Runs one ethlambda consensus node paired with one ethrex execution node over the Engine API. Every slot, ethlambda builds a block, asks ethrex to produce the execution payload (engine_forkchoiceUpdatedV3 + getPayloadV4), embeds it in the Lean block, and ethrex imports it (newPayloadV4). The chain advances and finalizes on both layers.

Single validator → finalizes solo. Good for a quick local demo.

• ethrex on PATH (v15+), or ETHREX=/path/to/ethrex.
• A dual-key (devnet5+) lean genesis bundle. By default the script looks in lean-quickstart/local-devnet/genesis. Generate one with:

  cd lean-quickstart && ./generate-genesis.sh local-devnet/genesis

  (needs the lean-quickstart main branch and Docker). To pay block rewards to a real address, add to validator-config.yaml's config block:

  suggested_fee_recipient: "0x00000000000000000000000000000000deadbeef"

• cargo (the script builds ethlambda in release) unless SKIP_BUILD=1.
• jq (optional, for the demo commands below).

scripts/engine-api-demo/run.sh          # build + start ethrex and ethlambda
scripts/engine-api-demo/run.sh stop     # stop both

Configurable via env vars (see the header of run.sh): ETHREX, LEAN_GENESIS_DIR, DATA_DIR, GENESIS_OFFSET, the four ports, SKIP_BUILD.

1. EL importing CL-built payloads — chain climbing, miner = the configured suggested_fee_recipient:

   curl -s -X POST http://127.0.0.1:8545 -H 'content-type: application/json' \
     --data '{"jsonrpc":"2.0","id":1,"method":"eth_getBlockByNumber","params":["latest",false]}' \
     | jq '.result | {number, hash, miner}'

2. ethlambda fork-choice tree (browser): http://127.0.0.1:5052/lean/v0/fork_choice/ui
3. Both layers in lockstep:

   tail -f "$DATA_DIR/ethlambda.log" | grep -E 'proposer|finalized|head updated'
   tail -f "$DATA_DIR/ethrex.log"    | grep -E 'BLOCK|executed|Fork choice'

The round-trip invariant: ethrex's FCU head equals ethlambda's block.body.execution_payload.block_hash.

ethrex builds payloads from its mempool, so anything submitted to its HTTP-RPC lands in the next slot's payload — and therefore inside the Lean block. The genesis prefunds the well-known hardhat/anvil dev account #0 (0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266, 10k ETH), and send-txs.sh signs EIP-1559 self-transfers from it (via uv run --with eth-account; no permanent install):

scripts/engine-api-demo/send-txs.sh 5      # sign + submit 5 transfers

One slot later (~4s), show the full tx → mempool → payload → Lean block → execution round-trip:

# The receipt: executed on the EL (status 0x1, note the block number N)
curl -s -X POST http://127.0.0.1:8545 -H 'content-type: application/json' \
  --data '{"jsonrpc":"2.0","id":1,"method":"eth_getTransactionReceipt","params":["<tx hash>"]}' \
  | jq '.result | {status, blockNumber}'

# The same transactions, raw, inside the Lean block's execution payload at slot N
curl -s http://127.0.0.1:5052/lean/v0/blocks/<N> \
  | jq '.body.execution_payload | {blockNumber, gasUsed, transactions}'

Override RPC_URL / KEY via env to use a different endpoint or sender.

• Fork level. ethlambda pins the V4 (Prague) Engine methods — the pre-Amsterdam, no-BAL path. Current ethrex's newPayloadV5 requires the EIP-7928 block-access-list (Amsterdam, off by default), so the EL genesis here stops at Prague. Fork-aware version selection (and V5/BAL support) is a future refinement.
• run.sh re-stamps GENESIS_TIME in the lean config.yaml on each run.
• Logs and data live under DATA_DIR (default $TMPDIR/ethlambda-el-demo).