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

[sergeykad]

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)

[herakles-dev]

I'd like to take this. The fix is straightforward — the pywin32 imports in src/mcp/os/win32/utilities.py (lines 19-23) should be wrapped in a try/except ImportError so they degrade gracefully when pywin32 isn't installed:

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

The downstream code (_create_job_object, _maybe_assign_process_to_job, terminate_windows_process_tree) already null-checks these modules before use, so the graceful degradation path is fully covered.

Will have a PR up shortly.