fix: gracefully terminate active Streamable HTTP sessions during shutdown (#2150)

[kaushik701]

Fixes #2150

Problem

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

ERROR: ASGI callable returned without completing response.

There were two compounding issues:

1. StreamableHTTPSessionManager.run() never called terminate() on active transports before cancelling the task group — it just cleared the dict, leaving EventSourceResponse coroutines killed mid-stream.
2. terminate() itself didn't close _sse_stream_writers. It only cleaned up _request_streams, but the SSE stream writers (which keep EventSourceResponse alive) were stored separately and never touched, so EventSourceResponse kept waiting on its reader.

Fix

src/mcp/server/streamable_http_manager.py — In the run() method's finally block, iterate over all active transports and call terminate() on each before cancelling the task group:

finally:
    logger.info("StreamableHTTP session manager shutting down")
    # Gracefully terminate all active sessions before cancelling tasks
    for transport in list(self._server_instances.values()):
        try:
            await transport.terminate()
        except Exception:
            logger.debug("Error terminating transport during shutdown", exc_info=True)
    # Cancel task group to stop all spawned tasks
    tg.cancel_scope.cancel()
    ...

src/mcp/server/streamable_http.py — In terminate(), close all _sse_stream_writers before cleaning up request streams, so active EventSourceResponse instances can complete gracefully:

# Close all SSE stream writers to allow EventSourceResponse to complete gracefully
for writer in list(self._sse_stream_writers.values()):
    writer.close()
self._sse_stream_writers.clear()

Notes

• Stateless mode is not affected (transports are already terminated per-request).
• • The _terminated flag ensures that any concurrent requests arriving during shutdown get a 404, not a partial response.
• • Using sync .close() on MemoryObjectSendStream (rather than await aclose()) is safe and avoids awaiting inside the shutdown path.