fix(streamable_http): use 499 (Client Closed Request) instead of 202 for ClientDisconnect

499 is the nginx convention for 'Client Closed Request' — more semantically
correct than 202 Accepted for a cancelled request.

Author: wiggzz

src/mcp/server/streamable_http.py

@@ -654,7 +654,8 @@ async def sse_writer():
             if writer is not None:
                 with suppress(Exception):
                     await writer.send(ClientDisconnect())
-            response = self._create_json_response(None, HTTPStatus.ACCEPTED)
+            # 499 = Client Closed Request (nginx convention, not in stdlib HTTPStatus)
+            response = self._create_json_response(None, 499)  # type: ignore[arg-type]
             with suppress(Exception):
                 await response(scope, receive, send)
             return

tests/server/streamable_http/test_client_disconnect_post.py

@@ -121,9 +121,9 @@ async def dummy_send(message):
         assert len(send_calls) >= 1, (
             f"Expected at least 1 ASGI send (response), got {len(send_calls)}"
         )
-        # First send should be http.response.start with 202
+        # First send should be http.response.start with 499 (Client Closed Request)
         assert send_calls[0]["type"] == "http.response.start"
-        assert send_calls[0]["status"] == 202
+        assert send_calls[0]["status"] == 499
 
     @pytest.mark.anyio
     async def test_client_disconnect_notifies_writer(self):
@@ -202,4 +202,4 @@ async def dummy_send(message):
         # Response is still sent even though writer was broken
         assert len(send_calls) >= 1
         assert send_calls[0]["type"] == "http.response.start"
-        assert send_calls[0]["status"] == 202
+        assert send_calls[0]["status"] == 499