fix: forward stdio server stderr through errlog

src/mcp/client/stdio.py

@@ -10,6 +10,7 @@
 
 import logging
 import os
+import subprocess
 import sys
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager, suppress
@@ -181,6 +182,21 @@ async def stdin_writer() -> None:
         finally:
             writer_done.set()
 
+    async def stderr_reader() -> None:
+        stderr = getattr(process, "stderr", None)
+        if stderr is None:
+            return
+
+        try:
+            async for chunk in TextReceiveStream(
+                stderr, encoding=server.encoding, errors=server.encoding_error_handler
+            ):
+                errlog.write(chunk)
+                errlog.flush()
+        except (anyio.ClosedResourceError, anyio.BrokenResourceError, ConnectionError, OSError):
+            if not shutting_down:  # pragma: no branch
+                logger.exception("Reading from the MCP server's stderr failed mid-session")
+
     async def shutdown() -> None:
         """Winds the transport down: stop traffic, flush, stop the server, release the streams."""
         # Unblock the reader into its drain: a server stuck writing stdout cannot
@@ -200,6 +216,7 @@ async def shutdown() -> None:
     async with anyio.create_task_group() as tg:
         tg.start_soon(stdout_reader)
         tg.start_soon(stdin_writer)
+        tg.start_soon(stderr_reader)
         try:
             yield read_stream, write_stream
         finally:
@@ -264,6 +281,9 @@ async def _stop_server_process(process: ServerProcess) -> None:
     close_process_job(process)
     # A kill survivor can hold the stdout pipe open; poison the reader anyway.
     await _close_pipe(process.stdout)
+    stderr = getattr(process, "stderr", None)
+    if stderr is not None:
+        await _close_pipe(stderr)
     _close_subprocess_transport(process)
 
 
@@ -342,7 +362,7 @@ async def _create_platform_compatible_process(
         return await anyio.open_process(
             [command, *args],
             env=env,
-            stderr=errlog,
+            stderr=subprocess.PIPE,
             cwd=cwd,
             start_new_session=True,
         )

tests/client/test_stdio.py

@@ -9,6 +9,7 @@
 
 import errno
 import gc
+import io
 import logging
 import math
 import os
@@ -25,7 +26,7 @@
 import pytest
 import trio
 import trio.testing
-from anyio.streams.memory import MemoryObjectReceiveStream
+from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 
 from mcp.client import stdio
 from mcp.client._transport import ReadStream
@@ -136,7 +137,9 @@ def __init__(
         stdin_send_gate: anyio.Event | None = None,
         stdout_eof_error: Exception | None = None,
         stdout_aclose_error: Exception | None = None,
+        stderr_eof_error: Exception | None = None,
         on_stdout_receive: Callable[[], None] | None = None,
+        with_stderr: bool = False,
     ) -> None:
         self._stdout_send, stdout_receive = anyio.create_memory_object_stream[bytes](math.inf)
         self.stdout = _FakeStdout(
@@ -145,6 +148,16 @@ def __init__(
             aclose_error=stdout_aclose_error,
             on_receive=self._dispatch_stdout_receive,
         )
+        self._stderr_send: MemoryObjectSendStream[bytes] | None = None
+        self.stderr: _FakeStdout | None = None
+        if with_stderr:
+            self._stderr_send, stderr_receive = anyio.create_memory_object_stream[bytes](math.inf)
+            self.stderr = _FakeStdout(
+                stderr_receive,
+                eof_error=stderr_eof_error,
+                aclose_error=None,
+                on_receive=lambda: None,
+            )
         self.pid = 424242
         self.written: list[bytes] = []
         self.stdin_closed = anyio.Event()
@@ -170,10 +183,21 @@ def close_stdout(self) -> None:
         """End the fake process's stdout, as the kernel does when it dies."""
         self._stdout_send.close()
 
+    async def feed_stderr(self, data: bytes) -> None:
+        """Make `data` readable on the fake process's stderr."""
+        assert self._stderr_send is not None
+        await self._stderr_send.send(data)
+
+    def close_stderr(self) -> None:
+        """End the fake process's stderr, as the kernel does when it dies."""
+        if self._stderr_send is not None:
+            self._stderr_send.close()
+
     def exit(self, code: int = 0) -> None:
         """Die: set the exit code and EOF stdout, as the kernel does."""
         self.returncode = code
         self.close_stdout()
+        self.close_stderr()
 
     def pending_stdout_chunks(self) -> int:
         """How many fed chunks the client has not yet pulled off the fake stdout."""
@@ -225,6 +249,42 @@ async def _next_message(read_stream: ReadStream[SessionMessage | Exception]) ->
     return received.message
 
 
+@pytest.mark.anyio
+async def test_server_stderr_is_forwarded_to_errlog(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Server stderr is piped and forwarded, so notebook-like errlogs still see it."""
+    process = FakeProcess(with_stderr=True)
+    install_fake_process(monkeypatch, process)
+    errlog = io.StringIO()
+
+    async with stdio_client(FAKE_PARAMS, errlog=errlog):
+        await process.feed_stderr(b"starting server\n")
+        with anyio.fail_after(1):
+            while errlog.getvalue() != "starting server\n":
+                await anyio.sleep(0)
+        process.exit()
+
+
+@pytest.mark.anyio
+async def test_mid_session_stderr_failure_is_logged(
+    monkeypatch: pytest.MonkeyPatch,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """A broken stderr pipe is logged, but does not escape the client context."""
+    process = FakeProcess(
+        on_stdin_close=lambda: process.exit(0),
+        stderr_eof_error=ConnectionError("stderr pipe failed"),
+        with_stderr=True,
+    )
+    install_fake_process(monkeypatch, process)
+
+    with anyio.fail_after(5):
+        async with stdio_client(FAKE_PARAMS):
+            process.close_stderr()
+            with anyio.fail_after(1):  # pragma: no branch
+                while "stderr failed mid-session" not in caplog.text:
+                    await anyio.sleep(0)
+
+
 @pytest.mark.anyio
 async def test_messages_split_and_packed_across_chunks_are_reframed(monkeypatch: pytest.MonkeyPatch) -> None:
     """Framing survives arbitrary chunk boundaries.