fix: gracefully terminate active sessions on shutdown

During StreamableHTTPSessionManager shutdown, iterate through all active
transports and call terminate() before cancelling the task group. This
allows SSE connections to receive a proper HTTP response instead of being
abruptly dropped, which caused Uvicorn to log 'ASGI callable returned
without completing response'.

Fixes #2150

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/server/streamable_http_manager.py

@@ -130,6 +130,15 @@ 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
+                # the task group so that SSE connections receive a proper
+                # HTTP response instead of being abruptly dropped.
+                for session_id, transport in list(self._server_instances.items()):
+                    try:
+                        await transport.terminate()
+                        logger.debug(f"Terminated session {session_id} during shutdown")
+                    except Exception:  # pragma: no cover
+                        logger.debug(f"Error terminating session {session_id} during shutdown", exc_info=True)
                 # Cancel task group to stop all spawned tasks
                 tg.cancel_scope.cancel()
                 self._task_group = None

tests/server/test_streamable_http_manager.py

@@ -410,3 +410,25 @@ def test_session_idle_timeout_rejects_non_positive():
 def test_session_idle_timeout_rejects_stateless():
     with pytest.raises(RuntimeError, match="not supported in stateless"):
         StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)
+
+
+@pytest.mark.anyio
+async def test_shutdown_terminates_active_sessions():
+    """Test that run() shutdown terminates active transports before cancelling tasks."""
+    app = Server("test-shutdown-terminate")
+    manager = StreamableHTTPSessionManager(app=app)
+
+    # We'll manually inject a mock transport into _server_instances
+    # and verify terminate() is called during shutdown.
+    mock_transport = AsyncMock(spec=StreamableHTTPServerTransport)
+    mock_transport.mcp_session_id = "test-session-1"
+    mock_transport.is_terminated = False
+
+    async with manager.run():
+        # Inject mock transport as if a session was created
+        manager._server_instances["test-session-1"] = mock_transport
+
+    # After exiting run(), terminate should have been called
+    mock_transport.terminate.assert_awaited_once()
+    # Server instances should be cleared
+    assert len(manager._server_instances) == 0