stdio_client with strict decoding withholds fatal read errors until peer stdout EOF #3714

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

Explicit encoding_error_handler="strict" decoding of invalid UTF-8 leaves the stdio consumer waiting until the peer closes stdout.

#2454 requests changing default decoding policy. This reproduction explicitly selects strict decoding, so changing the default to replacement would leave this failure-propagation case intact. Termination should become observable without requiring peer exit.

stdout_reader() enters its finally drain before closing the read-stream writer. _drain_stdout() waits for further raw bytes under a cancellation shield. A peer retaining stdout therefore blocks both stream closure and propagation of UnicodeDecodeError.

Example Code

Self-contained failing test reusing the module's existing in-memory FakeProcess and spawn monkeypatch. A byte peer is necessary because a typed server cannot emit malformed UTF-8. Test-only branch; no production changes.

After checking out that branch:

uv run --frozen pytest tests/client/test_stdio.py::test_strict_decode_failure_reaches_consumer_before_peer_stdout_closes -q

Event barriers prove consumption of malformed bytes. After tasks settle, the consumer has not failed. Releasing EOF then surfaces UnicodeDecodeError and completes clean shutdown. The explicit delayed-error assertion fails, without a timeout.

Python & MCP Python SDK

Python 3.14.7, Linux x86_64. Latest stable 2.3.0 (2118f14f8a19bc158d8a1cf90af58d85d187f849) and main 754254c08fe1c3eda18b69da3d336106fe8d72d6: three repetitions on asyncio and Trio all fail identically. The other 35 stdio tests pass against each revision.

AI-assisted investigation and test reproduction, with a human in the loop.