stdio_client fails with BrokenResourceError during initialization

[ParthibanRajasekaran]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

I'm trying to use stdio_client to spawn and communicate with a FastMCP server, but it consistently fails during the initialization handshake. The client times out after about 10 seconds with a BrokenResourceError.

The subprocess starts fine and I can see the server is running, but when the client tries to initialize the session, it hangs and eventually times out. Looking at the traceback, there's a BrokenResourceError happening in the stdout_reader coroutine at line 162 of mcp/client/stdio/init.py.

Expected behavior:
The client should successfully spawn the server, complete the initialize handshake, and be ready to call tools.

Actual behavior:
The client hangs during initialization and then times out with:

ExceptionGroup: unhandled errors in a TaskGroup (2 sub-exceptions)
BrokenResourceError at mcp/client/stdio/init.py:162 in stdout_reader
TimeoutError after waiting for initialize response

Additional cpontext:
Interesting finding: When I test the server directly by piping JSON to it, it works perfectly:

$ echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' | python test_server.py

The server immediately returns a valid response. So the server itself is working correctly - it can read from stdin and write to stdout just fine. The issue seems to be specifically with how stdio_client is handling the subprocess communication.

I've tried various things to fix it but nothing works:

• Running python with -u flag for unbuffered output
• Setting sys.stdout.reconfigure(line_buffering=True) in the server
• Using absolute paths
• Increasing timeout values
• Setting PYTHONUNBUFFERED=1

Note: HTTP/SSE transport works fine as an alternative.

Example Code

1. Create a simple FastMCP server (test_server.py):

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("TestServer")

@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers"""
    return a + b

if __name__ == "__main__":
    mcp.run()

2. Create a client that uses stdio_client (test_client.py):

import asyncio
from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client

async def test():
    params = StdioServerParameters(
        command="python",
        args=["test_server.py"]
    )
    
    async with stdio_client(params) as (stdio, write):
        session = ClientSession(stdio, write)
        await session.initialize()  # Hangs here
        
        result = await session.list_tools()
        print(f"Tools: {result.tools}")

asyncio.run(test())

3. Run: python test_client.py
4. Observe: hangs for about 10 seconds then fails with BrokenResourceError

Python & MCP Python SDK

- MCP SDK Version: 1.20.0
- Python Version: 3.12.7
- Operating System: macOS
- Installation method: pip install mcp==1.20.01

[madara-uchihaaa]

import asyncio
from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client


async def test():
    params = StdioServerParameters(command="python", args=["main.py"])

    async with stdio_client(params) as (stdio, write):
        async with ClientSession(stdio, write) as session:
            await session.initialize()

            result = await session.list_tools()
            print(f"Tools: {result.tools}")


asyncio.run(test())

can you try this @ParthibanRajasekaran