pyclass]
@@ -2063,7 +2074,7 @@ where
     #[cold]
     fn slot_iter(zelf: PyObjectRef, vm: &VirtualMachine) -> PyResult {
         let repr = zelf.repr(vm)?;
-        unreachable!("slot must be overridden for {}", repr.as_str());
+        unreachable!("slot must be overridden for {}", repr.as_wtf8());
     }
 
     #[cold]
diff --git a/crates/vm/src/types/structseq.rs b/crates/vm/src/types/structseq.rs
index 3b89a62b9ad..0ac73c0fc19 100644
--- a/crates/vm/src/types/structseq.rs
+++ b/crates/vm/src/types/structseq.rs
@@ -234,7 +234,7 @@ pub trait PyStructSequence: StaticType + PyClassImpl + Sized + 'static {
             match typ.get_attr(identifier!(vm.ctx, __module__)) {
                 Some(module) if module.downcastable::<PyStr>() => {
                     let module_str = module.downcast_ref::<PyStr>().unwrap();
-                    alloc::borrow::Cow::Owned(format!("{}.{}", module_str.as_str(), Self::NAME))
+                    alloc::borrow::Cow::Owned(format!("{}.{}", module_str.as_wtf8(), Self::NAME))
                 }
                 _ => alloc::borrow::Cow::Borrowed(Self::TP_NAME),
             }
diff --git a/crates/vm/src/utils.rs b/crates/vm/src/utils.rs
index db232e81949..b5117ddd8d1 100644
--- a/crates/vm/src/utils.rs
+++ b/crates/vm/src/utils.rs
@@ -1,4 +1,4 @@
-use rustpython_common::wtf8::Wtf8;
+use rustpython_common::wtf8::{Wtf8, Wtf8Buf};
 
 use crate::{
     PyObjectRef, PyResult, VirtualMachine,
@@ -43,33 +43,31 @@ pub(crate) fn collection_repr<'a, I>(
     suffix: &str,
     iter: I,
     vm: &VirtualMachine,
-) -> PyResult<String>
+) -> PyResult<Wtf8Buf>
 where
     I: core::iter::Iterator<Item = &'a PyObjectRef>,
 {
-    let mut repr = String::new();
+    let mut repr = Wtf8Buf::new();
     if let Some(name) = class_name {
         repr.push_str(name);
-        repr.push('(');
+        repr.push_char('(');
     }
     repr.push_str(prefix);
     {
         let mut parts_iter = iter.map(|o| o.repr(vm));
-        repr.push_str(
-            parts_iter
-                .next()
-                .transpose()?
-                .expect("this is not called for empty collection")
-                .as_str(),
-        );
+        let first = parts_iter
+            .next()
+            .transpose()?
+            .expect("this is not called for empty collection");
+        repr.push_wtf8(first.as_wtf8());
         for part in parts_iter {
             repr.push_str(", ");
-            repr.push_str(part?.as_str());
+            repr.push_wtf8(part?.as_wtf8());
         }
     }
     repr.push_str(suffix);
     if class_name.is_some() {
-        repr.push(')');
+        repr.push_char(')');
     }
 
     Ok(repr)
diff --git a/crates/vm/src/vm/context.rs b/crates/vm/src/vm/context.rs
index db5406f045f..41d47ccfe0a 100644
--- a/crates/vm/src/vm/context.rs
+++ b/crates/vm/src/vm/context.rs
@@ -3,7 +3,7 @@ use crate::{
     builtins::{
         PyByteArray, PyBytes, PyComplex, PyDict, PyDictRef, PyEllipsis, PyFloat, PyFrozenSet,
         PyInt, PyIntRef, PyList, PyListRef, PyNone, PyNotImplemented, PyStr, PyStrInterned,
-        PyTuple, PyTupleRef, PyType, PyTypeRef,
+        PyTuple, PyTupleRef, PyType, PyTypeRef, PyUtf8Str,
         bool_::PyBool,
         code::{self, PyCode},
         descriptor::{
@@ -447,6 +447,11 @@ impl Context {
         s.into().into_ref(self)
     }
 
