Unity shows this USS warning when loading the MCP window:

Unknown pseudo class "last-child" in StyleSheet Common
UnityEditor.AssetDatabase:LoadAssetAtPath<UnityEngine.UIElements.StyleSheet> (string)
MCPForUnity.Editor.Windows.MCPForUnityEditorWindow:CreateGUI ()

Unity UI Toolkit doesn't support the :last-child pseudo-class.
The USS file needs to be updated to remove or replace this selector.

Unity version: 6000.3.5f1
MCP package version: 6e9594da7766