fix: Add special handling for UIDocument serialization to prevent infinite loops (#586)

* fix: search inactive objects when setActive=true in modify

When trying to activate an inactive GameObject via manage_gameobject modify with setActive=true, the lookup would fail because inactive objects were not included in the search by default.

Now automatically sets searchInactive=true when setActive=true is specified, allowing inactive objects to be found and activated.

* fix: Add special handling for UIDocument serialization to prevent infinite loops (#585)

UIDocument.rootVisualElement contains circular parent/child references that
can cause infinite serialization loops. This adds special handling similar to
Transform and Camera components.

The fix:
- Safely serializes panelSettings, visualTreeAsset, sortingOrder, enabled, parentUI
- Explicitly skips rootVisualElement to prevent circular reference issues
- Includes a note explaining why rootVisualElement is skipped

Tested on Unity 2021.3 and Unity 6.3.

* refactor: Extract SerializeAssetReference helper and align UIDocument structure

- Add SerializeAssetReference() helper for consistent asset reference serialization
- UIDocument now uses same return structure as Camera (typeName, instanceID, properties)
- Reduces code duplication in special-case handlers
- Enhanced test coverage to verify structure matches Camera pattern

* fix: Handle UIDocument subclasses and add negative assertion for rootVisualElement

Address code review feedback:
- Add IsOrDerivedFrom() helper to detect UIDocument and any subclasses by walking
  the base-type chain, ensuring derived types also get special-case handling
- Add negative assertion verifying rootVisualElement is NOT in serialized output

MCPForUnity/Editor/Helpers/GameObjectSerializer.cs

@@ -114,6 +114,48 @@ namespace MCPForUnity.Editor.Helpers
         private static readonly Dictionary<Tuple<Type, bool>, CachedMetadata> _metadataCache = new Dictionary<Tuple<Type, bool>, CachedMetadata>();
         // --- End Metadata Caching ---
 
+        /// <summary>
+        /// Checks if a type is or derives from a type with the specified full name.
+        /// Used to detect special-case components including their subclasses.
+        /// </summary>
+        private static bool IsOrDerivedFrom(Type type, string baseTypeFullName)
+        {
+            Type current = type;
+            while (current != null)
+            {
+                if (current.FullName == baseTypeFullName)
+                    return true;
+                current = current.BaseType;
+            }
+            return false;
+        }
+
+        /// <summary>
+        /// Serializes a UnityEngine.Object reference to a dictionary with name, instanceID, and assetPath.
+        /// Used for consistent serialization of asset references in special-case component handlers.
+        /// </summary>
+        /// <param name="obj">The Unity object to serialize</param>
+        /// <param name="includeAssetPath">Whether to include the asset path (default true)</param>
+        /// <returns>A dictionary with the object's reference info, or null if obj is null</returns>
+        private static Dictionary<string, object> SerializeAssetReference(UnityEngine.Object obj, bool includeAssetPath = true)
+        {
+            if (obj == null) return null;
+            
+            var result = new Dictionary<string, object>
+            {
+                { "name", obj.name },
+                { "instanceID", obj.GetInstanceID() }
+            };
+            
+            if (includeAssetPath)
+            {
+                var assetPath = AssetDatabase.GetAssetPath(obj);
+                result["assetPath"] = string.IsNullOrEmpty(assetPath) ? null : assetPath;
+            }
+            
+            return result;
+        }
+
         /// <summary>
         /// Creates a serializable representation of a Component, attempting to serialize
         /// public properties and fields using reflection, with caching and control over non-public fields.
@@ -220,6 +262,72 @@ namespace MCPForUnity.Editor.Helpers
             }
             // --- End Special handling for Camera ---
 
+            // --- Special handling for UIDocument to avoid infinite loops from VisualElement hierarchy (Issue #585) ---
+            // UIDocument.rootVisualElement contains circular parent/child references that cause infinite serialization loops.
+            // Use IsOrDerivedFrom to also catch subclasses of UIDocument.
+            if (IsOrDerivedFrom(componentType, "UnityEngine.UIElements.UIDocument"))
+            {
+                var uiDocProperties = new Dictionary<string, object>();
+
+                try
+                {
+                    // Get panelSettings reference safely
+                    var panelSettingsProp = componentType.GetProperty("panelSettings");
+                    if (panelSettingsProp != null)
+                    {
+                        var panelSettings = panelSettingsProp.GetValue(c) as UnityEngine.Object;
+                        uiDocProperties["panelSettings"] = SerializeAssetReference(panelSettings);
+                    }
+
+                    // Get visualTreeAsset reference safely (the UXML file)
+                    var visualTreeAssetProp = componentType.GetProperty("visualTreeAsset");
+                    if (visualTreeAssetProp != null)
+                    {
+                        var visualTreeAsset = visualTreeAssetProp.GetValue(c) as UnityEngine.Object;
+                        uiDocProperties["visualTreeAsset"] = SerializeAssetReference(visualTreeAsset);
+                    }
+
+                    // Get sortingOrder safely
+                    var sortingOrderProp = componentType.GetProperty("sortingOrder");
+                    if (sortingOrderProp != null)
+                    {
+                        uiDocProperties["sortingOrder"] = sortingOrderProp.GetValue(c);
+                    }
+
+                    // Get enabled state (from Behaviour base class)
+                    var enabledProp = componentType.GetProperty("enabled");
+                    if (enabledProp != null)
+                    {
+                        uiDocProperties["enabled"] = enabledProp.GetValue(c);
+                    }
+
+                    // Get parentUI reference safely (no asset path needed - it's a scene reference)
+                    var parentUIProp = componentType.GetProperty("parentUI");
+                    if (parentUIProp != null)
+                    {
+                        var parentUI = parentUIProp.GetValue(c) as UnityEngine.Object;
+                        uiDocProperties["parentUI"] = SerializeAssetReference(parentUI, includeAssetPath: false);
+                    }
+
+                    // NOTE: rootVisualElement is intentionally skipped - it contains circular
+                    // parent/child references that cause infinite serialization loops
+                    uiDocProperties["_note"] = "rootVisualElement skipped to prevent circular reference loops";
+                }
+                catch (Exception e)
+                {
+                    McpLog.Warn($"[GetComponentData] Error reading UIDocument properties: {e.Message}");
+                }
+
+                // Return structure matches Camera special handling (typeName, instanceID, properties)
+                return new Dictionary<string, object>
+                {
+                    { "typeName", componentType.FullName },
+                    { "instanceID", c.GetInstanceID() },
+                    { "properties", uiDocProperties }
+                };
+            }
+            // --- End Special handling for UIDocument ---
+
             var data = new Dictionary<string, object>
             {
                 { "typeName", componentType.FullName },