+    #[inline]
+    pub fn new_utf8_str(&self, s: impl Into<PyUtf8Str>) -> PyRef<PyUtf8Str> {
+        s.into().into_ref(self)
+    }
+
     pub fn interned_or_new_str<S, M>(&self, s: S) -> PyRef<PyStr>
     where
         S: Into<PyStr> + AsRef<M>,
diff --git a/crates/vm/src/vm/interpreter.rs b/crates/vm/src/vm/interpreter.rs
index f6ce3448c03..f6d3ee4bc0a 100644
--- a/crates/vm/src/vm/interpreter.rs
+++ b/crates/vm/src/vm/interpreter.rs
@@ -562,7 +562,7 @@ mod tests {
             let b = vm.new_pyobj(4_i32);
             let res = vm._mul(&a, &b).unwrap();
             let value = res.downcast_ref::<PyStr>().unwrap();
-            assert_eq!(value.as_str(), "Hello Hello Hello Hello ")
+            assert_eq!(value.as_wtf8(), "Hello Hello Hello Hello ")
         })
     }
 }
diff --git a/crates/vm/src/vm/mod.rs b/crates/vm/src/vm/mod.rs
index 3285885eea2..9802832ceab 100644
--- a/crates/vm/src/vm/mod.rs
+++ b/crates/vm/src/vm/mod.rs
@@ -20,7 +20,7 @@ use crate::{
     AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult,
     builtins::{
         self, PyBaseExceptionRef, PyDict, PyDictRef, PyInt, PyList, PyModule, PyStr, PyStrInterned,
-        PyStrRef, PyTypeRef, PyWeak,
+        PyStrRef, PyTypeRef, PyUtf8Str, PyUtf8StrInterned, PyWeak,
         code::PyCode,
         dict::{PyDictItems, PyDictKeys, PyDictValues},
         pystr::AsPyStr,
@@ -34,7 +34,7 @@ use crate::{
     frozen::FrozenModule,
     function::{ArgMapping, FuncArgs, PySetterValue},
     import,
-    protocol::PyIterIter,
+    protocol::{PyIterIter, TraceEvent},
     scope::Scope,
     signal, stdlib,
     warn::WarningsState,
@@ -635,31 +635,28 @@ impl VirtualMachine {
             }
         };
 
-        // Format: "Exception ignored {msg} {object_repr}\n"
-        if let Some(msg) = msg {
-            write_to_stderr(&format!("Exception ignored {}", msg), &stderr, self);
+        let msg_str = if let Some(msg) = msg {
+            format!("{msg}: ")
         } else {
-            write_to_stderr("Exception ignored in: ", &stderr, self);
-        }
+            "Exception ignored in: ".to_owned()
+        };
+        write_to_stderr(&msg_str, &stderr, self);
 
-        if let Ok(repr) = object.repr(self) {
-            write_to_stderr(&format!("{}\n", repr.as_str()), &stderr, self);
-        } else {
-            write_to_stderr("<object repr failed>\n", &stderr, self);
-        }
+        let repr_result = object.repr(self);
+        let repr_wtf8 = repr_result
+            .as_ref()
+            .map_or("<object repr failed>".as_ref(), |s| s.as_wtf8());
+        write_to_stderr(&format!("{repr_wtf8}\n"), &stderr, self);
 
         // Write exception type and message
         let exc_type_name = e.class().name();
-        if let Ok(exc_str) = e.as_object().str(self) {
-            let exc_str = exc_str.as_str();
-            if exc_str.is_empty() {
-                write_to_stderr(&format!("{}\n", exc_type_name), &stderr, self);
-            } else {
-                write_to_stderr(&format!("{}: {}\n", exc_type_name, exc_str), &stderr, self);
+        let msg = match e.as_object().str(self) {
+            Ok(exc_str) if !exc_str.as_wtf8().is_empty() => {
+                format!("{}: {}\n", exc_type_name, exc_str.as_wtf8())
             }
-        } else {
-            write_to_stderr(&format!("{}\n", exc_type_name), &stderr, self);
-        }
+            _ => format!("{}\n", exc_type_name),
+        };
+        write_to_stderr(&msg, &stderr, self);
 
         // Flush stderr to ensure output is visible
         if let Some(ref stderr) = stderr {
@@ -810,7 +807,7 @@ impl VirtualMachine {
 
         for (key, value) in items {
             let name = key
-                .downcast_ref::<PyStr>()
+                .downcast_ref::<PyUtf8Str>()
                 .map(|s| s.as_str().to_owned())
                 .unwrap_or_default();
 
@@ -873,9 +870,9 @@ impl VirtualMachine {
                 continue;
             }
             if let Some(key_str) = key.downcast_ref::<PyStr>() {
-                let name = key_str.as_str();
-                if name.starts_with('_') && name != "__builtins__" {
-                    let _ = dict.set_item(name, none.clone(), vm);
+                let name = key_str.as_wtf8();
+                if name.starts_with("_") && name != "__builtins__" {
+                    let _ = dict.set_item(key_str, none.clone(), vm);
                 }
             }
         }
@@ -886,9 +883,9 @@ impl VirtualMachine {
                 continue;
             }
             if let Some(key_str) = key.downcast_ref::<PyStr>()
-                && key_str.as_str() != "__builtins__"
+                && key_str.as_bytes() != b"__builtins__"
             {
-                let _ = dict.set_item(key_str.as_str(), none.clone(), vm);
+                let _ = dict.set_item(key_str.as_wtf8(), none.clone(), vm);
             }
         }
     }
@@ -1078,7 +1075,6 @@ impl VirtualMachine {
                 crate::vm::thread::pop_thread_frame();
             }
 
