Fixes to stdio_client to support Windows more robustly (#372)

src/mcp/client/stdio/__init__.py

@@ -12,6 +12,12 @@ from pydantic import BaseModel, Field
 
 import mcp.types as types
 
+from .win32 import (
+    create_windows_process,
+    get_windows_executable_command,
+    terminate_windows_process,
+)
+
 # Environment variables to inherit by default
 DEFAULT_INHERITED_ENV_VARS = (
     [
@@ -101,14 +107,18 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
     read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
     write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 
-    process = await anyio.open_process(
-        [server.command, *server.args],
+    command = _get_executable_command(server.command)
+
+    # Open process with stderr piped for capture
+    process = await _create_platform_compatible_process(
+        command=command,
+        args=server.args,
         env=(
             {**get_default_environment(), **server.env}
             if server.env is not None
             else get_default_environment()
         ),
-        stderr=errlog,
+        errlog=errlog,
         cwd=server.cwd,
     )
 
@@ -159,4 +169,48 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
     ):
         tg.start_soon(stdout_reader)
         tg.start_soon(stdin_writer)
-        yield read_stream, write_stream
+        try:
+            yield read_stream, write_stream
+        finally:
+            # Clean up process to prevent any dangling orphaned processes
+            if sys.platform == "win32":
+                await terminate_windows_process(process)
+            else:
+                process.terminate()
+
+
+def _get_executable_command(command: str) -> str:
+    """
+    Get the correct executable command normalized for the current platform.
+
+    Args:
+        command: Base command (e.g., 'uvx', 'npx')
+
+    Returns:
+        str: Platform-appropriate command
+    """
+    if sys.platform == "win32":
+        return get_windows_executable_command(command)
+    else:
+        return command
+
+
+async def _create_platform_compatible_process(
+    command: str,
+    args: list[str],
+    env: dict[str, str] | None = None,
+    errlog: TextIO = sys.stderr,
+    cwd: Path | str | None = None,
+):
+    """
+    Creates a subprocess in a platform-compatible way.
+    Returns a process handle.
+    """
+    if sys.platform == "win32":
+        process = await create_windows_process(command, args, env, errlog, cwd)
+    else:
+        process = await anyio.open_process(
+            [command, *args], env=env, stderr=errlog, cwd=cwd
+        )
+
+    return process

src/mcp/client/stdio/win32.py

@@ -0,0 +1,109 @@
+"""
+Windows-specific functionality for stdio client operations.
+"""
+
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+from typing import TextIO
+
+import anyio
+from anyio.abc import Process
+
+
+def get_windows_executable_command(command: str) -> str:
+    """
+    Get the correct executable command normalized for Windows.
+
+    On Windows, commands might exist with specific extensions (.exe, .cmd, etc.)
+    that need to be located for proper execution.
+
+    Args:
+        command: Base command (e.g., 'uvx', 'npx')
+
+    Returns:
+        str: Windows-appropriate command path
+    """
+    try:
+        # First check if command exists in PATH as-is
+        if command_path := shutil.which(command):
+            return command_path
+
+        # Check for Windows-specific extensions
+        for ext in [".cmd", ".bat", ".exe", ".ps1"]:
+            ext_version = f"{command}{ext}"
+            if ext_path := shutil.which(ext_version):
+                return ext_path
+
+        # For regular commands or if we couldn't find special versions
+        return command
+    except OSError:
+        # Handle file system errors during path resolution
+        # (permissions, broken symlinks, etc.)
+        return command
+
+
+async def create_windows_process(
+    command: str,
+    args: list[str],
+    env: dict[str, str] | None = None,
+    errlog: TextIO = sys.stderr,
+    cwd: Path | str | None = None,
+):
+    """
+    Creates a subprocess in a Windows-compatible way.
+
+    Windows processes need special handling for console windows and
+    process creation flags.
+
+    Args:
+        command: The command to execute
+        args: Command line arguments
+        env: Environment variables
+        errlog: Where to send stderr output
+        cwd: Working directory for the process
+
+    Returns:
+        A process handle
+    """
+    try:
+        # Try with Windows-specific flags to hide console window
+        process = await anyio.open_process(
+            [command, *args],
+            env=env,
+            # Ensure we don't create console windows for each process
+            creationflags=subprocess.CREATE_NO_WINDOW  # type: ignore
+            if hasattr(subprocess, "CREATE_NO_WINDOW")
+            else 0,
+            stderr=errlog,
+            cwd=cwd,
+        )
+        return process
+    except Exception:
+        # Don't raise, let's try to create the process without creation flags
+        process = await anyio.open_process(
+            [command, *args], env=env, stderr=errlog, cwd=cwd
+        )
+        return process
+
+
+async def terminate_windows_process(process: Process):
+    """
+    Terminate a Windows process.
+
+    Note: On Windows, terminating a process with process.terminate() doesn't
+    always guarantee immediate process termination.
+    So we give it 2s to exit, or we call process.kill()
+    which sends a SIGKILL equivalent signal.
+
+    Args:
+        process: The process to terminate
+    """
+    try:
+        process.terminate()
+        with anyio.fail_after(2.0):
+            await process.wait()
+    except TimeoutError:
+        # Force kill if it doesn't terminate
+        process.kill()

uv.lock

@@ -1618,4 +1618,4 @@ wheels = [
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     { url = "https://files.pythonhosted.org/packages/68/a1/dcb68430b1d00b698ae7a7e0194433bce4f07ded185f0ee5fb21e2a2e91e/websockets-15.0.1-pp310-pypy310_pp73-win_amd64.whl", hash = "sha256:cad21560da69f4ce7658ca2cb83138fb4cf695a2ba3e475e0559e05991aa8122", size = 176884 },
     { url = "https://files.pythonhosted.org/packages/fa/a8/5b41e0da817d64113292ab1f8247140aac61cbf6cfd085d6a0fa77f4984f/websockets-15.0.1-py3-none-any.whl", hash = "sha256:f7a866fbc1e97b5c617ee4116daaa09b722101d4a3c170c787450ba409f9736f", size = 169743 },
-]
+]