fix(client): allow ClientSessionGroup to connect servers with no components (#3578)

Author: teddiesloco

src/mcp/client/session_group.py

@@ -414,11 +414,6 @@ async def _aggregate_components(self, server_info: types.Implementation, session
         except MCPError as err:  # pragma: no cover
             logging.warning(f"Could not fetch tools: {err}")
 
-        # Clean up exit stack for session if we couldn't retrieve anything
-        # from the server.
-        if not any((prompts_temp, resources_temp, tools_temp)):
-            del self._session_exit_stacks[session]  # pragma: no cover
-
         # Check for duplicates.
         matching_prompts = prompts_temp.keys() & self._prompts.keys()
         if matching_prompts:

tests/client/test_session_group.py

@@ -402,3 +402,27 @@ async def test_client_session_group_establish_session_parameterized(
             # 3. Assert returned values
             assert returned_server_info is mock_initialize_result.server_info
             assert returned_session is mock_entered_session
+
+
+@pytest.mark.anyio
+async def test_client_session_group_connect_empty_components_server():
+    """Test connecting a server that exposes no components (tools, prompts, or resources)."""
+    mock_server_info = mock.Mock(spec=types.Implementation)
+    mock_server_info.name = "EmptyServer"
+    mock_session = mock.AsyncMock(spec=mcp.ClientSession)
+    mock_session.list_tools.return_value = mock.AsyncMock(tools=[])
+    mock_session.list_resources.return_value = mock.AsyncMock(resources=[])
+    mock_session.list_prompts.return_value = mock.AsyncMock(prompts=[])
+
+    group = ClientSessionGroup()
+    # Should not raise KeyError when aggregating empty components
+    await group.connect_with_session(mock_server_info, mock_session)
+
+    assert mock_session in group._sessions
+    assert group.tools == {}
+    assert group.resources == {}
+    assert group.prompts == {}
+
+    # Disconnecting should also succeed cleanly
+    await group.disconnect_from_server(mock_session)
+    assert mock_session not in group._sessions