SSE Mode: Error when connecting

[eiseleMichael]

Hi everyone,

thanks a lot for this library!

I was just trying out the Echo demo with sse transport and there seems to be an error in the ASGI exception wrapper. The inspector can list the tools and resources, but on every request an error occurs inside starlet (TypeError: 'NoneType' object is not callable)

# demo.py
from fastmcp import FastMCP

mcp = FastMCP("Echo 🚀")

@mcp.resource("echo://{message}")
def echo_resource(message: str) -> str:
    """Echo a message as a resource"""
    return f"Resource echo: {message}"

@mcp.tool()
def echo_tool(message: str) -> str:
    """Echo a message as a tool"""
    return f"Tool echo: {message}"

@mcp.prompt()
def echo_prompt(message: str) -> str:
    """Create an echo prompt"""
    return f"Please process this message: {message}"

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

Then executing python demo.py and fastmcp dev demo.py and connect via localhost:8000/sse.

I included a minimal reproduction repo here: https://github.com/eiseleMichael/fastmcp-sse-test

This might be related: modelcontextprotocol/python-sdk#83

Let me know if you need more info or if I'm just using it wrong :-)