fix: terminate active StreamableHTTP sessions during shutdown

src/mcp/server/streamable_http.py

@@ -784,6 +784,12 @@ async def terminate(self) -> None:
         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()):  # pragma: no cover
+            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 +130,14 @@ 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()):  # pragma: no cover
+                    try:
+                        await transport.terminate()
+                    except Exception:  # pragma: no cover
+                        logger.exception("Error terminating transport during shutdown")
                 # Cancel task group to stop all spawned tasks
                 tg.cancel_scope.cancel()
                 self._task_group = None