-            use crate::protocol::TraceEvent;
             // Fire 'call' trace event after pushing frame
             // (current_frame() now returns the callee's frame)
             match self.trace_event(TraceEvent::Call, None) {
@@ -1624,11 +1620,11 @@ impl VirtualMachine {
         identifier!(self, utf_8)
     }
 
-    pub fn fs_encode_errors(&self) -> &'static PyStrInterned {
+    pub fn fs_encode_errors(&self) -> &'static PyUtf8StrInterned {
         if cfg!(windows) {
-            identifier!(self, surrogatepass)
+            identifier_utf8!(self, surrogatepass)
         } else {
-            identifier!(self, surrogateescape)
+            identifier_utf8!(self, surrogateescape)
         }
     }
 
@@ -1730,7 +1726,7 @@ fn frozen_origname_matches() {
                     .unwrap()
                     .try_into_value(vm)
                     .unwrap();
-                assert_eq!(origname.as_str(), expected);
+                assert_eq!(origname.as_wtf8(), expected);
             };
 
             check("_frozen_importlib", "importlib._bootstrap");
diff --git a/crates/vm/src/vm/vm_new.rs b/crates/vm/src/vm/vm_new.rs
index 9e33f430945..2c9320b4d87 100644
--- a/crates/vm/src/vm/vm_new.rs
+++ b/crates/vm/src/vm/vm_new.rs
@@ -13,6 +13,7 @@ use crate::{
     scope::Scope,
     vm::VirtualMachine,
 };
+use rustpython_common::wtf8::Wtf8Buf;
 use rustpython_compiler_core::SourceLocation;
 
 macro_rules! define_exception_fn {
@@ -24,7 +25,7 @@ macro_rules! define_exception_fn {
                     stringify!($python_repr),
                     " object.\nUseful for raising errors from python functions implemented in rust."
                 )]
-        pub fn $fn_name(&self, msg: impl Into<String>) -> PyBaseExceptionRef
+        pub fn $fn_name(&self, msg: impl Into<Wtf8Buf>) -> PyBaseExceptionRef
         {
             let err = self.ctx.exceptions.$attr.to_owned();
             self.new_exception_msg(err, msg.into())
@@ -150,7 +151,7 @@ impl VirtualMachine {
     /// type is passed in, it may not be fully initialized; try using
     /// [`vm.invoke_exception()`][Self::invoke_exception] or
     /// [`exceptions::ExceptionCtor`][crate::exceptions::ExceptionCtor] instead.
-    pub fn new_exception_msg(&self, exc_type: PyTypeRef, msg: String) -> PyBaseExceptionRef {
+    pub fn new_exception_msg(&self, exc_type: PyTypeRef, msg: Wtf8Buf) -> PyBaseExceptionRef {
         self.new_exception(exc_type, vec![self.ctx.new_str(msg).into()])
     }
 
@@ -162,7 +163,7 @@ impl VirtualMachine {
     pub fn new_exception_msg_dict(
         &self,
         exc_type: PyTypeRef,
-        msg: String,
+        msg: Wtf8Buf,
         dict: PyDictRef,
     ) -> PyBaseExceptionRef {
         PyRef::new_ref(
@@ -189,7 +190,7 @@ impl VirtualMachine {
         attribute_error
     }
 
-    pub fn new_name_error(&self, msg: impl Into<String>, name: PyStrRef) -> PyBaseExceptionRef {
+    pub fn new_name_error(&self, msg: impl Into<Wtf8Buf>, name: PyStrRef) -> PyBaseExceptionRef {
         let name_error_type = self.ctx.exceptions.name_error.to_owned();
         let name_error = self.new_exception_msg(name_error_type, msg.into());
         name_error.as_object().set_attr("name", name, self).unwrap();
@@ -619,7 +620,7 @@ impl VirtualMachine {
         if syntax_error_type.is(self.ctx.exceptions.tab_error) {
             msg = "inconsistent use of tabs and spaces in indentation".to_owned();
         }
-        let syntax_error = self.new_exception_msg(syntax_error_type, msg);
+        let syntax_error = self.new_exception_msg(syntax_error_type, msg.into());
         let (lineno, offset) = error.python_location();
         let lineno = self.ctx.new_int(lineno);
         let offset = self.ctx.new_int(offset);
@@ -689,10 +690,14 @@ impl VirtualMachine {
         self.new_syntax_error_maybe_incomplete(error, source, false)
     }
 
