Client tool call hangs forever if server crashes or connection dies when using streamable-http

[gyang-xai]

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

If the server dies during a tool call before it sends a response the client will get stuck forever.

Example Code

# server.py

from mcp.server.fastmcp import FastMCP
import sys

mcp = FastMCP("StatefulServer")

@mcp.tool()
def greet(name: str = "World") -> str:
 """Greet someone by name."""
 sys.exit(0)
 return f"Hello, {name}!"


if __name__ == "__main__":
 mcp.run(transport="streamable-http")


# client.py

import asyncio

from mcp import ClientSession
from mcp.client.streamable_http import streamablehttp_client


async def main():
 # Connect to a streamable HTTP server
 async with streamablehttp_client("http://localhost:8000/mcp") as (
 read_stream,
 write_stream,
 _,
 ):
 # Create a session using the client streams
 async with ClientSession(read_stream, write_stream) as session:
 # Initialize the connection
 await session.initialize()

 res = await session.call_tool("greet", {"name": "World"})
 print(res)


if __name__ == "__main__":
 asyncio.run(main())



# Run server.py then client.py


# client.py will hang forever in the tool call and never complete. I would expect it to throw an error since the connection died and it will never get a response.

Python & MCP Python SDK

Python 3.13.7
mcp 1.20.0

[gyang-xai]

I have a possible PR that fixes this here: #1578 for reference but not sure if this is the best solution.

[owenliang]

"I encountered the same issue: exceptions at the underlying connection layer are not being broadcast to every waiting caller. Currently, the only workaround is to set a timeout for call tool operations to actively give up."

[astro-angelfish]

Reproduced on my side, just made my framework hangs forever. Maybe a keep-alive mechanism is required for streamable-http.