Fix Windows stdio client issues: wrap stderr in FallbackProcess, correct PIPE usage, fix pyright errors

Author: Ashutosh0x

src/mcp/client/stdio.py

@@ -1,6 +1,9 @@
+from __future__ import annotations
+
 import logging
 import os
 import sys
+import subprocess
 from contextlib import asynccontextmanager
 from pathlib import Path
 from typing import Literal, TextIO
@@ -20,8 +23,8 @@
     get_windows_executable_command,
     terminate_windows_process_tree,
 )
-from mcp.shared.message import SessionMessage
 from mcp.shared.jupyter import is_jupyter
+from mcp.shared.message import SessionMessage
 
 logger = logging.getLogger(__name__)
 
@@ -123,7 +126,7 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
         # because sys.stderr redirection might not be reliable for subprocesses.
         actual_errlog = errlog
         if is_jupyter():
-            actual_errlog = anyio.lowlevel.PIPE
+            actual_errlog = subprocess.PIPE
 
         # Open process with stderr piped for capture
         process = await _create_platform_compatible_process(

src/mcp/os/win32/utilities.py

@@ -75,10 +75,11 @@ def __init__(self, popen_obj: subprocess.Popen[bytes]):
         self.popen: subprocess.Popen[bytes] = popen_obj
         self.stdin_raw = popen_obj.stdin  # type: ignore[assignment]
         self.stdout_raw = popen_obj.stdout  # type: ignore[assignment]
-        self.stderr = popen_obj.stderr  # type: ignore[assignment]
+        self.stderr_raw = popen_obj.stderr  # type: ignore[assignment]
 
         self.stdin = FileWriteStream(cast(BinaryIO, self.stdin_raw)) if self.stdin_raw else None
         self.stdout = FileReadStream(cast(BinaryIO, self.stdout_raw)) if self.stdout_raw else None
+        self.stderr = FileReadStream(cast(BinaryIO, self.stderr_raw)) if self.stderr_raw else None
 
     async def __aenter__(self):
         """Support async context manager entry."""
@@ -99,12 +100,14 @@ async def __aexit__(
             await self.stdin.aclose()
         if self.stdout:
             await self.stdout.aclose()
+        if self.stderr:
+            await self.stderr.aclose()
         if self.stdin_raw:
             self.stdin_raw.close()
         if self.stdout_raw:
             self.stdout_raw.close()
-        if self.stderr:
-            self.stderr.close()
+        if self.stderr_raw:
+            self.stderr_raw.close()
 
     async def wait(self):
         """Async wait for process completion."""