-    pub fn new_import_error(&self, msg: impl Into<String>, name: PyStrRef) -> PyBaseExceptionRef {
+    pub fn new_import_error(
+        &self,
+        msg: impl Into<Wtf8Buf>,
+        name: impl Into<PyStrRef>,
+    ) -> PyBaseExceptionRef {
         let import_error = self.ctx.exceptions.import_error.to_owned();
         let exc = self.new_exception_msg(import_error, msg.into());
-        exc.as_object().set_attr("name", name, self).unwrap();
+        exc.as_object().set_attr("name", name.into(), self).unwrap();
         exc
     }
 
@@ -733,7 +738,7 @@ impl VirtualMachine {
         } else {
             msg
         };
-        self.new_exception_msg(error_type.to_owned(), msg)
+        self.new_exception_msg(error_type.to_owned(), msg.into())
     }
 
     pub(crate) fn new_downcast_runtime_error(
diff --git a/crates/vm/src/warn.rs b/crates/vm/src/warn.rs
index 684630e6af0..7cefed3b1ae 100644
--- a/crates/vm/src/warn.rs
+++ b/crates/vm/src/warn.rs
@@ -392,23 +392,24 @@ pub(crate) fn warn_explicit(
     let action_str = PyStrRef::try_from_object(vm, action)
         .map_err(|_| vm.new_type_error("action must be a string".to_owned()))?;
 
-    if action_str.as_str() == "error" {
+    if action_str.as_bytes() == b"error" {
         let exc = PyBaseExceptionRef::try_from_object(vm, message)?;
         return Err(exc);
     }
-    if action_str.as_str() == "ignore" {
+    if action_str.as_bytes() == b"ignore" {
         return Ok(());
     }
 
     // For everything except "always"/"all", record in registry then
     // check per-action registries.
-    let already = if action_str.as_str() != "always" && action_str.as_str() != "all" {
+    let already = if action_str.as_wtf8() != "always" && action_str.as_wtf8() != "all" {
         if !vm.is_none(&registry) {
             registry.set_item(&*key, vm.ctx.true_value.clone().into(), vm)?;
         }
 
-        match action_str.as_str() {
-            "once" => {
+        let action_s = action_str.to_str();
+        match action_s {
+            Some("once") => {
                 let reg = if vm.is_none(&registry) {
                     get_once_registry(vm)?
                 } else {
@@ -416,17 +417,17 @@ pub(crate) fn warn_explicit(
                 };
                 update_registry(&reg, text.as_ref(), category.as_object(), false, vm)?
             }
-            "module" => {
+            Some("module") => {
                 if !vm.is_none(&registry) {
                     update_registry(&registry, text.as_ref(), category.as_object(), false, vm)?
                 } else {
                     false
                 }
             }
-            "default" => false,
-            other => {
+            Some("default") => false,
+            _ => {
                 return Err(vm.new_runtime_error(format!(
-                    "Unrecognized action ({other}) in warnings.filters:\n {other}"
+                    "Unrecognized action ({action_str}) in warnings.filters:\n {action_str}"
                 )));
             }
         }
@@ -517,11 +518,11 @@ fn show_warning(
 /// Check if a frame's filename starts with any of the given prefixes.
 fn is_filename_to_skip(frame: &crate::frame::Frame, prefixes: &PyTupleRef) -> bool {
     let filename = frame.f_code().co_filename();
-    let filename_s = filename.as_str();
+    let filename_bytes = filename.as_bytes();
     prefixes.iter().any(|prefix| {
         prefix
             .downcast_ref::<PyStr>()
-            .is_some_and(|s| filename_s.starts_with(s.as_str()))
+            .is_some_and(|s| filename_bytes.starts_with(s.as_bytes()))
     })
 }
 
diff --git a/crates/wasm/src/browser_module.rs b/crates/wasm/src/browser_module.rs
index d1eecce28a9..3ec319e10d0 100644
--- a/crates/wasm/src/browser_module.rs
+++ b/crates/wasm/src/browser_module.rs
@@ -64,22 +64,19 @@ mod _browser {
         } = args;
 
         let response_format = match response_format {
-            Some(s) => FetchResponseFormat::from_str(vm, s.as_str())?,
+            Some(s) => FetchResponseFormat::from_str(vm, s.expect_str())?,
             None => FetchResponseFormat::Text,
         };
 
         let opts = web_sys::RequestInit::new();
 
-        match method {
-            Some(s) => opts.set_method(s.as_str()),
-            None => opts.set_method("GET"),
-        };
+        opts.set_method(method.as_ref().map_or("GET", |s| s.expect_str()));
 
         if let Some(body) = body {
             opts.set_body(&convert::py_to_js(vm, body));
         }
 
