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

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

Start with the streamablehttp_client entry point and the ClientSession.call_tool path shown in client.py, then reproduce the server.py crash during a tool call. Done means the client reports a connection failure instead of waiting forever when the server exits before responding.