fix(lowlevel): log Disconnect/Cancel exceptions at DEBUG, not ERROR

The low-level server's generic case-Exception handler logs any exception
from the session stream at ERROR level. When the streamable HTTP transport
sends ClientDisconnect through the writer (to unblock the inner session
task), it was caught here and logged as ERROR — producing the same noise
we're trying to eliminate.

Add a guard clause matching Disconnect/Cancel exception types (by name,
to keep the low-level server transport-agnostic) and log at DEBUG instead.
Also skip sending 'Internal Server Error' to the client (there is none).

Author: wiggzz

src/mcp/server/lowlevel/server.py

@@ -703,6 +703,9 @@ async def _handle_message(
                         await self._handle_request(message, req, session, lifespan_context, raise_exceptions)
                 case types.ClientNotification(root=notify):
                     await self._handle_notification(notify)
+                case Exception() if "Disconnect" in type(message).__name__ or "Cancel" in type(message).__name__:  # pragma: no cover
+                    # Client went away (Disconnect) or task was cancelled — not a server error.
+                    logger.debug(f"Received {type(message).__name__} from stream")
                 case Exception():  # pragma: no cover
                     logger.error(f"Received exception from stream: {message}")
                     await session.send_log_message(

src/mcp/server/streamable_http.py

@@ -654,8 +654,9 @@ async def sse_writer():
             if writer is not None:
                 with suppress(Exception):
                     await writer.send(ClientDisconnect())
-            # 499 = Client Closed Request (nginx convention, not in stdlib HTTPStatus)
-            response = self._create_json_response(None, 499)  # type: ignore[arg-type]
+            # 499 = Client Closed Request (nginx convention, not in stdlib HTTPStatus).
+            # Build Response directly to avoid _create_json_response's HTTPStatus type hint.
+            response = Response(content=b"", status_code=499, media_type=CONTENT_TYPE_JSON)
             with suppress(Exception):
                 await response(scope, receive, send)
             return