-        let request = web_sys::Request::new_with_str_and_init(url.as_str(), &opts)
+        let request = web_sys::Request::new_with_str_and_init(url.expect_str(), &opts)
             .map_err(|err| convert::js_py_typeerror(vm, err))?;
 
         if let Some(headers) = headers {
@@ -87,7 +84,7 @@ mod _browser {
             for (key, value) in headers {
                 let key = key.str(vm)?;
                 let value = value.str(vm)?;
-                h.set(key.as_str(), value.as_str())
+                h.set(key.expect_str(), value.expect_str())
                     .map_err(|err| convert::js_py_typeerror(vm, err))?;
             }
         }
@@ -95,7 +92,7 @@ mod _browser {
         if let Some(content_type) = content_type {
             request
                 .headers()
-                .set("Content-Type", content_type.as_str())
+                .set("Content-Type", content_type.expect_str())
                 .map_err(|err| convert::js_py_typeerror(vm, err))?;
         }
 
@@ -171,7 +168,7 @@ mod _browser {
         fn query(&self, query: PyStrRef, vm: &VirtualMachine) -> PyResult {
             let elem = self
                 .doc
-                .query_selector(query.as_str())
+                .query_selector(query.expect_str())
                 .map_err(|err| convert::js_py_typeerror(vm, err))?
                 .map(|elem| Element { elem })
                 .to_pyobject(vm);
@@ -206,7 +203,7 @@ mod _browser {
             default: OptionalArg<PyObjectRef>,
             vm: &VirtualMachine,
         ) -> PyObjectRef {
-            match self.elem.get_attribute(attr.as_str()) {
+            match self.elem.get_attribute(attr.expect_str()) {
                 Some(s) => vm.ctx.new_str(s).into(),
                 None => default.unwrap_or_none(vm),
             }
@@ -215,7 +212,7 @@ mod _browser {
         #[pymethod]
         fn set_attr(&self, attr: PyStrRef, value: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
             self.elem
-                .set_attribute(attr.as_str(), value.as_str())
+                .set_attribute(attr.expect_str(), value.expect_str())
                 .map_err(|err| convert::js_py_typeerror(vm, err))
         }
     }
@@ -227,7 +224,7 @@ mod _browser {
         let opts = web_sys::RequestInit::new();
         opts.set_method("GET");
 
-        let request = web_sys::Request::new_with_str_and_init(path.as_str(), &opts)
+        let request = web_sys::Request::new_with_str_and_init(path.expect_str(), &opts)
             .map_err(|err| convert::js_py_typeerror(vm, err))?;
 
         let window = window();
@@ -244,7 +241,7 @@ mod _browser {
                 .expect("that the vm is valid when the promise resolves");
             stored_vm.interp.enter(move |vm| {
                 let resp_text = text.as_string().unwrap();
-                let res = import_source(vm, module.as_str(), &resp_text);
+                let res = import_source(vm, module.expect_str(), &resp_text);
                 match res {
                     Ok(_) => Ok(JsValue::null()),
                     Err(err) => Err(convert::py_err_to_js_err(vm, &err)),
diff --git a/crates/wasm/src/convert.rs b/crates/wasm/src/convert.rs
index f84b0d46239..a0186ce2834 100644
--- a/crates/wasm/src/convert.rs
+++ b/crates/wasm/src/convert.rs
@@ -56,7 +56,7 @@ pub fn py_err_to_js_err(vm: &VirtualMachine, py_err: &Py<PyBaseException>) -> Js
 }
 
 pub fn js_py_typeerror(vm: &VirtualMachine, js_err: JsValue) -> PyBaseExceptionRef {
-    let msg = js_err.unchecked_into::<js_sys::Error>().to_string();
+    let msg: String = js_err.unchecked_into::<js_sys::Error>().to_string().into();
     vm.new_type_error(msg)
 }
 
@@ -70,11 +70,11 @@ pub fn js_err_to_py_err(vm: &VirtualMachine, js_err: &JsValue) -> PyBaseExceptio
                 _ => vm.ctx.exceptions.exception_type,
             }
             .to_owned();
-            vm.new_exception_msg(exc_type, err.message().into())
+            vm.new_exception_msg(exc_type, String::from(err.message()).into())
         }
         None => vm.new_exception_msg(
             vm.ctx.exceptions.exception_type.to_owned(),
-            format!("{js_err:?}"),
+            format!("{js_err:?}").into(),
         ),
     }
 }
diff --git a/crates/wasm/src/js_module.rs b/crates/wasm/src/js_module.rs
index 750e85994a1..314e2adbee0 100644
--- a/crates/wasm/src/js_module.rs
+++ b/crates/wasm/src/js_module.rs
@@ -83,7 +83,7 @@ mod _js {
     impl JsProperty {
         fn into_js_value(self) -> JsValue {
             match self {
-                JsProperty::Str(s) => s.as_str().into(),
+                JsProperty::Str(s) => s.expect_str().into(),
                 JsProperty::Js(value) => value.value.clone(),
             }
         }
@@ -109,7 +109,7 @@ mod _js {
 
