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Title: Fix Windows subprocess NotImplementedError (STDIO clients) by theailanguage · Pull Request #596 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: Fix Windows subprocess NotImplementedError (STDIO clients) by theailanguage · Pull Request #596 · modelcontextprotocol/python-sdk

X Title: Fix Windows subprocess NotImplementedError (STDIO clients) by theailanguage · Pull Request #596 · modelcontextprotocol/python-sdk

Description: Fix subprocess creation errors on Windows for STDIO client connections. This updates create_windows_process to properly handle the case where asyncio cannot create subprocess transports on Windows by falling back to using the subprocess module manually. Additionally, minor import sorting and style issues have been fixed according to ruff linter checks. Motivation and Context On Windows systems, MCP clients that connect via STDIO were failing with a NotImplementedError during anyio.open_process(). This is because asyncio on Windows does not implement full support for subprocess transports. This change provides a clean workaround: Attempt async process creation first. If not supported, fall back to classic subprocess.Popen with asyncio wrapping. This allows MCP STDIO clients (e.g., streamlit_client_ui.py) to work successfully on Windows without changing user workflows. edit: the default event loop comes out as (Windows 11, Python 3.13) - My understanding is that #555 might work under the ProactorEventLoop, but my patch "might" help to support environments where that’s not the default Frameworks like Streamlit, which may run their own event loop internally Python 3.13 and above (where SelectorEventLoop is often the default on Windows) CI setups or test runners that don’t customize the loop policy (note - personally I have faced this with my Streamlit UI only) How Has This Been Tested? Tested manually on Windows 11, Python 3.13. Ran streamlit_client_ui.py to launch the MCP client. Successfully connected to STDIO MCP servers without errors. Validated tool loading and interaction without subprocess creation issues. Verified no new ruff or pyright issues for the updated file. Breaking Changes No breaking changes. The fallback mechanism is backward compatible. Linux/macOS users are unaffected. Windows users gain compatibility without needing to change their code. Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update (style improvements, minor doc updates) Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed Additional context This fix is especially important for making MCP tools and agents accessible on all platforms, including Windows, thereby helping broader adoption.

Open Graph Description: Fix subprocess creation errors on Windows for STDIO client connections. This updates create_windows_process to properly handle the case where asyncio cannot create subprocess transports on Windows ...

X Description: Fix subprocess creation errors on Windows for STDIO client connections. This updates create_windows_process to properly handle the case where asyncio cannot create subprocess transports on Windows ...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/596

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9f5eecd Fix Windows subprocess compatibility for STDIO mode with async streams theailanguage Apr 28, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/9f5eecd8b373d268ee4eafc2b432dcc3a7803b48
306b28d Fix: Windows stdio subprocess compatibility with type hints and fallb… theailanguage Apr 28, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/306b28dcf75b4bf380050781fd0828a65991b015
2f26efd style(win32): fix import sorting and formatting issues theailanguage Apr 28, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/2f26efd414aa04e07d051cd71b2415942f546586
5b98439 style(stdio): format imports and wrap long lines for ruff compliance theailanguage Apr 28, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/5b98439b61b9643d849c004848c3e2af2c6ddbfc
02b3d3c updated tests - ignored test_stdio_context_manager_exiting, test_stdi… theailanguage May 29, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/02b3d3c3802bb4b7145c7f7a85bce1da1f56c06e
a550b02 Revert "updated tests - ignored test_stdio_context_manager_exiting, t… theailanguage May 29, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/a550b021a9cc5dc020fb702aa33a5a28a78c8158
5815e6c fix: rebase on main and resolve merge conflicts in stdio subprocess l… theailanguage May 29, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/5815e6c026c888c29c465aacd101cc2939be02f7
dddff2b Fix Windows compatibility for STDIO with async DummyProcess theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/dddff2b7d5e0252841965e68f7fb1e27092e712c
9e38ab5 added 6a703a4 closing file handles in __aexit__ in DummyProcess - Fix… theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/9e38ab5d250c75f194b77329f69c85cff4acde66
249d81e pre_commit run issues fixed theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/249d81e2c4a5265da54e4589d8a134fd7db2b76e
d5e46c6 Removed -c to simulate a real file not found error instead of executi… theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/d5e46c6f5e7a382346e633fbbd9e4906c6dae08e
0629a3f On Linux, terminating a process that has already exited raises Proces… theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/0629a3fb3146d08ebbc18b294fb904e7df84fd6a
c8eaeba Revert README, stdio/__init__.py, and test_stdio.py to match upstream… theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/c8eaebaf593fc4bdf0f8f6e8e9870ceba1dddca8
d9863c3 emtpy commit to retrigger checks theailanguage Jun 24, 2025 https://github.com/modelcontextprotocol/python-sdk/pull/596/commits/d9863c31a0e1084e33ddd59c8e716f7b55715a73
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