SDK Parity: Avoid Parsing Server Response for non-JsonRPCMessage Requests (#1290)

Author: justin-yi-wang

src/mcp/client/streamable_http.py

@@ -279,17 +279,19 @@ 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()
-
-            if content_type.startswith(JSON):
-                await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
-            elif content_type.startswith(SSE):
-                await self._handle_sse_response(response, ctx, is_initialization)
-            else:
-                await self._handle_unexpected_content_type(
-                    content_type,
-                    ctx.read_stream_writer,
-                )
+            # 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):
+                    await self._handle_sse_response(response, ctx, is_initialization)
+                else:
+                    await self._handle_unexpected_content_type(
+                        content_type,
+                        ctx.read_stream_writer,
+                    )
 
     async def _handle_json_response(
         self,

tests/client/test_notification_response.py

@@ -0,0 +1,150 @@
+"""
+Tests for StreamableHTTP client transport with non-SDK servers.
+
+These tests verify client behavior when interacting with servers
+that don't follow SDK conventions.
+"""
+
+import json
+import multiprocessing
+import socket
+import time
+from collections.abc import Generator
+
+import pytest
+import uvicorn
+from starlette.applications import Starlette
+from starlette.requests import Request
+from starlette.responses import JSONResponse, Response
+from starlette.routing import Route
+
+from mcp import ClientSession, types
+from mcp.client.streamable_http import streamablehttp_client
+from mcp.shared.session import RequestResponder
+from mcp.types import ClientNotification, RootsListChangedNotification
+
+
+def create_non_sdk_server_app() -> Starlette:
+    """Create a minimal server that doesn't follow SDK conventions."""
+
+    async def handle_mcp_request(request: Request) -> Response:
+        """Handle MCP requests with non-standard responses."""
+        try:
+            body = await request.body()
+            data = json.loads(body)
+
+            # Handle initialize request normally
+            if data.get("method") == "initialize":
+                response_data = {
+                    "jsonrpc": "2.0",
+                    "id": data["id"],
+                    "result": {
+                        "serverInfo": {"name": "test-non-sdk-server", "version": "1.0.0"},
+                        "protocolVersion": "2024-11-05",
+                        "capabilities": {},
+                    },
+                }
+                return JSONResponse(response_data)
+
+            # For notifications, return 204 No Content (non-SDK behavior)
+            if "id" not in data:
+                return Response(status_code=204, headers={"Content-Type": "application/json"})
+
+            # Default response for other requests
+            return JSONResponse(
+                {"jsonrpc": "2.0", "id": data.get("id"), "error": {"code": -32601, "message": "Method not found"}}
+            )
+
+        except Exception as e:
+            return JSONResponse({"error": f"Server error: {str(e)}"}, status_code=500)
+
+    app = Starlette(
+        debug=True,
+        routes=[
+            Route("/mcp", handle_mcp_request, methods=["POST"]),
+        ],
+    )
+    return app
+
+
+def run_non_sdk_server(port: int) -> None:
+    """Run the non-SDK server in a separate process."""
+    app = create_non_sdk_server_app()
+    config = uvicorn.Config(
+        app=app,
+        host="127.0.0.1",
+        port=port,
+        log_level="error",  # Reduce noise in tests
+    )
+    server = uvicorn.Server(config=config)
+    server.run()
+
+
+@pytest.fixture
+def non_sdk_server_port() -> int:
+    """Get an available port for the test server."""
+    with socket.socket() as s:
+        s.bind(("127.0.0.1", 0))
+        return s.getsockname()[1]
+
+
+@pytest.fixture
+def non_sdk_server(non_sdk_server_port: int) -> Generator[None, None, None]:
+    """Start a non-SDK server for testing."""
+    proc = multiprocessing.Process(target=run_non_sdk_server, kwargs={"port": non_sdk_server_port}, daemon=True)
+    proc.start()
+
+    # Wait for server to be ready
+    start_time = time.time()
+    while time.time() - start_time < 10:
+        try:
+            with socket.create_connection(("127.0.0.1", non_sdk_server_port), timeout=0.1):
+                break
+        except (TimeoutError, ConnectionRefusedError):
+            time.sleep(0.1)
+    else:
+        proc.kill()
+        proc.join(timeout=2)
+        pytest.fail("Server failed to start within 10 seconds")
+
+    yield
+
+    proc.kill()
+    proc.join(timeout=2)
+
+
+@pytest.mark.anyio
+async def test_non_compliant_notification_response(non_sdk_server: None, non_sdk_server_port: int) -> None:
+    """
+    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
+
+    async def message_handler(
+        message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
+    ):
+        nonlocal returned_exception
+        if isinstance(message, Exception):
+            returned_exception = message
+
+    async with streamablehttp_client(server_url) as (read_stream, write_stream, _):
+        async with ClientSession(
+            read_stream,
+            write_stream,
+            message_handler=message_handler,
+        ) as session:
+            # Initialize should work normally
+            await session.initialize()
+
+            # The test server returns a 204 instead of the expected 202
+            await session.send_notification(
+                ClientNotification(RootsListChangedNotification(method="notifications/roots/list_changed"))
+            )
+
+    if returned_exception:
+        pytest.fail(f"Server encountered an exception: {returned_exception}")