         #[pymethod]
         fn new_from_str(&self, s: PyStrRef) -> PyJsValue {
-            PyJsValue::new(s.as_str())
+            PyJsValue::new(s.expect_str())
         }
 
         #[pymethod]
diff --git a/crates/wasm/src/wasm_builtins.rs b/crates/wasm/src/wasm_builtins.rs
index 79423fc250e..efbc03c39ce 100644
--- a/crates/wasm/src/wasm_builtins.rs
+++ b/crates/wasm/src/wasm_builtins.rs
@@ -33,7 +33,7 @@ pub fn make_stdout_object(
         "write",
         cls,
         move |_self: PyObjectRef, data: PyStrRef, vm: &VirtualMachine| -> PyResult<()> {
-            write_f(data.as_str(), vm)
+            write_f(data.expect_str(), vm)
         },
     );
     let flush_method = vm.new_method("flush", cls, |_self: PyObjectRef| {});
diff --git a/crates/wtf8/src/concat.rs b/crates/wtf8/src/concat.rs
new file mode 100644
index 00000000000..055a401d69f
--- /dev/null
+++ b/crates/wtf8/src/concat.rs
@@ -0,0 +1,143 @@
+use alloc::borrow::{Cow, ToOwned};
+use alloc::boxed::Box;
+use alloc::string::String;
+use core::fmt;
+use fmt::Write;
+
+use crate::{CodePoint, Wtf8, Wtf8Buf};
+
+impl fmt::Write for Wtf8Buf {
+    #[inline]
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.push_str(s);
+        Ok(())
+    }
+}
+
+/// Trait for types that can be appended to a [`Wtf8Buf`], preserving surrogates.
+pub trait Wtf8Concat {
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf);
+}
+
+impl Wtf8Concat for Wtf8 {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_wtf8(self);
+    }
+}
+
+impl Wtf8Concat for Wtf8Buf {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_wtf8(self);
+    }
+}
+
+impl Wtf8Concat for str {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_str(self);
+    }
+}
+
+impl Wtf8Concat for String {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_str(self);
+    }
+}
+
+impl Wtf8Concat for char {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push_char(*self);
+    }
+}
+
+impl Wtf8Concat for CodePoint {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        buf.push(*self);
+    }
+}
+
+/// Wrapper that appends a [`fmt::Display`] value to a [`Wtf8Buf`].
+///
+/// Note: This goes through UTF-8 formatting, so lone surrogates in the
+/// display output will be replaced with U+FFFD. Use direct [`Wtf8Concat`]
+/// impls for surrogate-preserving concatenation.
+#[allow(dead_code)]
+pub struct DisplayAsWtf8<T>(pub T);
+
+impl<T: fmt::Display> Wtf8Concat for DisplayAsWtf8<T> {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        write!(buf, "{}", self.0).unwrap();
+    }
+}
+
+macro_rules! impl_wtf8_concat_for_int {
+    ($($t:ty),*) => {
+        $(impl Wtf8Concat for $t {
+            #[inline]
+            fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+                write!(buf, "{}", self).unwrap();
+            }
+        })*
+    };
+}
+
+impl_wtf8_concat_for_int!(
+    u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize, f32, f64
+);
+
+impl<T: Wtf8Concat + ?Sized> Wtf8Concat for &T {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        (**self).fmt_wtf8(buf);
+    }
+}
+
+impl<T: Wtf8Concat + ?Sized> Wtf8Concat for &mut T {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        (**self).fmt_wtf8(buf);
+    }
+}
+
+impl<T: Wtf8Concat + ?Sized> Wtf8Concat for Box<T> {
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        (**self).fmt_wtf8(buf);
+    }
+}
+
+impl<T: Wtf8Concat + ?Sized> Wtf8Concat for Cow<'_, T>
+where
+    T: ToOwned,
+{
+    #[inline]
+    fn fmt_wtf8(&self, buf: &mut Wtf8Buf) {
+        (**self).fmt_wtf8(buf);
+    }
+}
+
+/// Concatenate values into a [`Wtf8Buf`], preserving surrogates.
+///
+/// Each argument must implement [`Wtf8Concat`]. String literals (`&str`),
+/// [`Wtf8`], [`Wtf8Buf`], [`char`], and [`CodePoint`] are all supported.
+///
+/// ```
+/// use rustpython_wtf8::Wtf8Buf;
+/// let name = "world";
+/// let result = rustpython_wtf8::wtf8_concat!("hello, ", name, "!");
+/// assert_eq!(result, Wtf8Buf::from("hello, world!"));
+/// ```
+#[macro_export]
+macro_rules! wtf8_concat {
+    ($($arg:expr),* $(,)?) => {{
+        let mut buf = $crate::Wtf8Buf::new();
+        $($crate::Wtf8Concat::fmt_wtf8(&$arg, &mut buf);)*
+        buf
+    }};
+}
diff --git a/crates/wtf8/src/lib.rs b/crates/wtf8/src/lib.rs
index 6614ca83572..772a2879944 100644
--- a/crates/wtf8/src/lib.rs
+++ b/crates/wtf8/src/lib.rs
@@ -330,6 +330,24 @@ impl Wtf8Buf {
         }
     }
 
