nemo-relay-plugin is the Python authoring SDK for NeMo Relay out-of-process dynamic worker plugins. Use it when plugin code should run in its own Python process and communicate with Relay through the versioned grpc-v1 worker protocol.

Relay 0.8 establishes canonical tool results as the grpc-v1 baseline. The protocol name remains grpc-v1, while generated ToolNext responses and tool execution outcomes use structural protobuf ToolExecutionResult and ToolExecutionInterceptOutcome messages instead of schema-tagged JSON envelopes. Workers and custom bindings built for an earlier Relay release must regenerate their protobuf bindings, rebuild with this SDK, and declare a compat.relay range beginning at 0.8.0 or later.

• Isolate plugin dependencies: Run custom policy, middleware, or exporter code outside the Relay host process.
• Use the shared runtime contract: Register subscribers, guardrails, and intercepts through WorkerPlugin and PluginContext.
• Call back into Relay safely: Emit marks, create scopes, and continue managed execution through the host runtime handle.
• Keep worker lifecycle managed: Let Relay provision the worker environment, start the entrypoint, and supply authenticated local endpoints.

• WorkerPlugin and PluginContext: The plugin validation and registration contract for worker-owned runtime behavior.
• serve_plugin: An AsyncIO gRPC server wired to the Relay-managed worker environment.
• Typed runtime helpers: JSON, event, scope, middleware, continuation, and diagnostic types shared with Relay.
• Canonical tool results: ToolNext.call() returns ToolExecutionResult, preserving opaque annotations separately from application result JSON.
• Generated transport bindings: Private protobuf bindings included in built wheels; published-wheel installation does not require protoc or grpcio-tools.

Add the SDK to the Python worker project's dependencies:

uv add nemo-relay-plugin

If you are not using uv, install it with pip:

pip install nemo-relay-plugin

Declare a module:function entrypoint that starts the worker with serve_plugin. Register the plugin manifest through the CLI; Relay creates a per-plugin virtual environment, installs source.manifest_root, and records that environment for activation:

nemo-relay plugins add ./relay-plugin.toml
nemo-relay plugins enable <plugin_id>

Python workers cannot be loaded directly from plugins.toml. They must be registered through plugins add, which provisions the required managed environment. plugins remove <plugin_id> deletes that environment.

A minimal worker plugin looks like this:

from nemo_relay_plugin import Json, PluginContext, WorkerPlugin, serve_plugin


class PolicyPlugin(WorkerPlugin):
    plugin_id = "acme.policy"

    def register(self, ctx: PluginContext, config: Json) -> None:
        async def tag_tool_request(tool_name: str, args: Json) -> Json:
            await ctx.runtime.emit_mark("acme.policy.tool_request", {"tool_name": tool_name})
            if isinstance(args, dict):
                return {**args, "policy": "checked"}
            return {"value": args, "policy": "checked"}

        ctx.register_tool_request_intercept("tag_tool_request", tag_tool_request)


async def main() -> None:
    await serve_plugin(PolicyPlugin())

Set load.entrypoint to your_module:main in relay-plugin.toml. Relay imports that function and awaits the returned coroutine when it starts the worker process.

For a complete manifest and runnable plugin, see the Python gRPC worker plugin example.

LLM request intercepts return one canonical outcome:

from nemo_relay_plugin import LlmRequestInterceptOutcome, PendingMarkSpec


def intercept(model_name, request, annotated):
    del model_name
    headers = {**request.get("headers", {}), "x-policy": "checked"}
    return LlmRequestInterceptOutcome(
        request={**request, "headers": headers},
        annotated_request=annotated,
        pending_marks=[PendingMarkSpec("acme.policy.checked")],
    )

When annotated is present, it is authoritative for provider-body content: leave raw request["content"] unchanged, edit normalized fields or provider extensions through the annotation, and use request["headers"] for transport headers.

The gRPC AsyncIO server can keep multiple RPCs in flight. Callback execution is cooperative: asynchronous callbacks overlap only when they yield control at an await. Synchronous callbacks and synchronous stream iterators run on the worker event-loop thread. Blocking I/O, time.sleep, or long-running CPU work in those callbacks stalls all worker RPCs. Wrap blocking work in an asynchronous callback and offload it with asyncio.to_thread() or another appropriate executor.

The SDK does not configure maximum_concurrent_rpcs, so gRPC does not enforce an application-level RPC admission limit.

Relay assigns every unary and streaming callback an invocation ID. The host sends CancelInvocation when its managed caller is cancelled, its worker RPC times out, or it stops consuming a worker-backed stream. The SDK cancels the matching asyncio.Task and reports a structured worker.cancelled result.

Cancellation is idempotent. The first request that matches an active callback returns accepted = true; requests for unknown, completed, or already cancelled IDs return accepted = false. Treat acceptance as confirmation that the SDK found and cancelled the task, not as proof that arbitrary user code has stopped.

Python task cancellation is cooperative. Async callbacks should allow asyncio.CancelledError to propagate and use try/finally for cleanup. Synchronous callbacks run on the event-loop thread and cannot be preempted by task cancellation. A blocking synchronous callback can delay both the cancellation RPC and all other worker RPCs, so offload blocking work and make its own cancellation behavior explicit.

grpc-v1 workers are expected to implement this best-effort cancellation contract. When a worker returns accepted = false, Relay still drops the transport request, but it cannot guarantee worker-side interruption.

Windows ARM64 is not currently supported because grpcio does not publish a usable wheel for that platform. The NeMo Relay workspace skips installation and tests for this SDK on Windows ARM64 rather than creating a package without its required gRPC runtime.

• NeMo Relay documentation
• Build Plugins guide
• Python gRPC worker plugin example