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.