pywin32 installation fails on Windows — hard dep only needed for client stdio, but pulled in for all users via eager import in __init__.py

mcp/init.py から始め、mcp/client/stdio と mcp/os/win32/utilities.py への eager import を追跡します。既存の null-check が pywin32 の欠如をどのように処理しているかを確認し、その後、Windows で mcp.server.stdio を import してもサーバーのみを使用するユーザーには pywin32 が不要であり、client stdio のクリーンアップは引き続きサポートされることを検証します。

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

pywin32>=311 is a required Windows dependency used exclusively in mcp.client.stdio (Windows Job Objects for child process cleanup). However, mcp/__init__.py eagerly imports from .client.stdio import StdioServerParameters, stdio_client, which causes mcp/os/win32/utilities.py to unconditionally execute:

if sys.platform == "win32":
    import pywintypes
    import win32api
    import win32con
    import win32job

This means pywin32 is a hard runtime requirement on Windows for all users — including those running a pure MCP server that never uses mcp.client.stdio.

The pywin32 wheel contains a .data directory with Windows DLLs. On Windows, AV software (e.g. Windows Defender) locks newly-extracted DLLs before uv can clean up its temp directory, causing installation to fail every time with:

error: Failed to install: pywin32-311-cp313-cp313-win_amd64.whl (pywin32==311)
  Caused by: failed to remove directory `...\uv\cache\builds-v0\.tmp\Lib\site-packages\pywin32-311.data`
  os error 32 (file in use by another process)

This blocks server-only deployments like ha-mcp entirely on Windows (tracked in homeassistant-ai/ha-mcp#672).

The null-checks already in _create_job_object() and _maybe_assign_process_to_job() handle win32job = None gracefully — so the only missing piece is allowing the import to fail softly:

if sys.platform == "win32":
    try:
        import pywintypes, win32api, win32con, win32job
    except ImportError:
        win32api = win32con = win32job = pywintypes = None

Or alternatively, lazily import mcp.client.stdio from mcp/__init__.py so server-only users don't trigger this code path at all.

Example Code

# This alone triggers the pywin32 import on Windows:
from mcp.server.stdio import stdio_server

Python & MCP Python SDK

Python 3.13, MCP Python SDK 1.26.0 (latest)