fix(client): allow ClientSessionGroup to connect to servers without components

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

@@ -295,6 +295,31 @@ async def test_client_session_group_disconnect_non_existent_server():
         await group.disconnect_from_server(session)
 
 
+@pytest.mark.anyio
+async def test_client_session_group_connect_empty_server_succeeds():
+    """Test connecting to a server that exposes no tools, resources, or prompts."""
+    group = ClientSessionGroup()
+    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=[])
+
+    server_info = types.Implementation(name="empty_server", version="1.0")
+
+    # Should not raise KeyError when connecting via connect_with_session
+    connected_session = await group.connect_with_session(server_info, mock_session)
+    assert connected_session is mock_session
+    assert mock_session in group._sessions
+    assert len(group.tools) == 0
+    assert len(group.resources) == 0
+    assert len(group.prompts) == 0
+
+    # Should disconnect cleanly
+    await group.disconnect_from_server(mock_session)
+    assert mock_session not in group._sessions
+
+
+
 # TODO(Marcelo): This is horrible. We should drop this test.
 @pytest.mark.anyio
 @pytest.mark.parametrize(