+    pub fn join<I, S>(sep: impl AsRef<Wtf8>, iter: I) -> Wtf8Buf
+    where
+        I: IntoIterator<Item = S>,
+        S: AsRef<Wtf8>,
+    {
+        let sep = sep.as_ref();
+        let mut iter = iter.into_iter();
+        let mut buf = match iter.next() {
+            Some(first) => first.as_ref().to_owned(),
+            None => return Wtf8Buf::new(),
+        };
+        for part in iter {
+            buf.push_wtf8(sep);
+            buf.push_wtf8(part.as_ref());
+        }
+        buf
+    }
+
     pub fn clear(&mut self) {
         self.bytes.clear();
     }
@@ -979,6 +997,28 @@ impl Wtf8 {
         }
     }
 
+    pub fn to_lowercase(&self) -> Wtf8Buf {
+        let mut buf = Wtf8Buf::with_capacity(self.len());
+        for chunk in self.chunks() {
+            match chunk {
+                Wtf8Chunk::Utf8(s) => buf.push_str(&s.to_lowercase()),
+                Wtf8Chunk::Surrogate(c) => buf.push(c),
+            }
+        }
+        buf
+    }
+
+    pub fn to_uppercase(&self) -> Wtf8Buf {
+        let mut buf = Wtf8Buf::with_capacity(self.len());
+        for chunk in self.chunks() {
+            match chunk {
+                Wtf8Chunk::Utf8(s) => buf.push_str(&s.to_uppercase()),
+                Wtf8Chunk::Surrogate(c) => buf.push(c),
+            }
+        }
+        buf
+    }
+
     #[inline]
     pub const fn is_ascii(&self) -> bool {
         self.bytes.is_ascii()
@@ -1114,23 +1154,23 @@ impl Wtf8 {
         }
     }
 
-    pub fn ends_with(&self, w: &Wtf8) -> bool {
-        self.bytes.ends_with_str(w)
+    pub fn ends_with(&self, w: impl AsRef<Wtf8>) -> bool {
+        self.bytes.ends_with_str(w.as_ref())
     }
 
-    pub fn starts_with(&self, w: &Wtf8) -> bool {
-        self.bytes.starts_with_str(w)
+    pub fn starts_with(&self, w: impl AsRef<Wtf8>) -> bool {
+        self.bytes.starts_with_str(w.as_ref())
     }
 
-    pub fn strip_prefix(&self, w: &Wtf8) -> Option<&Self> {
+    pub fn strip_prefix(&self, w: impl AsRef<Wtf8>) -> Option<&Self> {
         self.bytes
-            .strip_prefix(w.as_bytes())
+            .strip_prefix(w.as_ref().as_bytes())
             .map(|w| unsafe { Wtf8::from_bytes_unchecked(w) })
     }
 
-    pub fn strip_suffix(&self, w: &Wtf8) -> Option<&Self> {
+    pub fn strip_suffix(&self, w: impl AsRef<Wtf8>) -> Option<&Self> {
         self.bytes
-            .strip_suffix(w.as_bytes())
+            .strip_suffix(w.as_ref().as_bytes())
             .map(|w| unsafe { Wtf8::from_bytes_unchecked(w) })
     }
 
