fix(client): support stderr logging without file descriptors

src/mcp/client/stdio.py

@@ -8,8 +8,10 @@
 process nor hang on one.
 """
 
+import io
 import logging
 import os
+import subprocess
 import sys
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager, suppress
@@ -122,11 +124,19 @@ async def stdio_client(
     """
     command = _get_executable_command(server.command)
 
+    stderr = errlog
+    pipe_stderr = False
+    try:
+        errlog.fileno()
+    except (AttributeError, io.UnsupportedOperation, OSError):
+        stderr = subprocess.PIPE
+        pipe_stderr = True
+
     process = await _create_platform_compatible_process(
         command=command,
         args=server.args,
         env=get_default_environment() | (server.env or {}),
-        errlog=errlog,
+        errlog=stderr,
         cwd=server.cwd,
     )
 
@@ -181,6 +191,16 @@ async def stdin_writer() -> None:
         finally:
             writer_done.set()
 
+    async def stderr_reader() -> None:
+        process_stderr = getattr(process, "stderr", None)
+        if process_stderr is None or not pipe_stderr:
+            return
+        with suppress(anyio.EndOfStream, anyio.ClosedResourceError, anyio.BrokenResourceError, OSError):
+            while True:
+                chunk = await process_stderr.receive()
+                errlog.write(chunk.decode(server.encoding, errors=server.encoding_error_handler))
+                errlog.flush()
+
     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 +220,7 @@ 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 +285,9 @@ 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)
+    process_stderr = getattr(process, "stderr", None)
+    if process_stderr is not None:
+        await _close_pipe(process_stderr)
     _close_subprocess_transport(process)
 
 
@@ -329,7 +353,7 @@ async def _create_platform_compatible_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO = sys.stderr,
+    errlog: TextIO | int = sys.stderr,
     cwd: Path | str | None = None,
 ) -> ServerProcess:
     """Spawns the server in its own kill scope.

src/mcp/os/win32/utilities.py

@@ -137,7 +137,7 @@ async def create_windows_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO | None = sys.stderr,
+    errlog: TextIO | int | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> Process | FallbackProcess:
     """Creates a subprocess with Job Object support for tree termination.
@@ -177,7 +177,7 @@ async def _create_windows_fallback_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO | None = sys.stderr,
+    errlog: TextIO | int | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> FallbackProcess:
     """Spawns via subprocess.Popen and wraps it in FallbackProcess."""

tests/client/test_stdio.py

@@ -9,10 +9,12 @@
 
 import errno
 import gc
+import io
 import logging
 import math
 import os
 import signal
+import subprocess
 import sys
 from collections.abc import Callable
 from contextlib import AsyncExitStack, suppress
@@ -222,6 +224,30 @@ async def fake_terminate_tree(proc: FakeProcess) -> None:
 FAKE_PARAMS = StdioServerParameters(command="fake-server")
 
 
+@pytest.mark.anyio
+async def test_stdio_client_pipes_stderr_for_non_file_errlog(monkeypatch: pytest.MonkeyPatch) -> None:
+    process = FakeProcess(on_stdin_close=lambda: process.exit(0))
+    seen_errlog: list[object] = []
+
+    async def fake_spawn(
+        command: str,
+        args: list[str],
+        env: dict[str, str] | None = None,
+        errlog: TextIO = sys.stderr,
+        cwd: Path | str | None = None,
+    ) -> FakeProcess:
+        seen_errlog.append(errlog)
+        return process
+
+    monkeypatch.setattr(stdio, "_create_platform_compatible_process", fake_spawn)
+    monkeypatch.setattr(stdio, "_terminate_process_tree", lambda proc: proc.exit(-15))
+
+    async with stdio_client(FAKE_PARAMS, errlog=io.StringIO()):
+        pass
+
+    assert seen_errlog == [subprocess.PIPE]
+
+
 def _line(message: JSONRPCMessage) -> bytes:
     """The wire form of `message`: one JSON document on its own line."""
     return (message.model_dump_json(by_alias=True, exclude_unset=True) + "\n").encode()