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

streamablehttp_client エントリーポイントと client.py に示されている ClientSession.call_tool のパスから始め、次にツール呼び出し中に発生する server.py のクラッシュを再現します。サーバーが応答する前に終了した場合、クライアントがいつまでも待機するのではなく接続失敗を報告すれば完了です。