Gracefully terminate active sessions during StreamableHTTP shutdown

During server shutdown, StreamableHTTPSessionManager.run() cancelled
the task group without first terminating active sessions. This killed
EventSourceResponse coroutines mid-stream, causing Uvicorn to log
"ASGI callable returned without completing response" errors.

Two changes fix this:

1. In StreamableHTTPSessionManager.run(), iterate over all active
   transports and call terminate() on each before cancelling the
   task group.

2. In StreamableHTTPServerTransport.terminate(), close all SSE stream
   writers (_sse_stream_writers) so that active EventSourceResponse
   instances complete gracefully. Also add an early return guard for
   idempotency since terminate() may now be called from both the
   manager shutdown path and the per-session cleanup path.

Github-Issue: #2150
Reported-by: emmahoggan

Author: Bortlesboat

src/mcp/server/streamable_http.py

@@ -766,9 +766,17 @@ async def terminate(self) -> None:
         Once terminated, all requests with this session ID will receive 404 Not Found.
         """
 
+        if self._terminated:
+            return
         self._terminated = True
         logger.info(f"Terminating session: {self.mcp_session_id}")
 
+        # Close all SSE stream writers so that active EventSourceResponse
+        # coroutines complete gracefully instead of being cancelled mid-stream.
+        for writer in list(self._sse_stream_writers.values()):
+            writer.close()
+        self._sse_stream_writers.clear()
+
         # We need a copy of the keys to avoid modification during iteration
         request_stream_keys = list(self._request_streams.keys())
 

src/mcp/server/streamable_http_manager.py

@@ -130,6 +130,17 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 yield  # Let the application run
             finally:
                 logger.info("StreamableHTTP session manager shutting down")
+                # Gracefully terminate all active sessions before cancelling
+                # tasks so that EventSourceResponse coroutines can complete
+                # and Uvicorn does not log ASGI-incomplete-response errors.
+                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()
                 self._task_group = None