Hi team!

This is my first pull request. PR fixes Windows compatibility for MCP STDIO clients by introducing a fallback to subprocess.Popen wrapped with async I/O streams. It ensures that Windows users can now use streamlit and similar STDIO clients without errors.

Please let me know if you'd like any further changes. Thank you for your time and for maintaining such an awesome project!

Regards
[Kartik / theailanguage]

@dsp-ant @jerome3o-anthropic would appreciate any kind of feedback on this if possible from your end. Thanks a ton!

Regards
[Kartik / theailanguage]

#555 solves this in a much cleaner way

#555 solves this in a much cleaner way

Thanks for sharing 🙏 I just tested it on my setup (Windows 11 + Python 3.13), and I’m still hitting the same error:
raise NotImplementedError

I think I have tested #555 well by updating the init and win32 python files in my .venv mcp path with the edits suggested here.

Baically #555 still uses await anyio.open_process(...) in both primary and fallback paths, which internally relies on asyncio.create_subprocess_exec(). Unfortunately, that call raises NotImplementedError on my machine, so the fallback logic never helps.

In my fork, I catch the failure and bypass the entire async subprocess path by:
Falling back to a sync subprocess.Popen
Then manually wrapping stdin, stdout in FileWriteStream and FileReadStream
This makes it work regardless of the event loop or OS internals

So for Windows users with newer Python versions or CI environments, my fallback is the only one that works when the async event loop doesn’t support subprocesses.

Let me know what you think. Have you tested this in event loop scenarios like the one pointed out in this PR?

Thanks
Kartik / theailanguage

I am on windows 11; also using it in an event loop. are you using windows proactor for the async loop?

edit:
I also have this change in the version i am using

mcp\client\session.py,

import asyncio
import os
if os.name == 'nt':
  asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())

I am on windows 11; also using it in an event loop. are you using windows proactor for the async loop?

edit: I also have this change in the version i am using

mcp\client\session.py,

import asyncio
import os
if os.name == 'nt':
  asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())

Aah that explains it :)

For me it is
<class 'asyncio.windows_events._WindowsSelectorEventLoop'>

This I think is the default.

I actually tried to add your session.py code but cannot get the event loop to change for some reason (I guess due to Streamlit's own eventloop).

I understand #555 might work perfectly under the ProactorEventLoop, but this 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)

That said, I really appreciate the clean structure of the other solution.

Thanks
Kartik / theailanguage

One more thing, in case you'd want to look at the implementation of the streamlit code - you can have a look at

Repo - https://github.com/theailanguage/mcp_client
UI code - https://github.com/theailanguage/mcp_client/blob/main/streamlit_client_ui.py

This is a tutorial video for the same that I built for a course
https://youtu.be/Ln-Tgz8Pmek