Active Streamable HTTP sessions are not terminated during shutdown

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

Context

This is primarily a repost of this fastmcp issue as it seems to actually be a bug in the mcp python sdk and I want to make sure we've got visibility over here.

When shutting down a FastMCP server (CTRL+C) while clients have active streaming connections, Uvicorn logs:

ERROR: ASGI callable returned without completing response.

The error occurs because StreamableHTTPSessionManager cancels the task group without first gracefully terminating active SSE/streaming connections.

Expected behavior would be the active connections being closed with a proper http response before the rest of shutdown.

Example Code

from fastmcp import FastMCP

mcp = FastMCP("Test Server")

@mcp.tool
def hello() -> str:
  return "Hello, World!"

if __name__ == "__main__":
  mcp.run(transport="streamable-http", port=8000)

Python & MCP Python SDK

Python 3.12
MCP Python SDK 1.26.0

Commencez par la gestion du shutdown de StreamableHTTPSessionManager et reproduisez le problème avec l’exemple streamable-http sous Python 3.12. Suivez la manière dont les connexions SSE ou de streaming actives sont annulées, puis vérifiez que le shutdown les ferme avec une réponse HTTP correcte, sans l’erreur Uvicorn ASGI.