@@ -1520,6 +1560,14 @@ impl From<&Wtf8> for Box<Wtf8> {
     }
 }
 
+impl<'a> From<&'a str> for &'a Wtf8 {
+    #[inline]
+    fn from(s: &'a str) -> &'a Wtf8 {
+        // Valid UTF-8 is always valid WTF-8
+        unsafe { Wtf8::from_bytes_unchecked(s.as_bytes()) }
+    }
+}
+
 impl From<&str> for Box<Wtf8> {
     fn from(s: &str) -> Self {
         Box::<str>::from(s).into()
@@ -1561,3 +1609,6 @@ impl From<String> for Box<Wtf8> {
         s.into_boxed_str().into()
     }
 }
+
+mod concat;
+pub use concat::Wtf8Concat;
diff --git a/example_projects/wasm32_without_js/rustpython-without-js/src/lib.rs b/example_projects/wasm32_without_js/rustpython-without-js/src/lib.rs
index 0a8695fd7fb..a152ebe1c4a 100644
--- a/example_projects/wasm32_without_js/rustpython-without-js/src/lib.rs
+++ b/example_projects/wasm32_without_js/rustpython-without-js/src/lib.rs
@@ -25,7 +25,7 @@ pub unsafe extern "C" fn eval(s: *const u8, l: usize) -> i32 {
             Err(_) => return Err(-1), // Python execution error
         };
         let repr_str = match res.repr(vm) {
-            Ok(repr) => repr.as_str().to_string(),
+            Ok(repr) => repr.to_string(),
             Err(_) => return Err(-1), // Failed to get string representation
         };
         Ok(repr_str)
diff --git a/src/lib.rs b/src/lib.rs
index 60b66d83b3d..07bfc45e59f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -33,7 +33,7 @@
 //!     #[pyfunction]
 //!     fn other_thing(s: PyStrRef) -> (String, usize) {
 //!         let new_string = format!("hello from rust, {}!", s);
-//!         let prev_len = s.as_str().len();
+//!         let prev_len = s.byte_len();
 //!         (new_string, prev_len)
 //!     }
 //! }
@@ -135,7 +135,7 @@ __import__("io").TextIOWrapper(
         .downcast()
         .expect("TextIOWrapper.read() should return str");
     eprintln!("running get-pip.py...");
-    vm.run_string(scope, getpip_code.as_str(), "get-pip.py".to_owned())?;
+    vm.run_string(scope, getpip_code.expect_str(), "get-pip.py".to_owned())?;
     Ok(())
 }
 
@@ -143,7 +143,7 @@ fn install_pip(installer: InstallPipMode, scope: Scope, vm: &VirtualMachine) ->
     if !cfg!(feature = "ssl") {
         return Err(vm.new_exception_msg(
             vm.ctx.exceptions.system_error.to_owned(),
-            "install-pip requires rustpython be build with '--features=ssl'".to_owned(),
+            "install-pip requires rustpython be build with '--features=ssl'".into(),
         ));
     }
 
diff --git a/src/shell.rs b/src/shell.rs
index d2e54c490a8..6c75e94572c 100644
--- a/src/shell.rs
+++ b/src/shell.rs
@@ -142,7 +142,7 @@ pub fn run_shell(vm: &VirtualMachine, scope: Scope) -> PyResult<()> {
             .get_attr(prompt_name, vm)
             .and_then(|prompt| prompt.str(vm));
         let prompt = match prompt {
-            Ok(ref s) => s.as_str(),
+            Ok(ref s) => s.expect_str(),
             Err(_) => "",
         };
 
@@ -211,8 +211,10 @@ pub fn run_shell(vm: &VirtualMachine, scope: Scope) -> PyResult<()> {
             }
             #[cfg(unix)]
             ReadlineResult::OsError(num) => {
-                let os_error =
-                    vm.new_exception_msg(vm.ctx.exceptions.os_error.to_owned(), format!("{num:?}"));
+                let os_error = vm.new_exception_msg(
+                    vm.ctx.exceptions.os_error.to_owned(),
+                    format!("{num:?}").into(),
+                );
                 vm.print_exception(os_error);
                 break;
             }
diff --git a/src/shell/helper.rs b/src/shell/helper.rs
index 493dc578469..944e936397d 100644
--- a/src/shell/helper.rs
+++ b/src/shell/helper.rs
@@ -108,7 +108,7 @@ impl<'vm> ShellHelper<'vm> {
             .filter(|res| {
                 res.as_ref()
                     .ok()
-                    .is_none_or(|s| s.as_str().starts_with(word_start))
+                    .is_none_or(|s| s.as_bytes().starts_with(word_start.as_bytes()))
             })
             .collect::<Result<Vec<_>, _>>()
             .ok()?;
@@ -120,7 +120,7 @@ impl<'vm> ShellHelper<'vm> {
             // only the completions that don't start with a '_'
             let no_underscore = all_completions
                 .iter()
-                .filter(|&s| !s.as_str().starts_with('_'))
+                .filter(|&s| !s.as_bytes().starts_with(b"_"))
                 .cloned()
                 .collect::<Vec<_>>();
 
@@ -134,13 +134,13 @@ impl<'vm> ShellHelper<'vm> {
         };
 
         // sort the completions alphabetically
-        completions.sort_by(|a, b| std::cmp::Ord::cmp(a.as_str(), b.as_str()));
+        completions.sort_by(|a, b| a.as_wtf8().cmp(b.as_wtf8()));
 
         Some((
             startpos,
             completions
                 .into_iter()
-                .map(|s| s.as_str().to_owned())
+                .map(|s| s.expect_str().to_owned())
                 .collect(),
         ))
     }