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

[isakulaksiz]

Describe the bug
When using MCP client with Python 3.12 on Windows, it fails to connect to the server with a NotImplementedError. This is happening because asyncio subprocess functionality is not properly implemented in Python 3.12 on Windows.

To Reproduce
Steps to reproduce the behavior:

1. Install Python 3.12 on Windows
2. Install MCP and required packages
3. Create a simple MCP server script and MCP client
4. Try to connect to the server using the client
5. See error: NotImplementedError is raised in asyncio's _make_subprocess_transport method

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Desktop

• OS: [Windows 10]
• Browser [chrome]
• Package versions: anyio, mcp, asyncio standard library

Additional context
it works fine with macos.

[jayanthma]

Hi @isakulaksiz , the issue was persisting in the jupyter notebook so I converted it to .py and it ran successfully. Find the code below.

from langchain_mcp_adapters.client import MultiServerMCPClient

from langgraph.prebuilt import create_react_agent

from langchain_aws import ChatBedrockConverse
import boto3

bedrock_client=boto3.client(service_name='bedrock-runtime', region_name="us-east-1")

model = ChatBedrockConverse(
client=bedrock_client,
model="us.meta.llama3-3-70b-instruct-v1:0",
temperature=0,
)

async def test_mcp_client():
async with MultiServerMCPClient(
{
"math": {
"command": "C:/Users/jay/coding_projects/LangGraph/venv/Scripts/python.exe",

Make sure to update to the full absolute path to your math_server.py file

"args": ["C:/Users/jay/coding_projects/LangGraph/mcp/math_server.py"],
"transport": "stdio",
},
"weather": {

make sure you start your weather server on port 8000

"url": "http://localhost:8000/sse",
"transport": "sse",
}
}
) as client:
tools = client.get_tools()
print("Tools: ", tools)
agent = create_react_agent(model, client.get_tools(), debug=True)
math_response = await agent.ainvoke({"messages": "what's 3*175"})
weather_response = await agent.ainvoke({"messages": "what is the weather in nyc?"})

return math_response, weather_response
if name == "main":
import asyncio
math_response, weather_response = asyncio.run(test_mcp_client())
print(math_response)
print(weather_response)

Hope it helps!!