fix: Raise exceptions in default ClientSession message handler (#1401)

Author: matthew-gries

src/mcp/client/session.py

@@ -58,6 +58,9 @@ async def __call__(
 async def _default_message_handler(
     message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
 ) -> None:
+    if isinstance(message, Exception):
+        logging.exception("Exception in MCP client message handler")
+        raise message
     await anyio.lowlevel.checkpoint()
 
 

src/mcp/shared/session.py

@@ -415,8 +415,9 @@ async def _receive_loop(self) -> None:
                         if stream:
                             await stream.send(message.message.root)
                         else:
-                            await self._handle_incoming(
-                                RuntimeError(f"Received response with an unknown request ID: {message}")
+                            logging.warning(
+                                f"Received response for unknown request ID {message.message.root.id}. "
+                                f"Response was: {message.message.root}"
                             )
 
             except anyio.ClosedResourceError:

tests/client/test_session.py

@@ -688,3 +688,53 @@ async def mock_server():
         await session.initialize()
 
         await session.call_tool(name=mocked_tool.name, arguments={"foo": "bar"}, meta=meta)
+
+
+@pytest.mark.anyio
+async def test_default_message_handler_raises_exception(caplog: pytest.LogCaptureFixture):
+    """Test that default message handler raises exceptions it receives"""
+    client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
+    server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
+
+    async def mock_server():
+        session_message = await client_to_server_receive.receive()
+        jsonrpc_request = session_message.message
+        assert isinstance(jsonrpc_request.root, JSONRPCRequest)
+
+        result = ServerResult(
+            InitializeResult(
+                protocolVersion=LATEST_PROTOCOL_VERSION,
+                capabilities=ServerCapabilities(),
+                serverInfo=Implementation(name="mock-server", version="0.1.0"),
+            )
+        )
+
+        await server_to_client_send.send(
+            SessionMessage(
+                JSONRPCMessage(
+                    JSONRPCResponse(
+                        jsonrpc="2.0",
+                        id=jsonrpc_request.root.id,
+                        result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
+                    )
+                )
+            )
+        )
+
+        await client_to_server_receive.receive()
+        await server_to_client_send.send(ValueError("Test error from transport"))
+
+    async with (
+        ClientSession(server_to_client_receive, client_to_server_send) as session,
+        anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        tg.start_soon(mock_server)
+        await session.initialize()
+        await anyio.sleep(0.1)
+
+    assert "Exception in MCP client message handler" in caplog.text
+    assert "Unhandled exception in receive loop" in caplog.text