Client hangs forever

[afarntrog]

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

Client hangs forever when sse_read_timeout is specified unless read_timeout_seconds in session.call_tool is also populated.

Example Code

import threading
import time
import asyncio
from typing import Literal
from datetime import timedelta

from mcp import ClientSession
from mcp.client.streamable_http import streamablehttp_client
from mcp.types import ImageContent as MCPImageContent


def start_comprehensive_mcp_server(transport: Literal["sse", "streamable-http"], port=int):
 from mcp.server import FastMCP

 mcp = FastMCP("Comprehensive MCP Server", port=port)

 @mcp.tool(description="Tool that will timeout")
 def timeout_tool() -> str:
 time.sleep(10)
 return "This tool has timed out"

 @mcp.tool(description="Calculator tool which performs calculations")
 def calculator(x: int, y: int) -> int:
 return x + y

 mcp.run(transport=transport)


# Start server
server_thread = threading.Thread(
 target=start_comprehensive_mcp_server, 
 kwargs={"transport": "streamable-http", "port": 8001}, 
 daemon=True
)
server_thread.start()
time.sleep(3)
print("SERVER STARTED", flush=True)


async def test_raw_mcp():
 print("CREATING RAW MCP CONNECTION...", flush=True)
 
 async with streamablehttp_client(
 url="http://127.0.0.1:8001/mcp",
 sse_read_timeout=2, # 2 second timeout
 ) as (read_stream, write_stream, get_session_id):
 print("TRANSPORT CONNECTED", flush=True)
 
 async with ClientSession(read_stream, write_stream) as session:
 print("SESSION CREATED", flush=True)
 
 await session.initialize()
 print("SESSION INITIALIZED", flush=True)
 
 print("CALLING TOOL (with 2s timeout)...", flush=True)
 try:
 result = await session.call_tool(
 "timeout_tool", 
 arguments={},
 # read_timeout_seconds=timedelta(seconds=5), # This needs to be provided or the client will hang forever
 )
 print(f"TOOL RESULT: {result}", flush=True)
 except Exception as e:
 print(f"TOOL CALL EXCEPTION: {type(e).__name__}: {e}", flush=True)
 
 print("DONE", flush=True)


print("RUNNING TEST...", flush=True)
asyncio.run(test_raw_mcp())
print("TEST COMPLETE", flush=True)

Python & MCP Python SDK

1.24.0

[maxisbey]

Thanks for the detailed reproduction! This is a duplicate of #1401. I've added your root cause analysis there - the infinite reconnection loop you found (attempt = 0 reset) is a related bug that the fix should address.

Tracking in #1401.

_{AI Disclaimer}