stdio_client not starting in windows: getting NotImplementedError

[JexinSam]

anyio.open_process in stdio_client not working in windows. It raises NotImplementedError.

async with stdio_client(server_params) as (read, write):
        async with ClientSession(
            read, write, sampling_callback=handle_sampling_message
        ) as session:
            # Initialize the connection
            await session.initialize()

It gives out error:

   async with stdio_client(server_param) as (read, write):
  File "C:\Python311\Lib\contextlib.py", line 204, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio.py", line 104, in stdio_client
    process = await anyio.open_process(
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_core\_subprocesses.py", line 190, in open_process
    return await get_async_backend().open_process(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_backends\_asyncio.py", line 2561, in open_process
    process = await asyncio.create_subprocess_exec(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\subprocess.py", line 218, in create_subprocess_exec
    transport, protocol = await loop.subprocess_exec(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 1680, in subprocess_exec
    transport = await self._make_subprocess_transport(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 502, in _make_subprocess_transport
    raise NotImplementedError
NotImplementedError

Is there a solution to this?

[Kludex]

Hmmm... I think we need to choose the spawn mode somewhere...

[saqadri]

@dsp-ant I had several users report issues with mcp-agent on Windows at least in part due to this. I got around to spending several hours to fix this, and figured I'd contribute the fix back upstream (or downstream? 🙃 ). #372 should cover most cases.

[isakulaksiz]

NotImplementedError in asyncio subprocess_exec when using MCP client on Windows with Python 3.12 #369

[JexinSam]

@saqadri #372 PR still giving me the same error in windows: NotImplementedError

return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\__init__.py", line 113, in stdio_client
    process = await _create_platform_compatible_process(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\__init__.py", line 210, in _create_platform_compatible_process
    process = await create_windows_process(command, args, env, errlog, cwd)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\win32.py", line 85, in create_windows_process
    process = await anyio.open_process(
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_core\_subprocesses.py", line 190, in open_process
    return await get_async_backend().open_process(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_backends\_asyncio.py", line 2561, in open_process
    process = await asyncio.create_subprocess_exec(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\subprocess.py", line 218, in create_subprocess_exec
    transport, protocol = await loop.subprocess_exec(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 1680, in subprocess_exec
    transport = await self._make_subprocess_transport(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 502, in _make_subprocess_transport
    raise NotImplementedError
NotImplementedError

[saqadri]

@saqadri #372 PR still giving me the same error in windows: NotImplementedError

return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\__init__.py", line 113, in stdio_client
    process = await _create_platform_compatible_process(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\__init__.py", line 210, in _create_platform_compatible_process
    process = await create_windows_process(command, args, env, errlog, cwd)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\mcp\client\stdio\win32.py", line 85, in create_windows_process
    process = await anyio.open_process(
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_core\_subprocesses.py", line 190, in open_process
    return await get_async_backend().open_process(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\site-packages\anyio\_backends\_asyncio.py", line 2561, in open_process
    process = await asyncio.create_subprocess_exec(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\subprocess.py", line 218, in create_subprocess_exec
    transport, protocol = await loop.subprocess_exec(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 1680, in subprocess_exec
    transport = await self._make_subprocess_transport(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Python311\Lib\asyncio\base_events.py", line 502, in _make_subprocess_transport
    raise NotImplementedError
NotImplementedError

@JexinSam I'll investigate - is there anything special about your setup. For example are you running in a Jupyter notebook or regular Python script? A snippet and repro steps would be super helpful.

[JexinSam]

I am trying to run mcp client in fastapi server.

Here is a sample code:

mcp-server-config.json

{
  "mcpServers": {
    "mysql": {
      "command": "python",
      "args": ["path/to/mcp-host/mysql-server"],
      "env": {},
      "transport": "stdio"
    }
  }
}

app.py

from mcp import ClientSession, StdioServerParameters, types
from typing import List
from mcp.client.stdio import stdio_client
import os, json
from fastapi import FastAPI, HTTPException

app = FastAPI()

CONFIG_FILE = 'mcp-server-config.json'

def load_server_config() -> dict:
    # Load server configuration from the config file
    if os.path.exists(CONFIG_FILE):
        with open(CONFIG_FILE, "r", encoding="utf-8") as f:
            return json.load(f)  # Load server configuration
    raise FileNotFoundError(f"Could not find config file {CONFIG_FILE}")


def create_server_parameters(server_config: dict) -> List[StdioServerParameters]:
    """Create server parameters from the server configuration."""
    server_parameters = []
    # Create server parameters for each server configuration
    for config in server_config["mcpServers"].values():
        server_parameter = StdioServerParameters(
            command=config["command"],
            args=config.get("args", []),
            env={**config.get("env", {}), "PATH": os.getenv("PATH")}
        )
        # Add environment variables from the system if not provided
        for key, value in server_parameter.env.items():
            if len(value) == 0 and key in os.environ:
                server_parameter.env[key] = os.getenv(key)
        server_parameters.append(server_parameter)
    return server_parameters

async def get_mcp_tools(server_param: StdioServerParameters) -> List:
    """Asynchronously retrieves and converts tools from a server using specified parameters"""
    mcp_tools = []

    async with stdio_client(server_param) as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()  # Initialize the session
            tools: types.ListToolsResult = await session.list_tools()  # Retrieve tools from the server

            for tool_data in tools.tools:
                mcp_tools.append(tool_data.__dict__)

    return mcp_tools

@app.get("/api/mcp-tools")
async def get_mcp_tools_api():
    """API endpoint to retrieve MCP tools."""
    server_config = load_server_config()  # Load server configuration
    server_params = create_server_parameters(server_config)
    tools = await get_mcp_tools(server_params[0])  # Get tools from the first server
    return {"tools": tools}

fastapi dev app.py
# accessing /api/mcp-tools is resulting in NotImplementedError

[saqadri]

@JexinSam thanks for the repro steps. I can try looking into this later today. In the meantime, can you please try setting this to see if it helps?

import asyncio
asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())

It is a hunch, but I noticed this issue in windows when using streamlit and jupyter as well, and at least for the streamlit case having the above resolved the issue.

[JexinSam]

Yes the issue is related to asyncio in windows. But the above code doesn't solve it for me.
fastapi/fastapi#7770

Turns out it happens only when --reload is enabled in fastapi.