address comments

Author: justin-yi-wang

src/mcp/client/streamable_http.py

@@ -279,11 +279,10 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
             if is_initialization:
                 self._maybe_extract_session_id_from_response(response)
 
-            content_type = response.headers.get(CONTENT_TYPE, "").lower()
-
             # Per https://modelcontextprotocol.io/specification/2025-06-18/basic#notifications:
             # The server MUST NOT send a response to notifications.
             if isinstance(message.root, JSONRPCRequest):
+                content_type = response.headers.get(CONTENT_TYPE, "").lower()
                 if content_type.startswith(JSON):
                     await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
                 elif content_type.startswith(SSE):

tests/client/test_notification_response.py

@@ -114,11 +114,13 @@ def non_sdk_server(non_sdk_server_port: int) -> Generator[None, None, None]:
 
 
 @pytest.mark.anyio
-async def test_notification_with_204_response(non_sdk_server: None, non_sdk_server_port: int) -> None:
+async def test_non_compliant_notification_response(non_sdk_server: None, non_sdk_server_port: int) -> None:
     """
-    This test verifies that the client does not parse responses to non-JsonRPCRequests, which matches the
-    behavior of the TypeScript SDK. The test uses a 204 No Content (commonly seen from servers), but in reality
-    any 2xx response should be handled the same way.
+   This test verifies that the client ignores unexpected responses to notifications: the spec states they should
+   either be 202 + no response body, or 4xx + optional error body
+   (https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server),
+   but some servers wrongly return other 2xx codes (e.g. 204). For now we simply ignore unexpected responses
+   (aligning behaviour w/ the TS SDK).
     """
     server_url = f"http://127.0.0.1:{non_sdk_server_port}/mcp"
     returned_exception = None