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

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)

Bắt đầu với mcp/init.py và lần theo eager import của nó vào mcp/client/stdio và mcp/os/win32/utilities.py. Kiểm tra cách các null-check hiện có xử lý trường hợp thiếu pywin32, sau đó xác minh rằng việc import mcp.server.stdio trên Windows không còn yêu cầu pywin32 đối với người dùng chỉ sử dụng server, trong khi việc dọn dẹp client stdio vẫn được hỗ trợ.