TestProjects/UnityMCPTests/Assets/Tests/EditMode/Tools/UIDocumentSerializationTests.cs

@@ -0,0 +1,247 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.UIElements;
+using UnityEditor;
+using MCPForUnity.Editor.Helpers;
+
+namespace MCPForUnityTests.Editor.Tools
+{
+    /// <summary>
+    /// Tests for UIDocument component serialization.
+    /// Reproduces issue #585: UIDocument component causes infinite loop when serializing components
+    /// due to circular parent/child references in rootVisualElement.
+    /// </summary>
+    public class UIDocumentSerializationTests
+    {
+        private GameObject testGameObject;
+        private PanelSettings testPanelSettings;
+        private VisualTreeAsset testVisualTreeAsset;
+
+        [SetUp]
+        public void SetUp()
+        {
+            // Create a test GameObject
+            testGameObject = new GameObject("UIDocumentTestObject");
+            
+            // Create PanelSettings asset (required for UIDocument to have a rootVisualElement)
+            testPanelSettings = ScriptableObject.CreateInstance<PanelSettings>();
+            
+            // Create a minimal VisualTreeAsset
+            // Note: VisualTreeAsset cannot be created via CreateInstance, we need to use AssetDatabase
+            // For the test, we'll create a temporary UXML file
+            CreateTestVisualTreeAsset();
+        }
+
+        [TearDown]
+        public void TearDown()
+        {
+            // Clean up test GameObject
+            if (testGameObject != null)
+            {
+                UnityEngine.Object.DestroyImmediate(testGameObject);
+            }
+            
+            // Clean up ScriptableObject instances
+            if (testPanelSettings != null)
+            {
+                UnityEngine.Object.DestroyImmediate(testPanelSettings);
+            }
+            
+            // Clean up temporary UXML file
+            CleanupTestVisualTreeAsset();
+        }
+
+        private void CreateTestVisualTreeAsset()
+        {
+            // Create a minimal UXML file for testing
+            string uxmlPath = "Assets/Tests/EditMode/Tools/TestUIDocument.uxml";
+            string uxmlContent = @"<ui:UXML xmlns:ui=""UnityEngine.UIElements"">
+    <ui:VisualElement name=""root"">
+        <ui:Label text=""Test Label"" />
+    </ui:VisualElement>
+</ui:UXML>";
+            
+            // Ensure directory exists
+            string directory = System.IO.Path.GetDirectoryName(uxmlPath);
+            if (!System.IO.Directory.Exists(directory))
+            {
+                System.IO.Directory.CreateDirectory(directory);
+            }
+            
+            System.IO.File.WriteAllText(uxmlPath, uxmlContent);
+            AssetDatabase.Refresh();
+            
+            testVisualTreeAsset = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>(uxmlPath);
+        }
+
+        private void CleanupTestVisualTreeAsset()
+        {
+            string uxmlPath = "Assets/Tests/EditMode/Tools/TestUIDocument.uxml";
+            if (System.IO.File.Exists(uxmlPath))
+            {
+                AssetDatabase.DeleteAsset(uxmlPath);
+            }
+        }
+
+        /// <summary>
+        /// Test that UIDocument component can be serialized without infinite loops.
+        /// This test reproduces issue #585 where UIDocument causes infinite loop 
+        /// when both visualTreeAsset and panelSettings are assigned.
+        /// 
+        /// The bug: UIDocument.rootVisualElement returns a VisualElement with circular
+        /// parent/child references (children[] → child elements → parent → back to parent).
+        /// 
+        /// Note: NUnit [Timeout] will fail this test if serialization hangs.
+        /// </summary>
+        [Test]
+        [Timeout(10000)] // 10 second timeout - if serialization hangs, test fails
+        public void GetComponentData_UIDocument_WithBothAssetsAssigned_DoesNotHang()
+        {
+            // Skip test if we couldn't create the VisualTreeAsset
+            if (testVisualTreeAsset == null)
+            {
+                Assert.Inconclusive("Could not create test VisualTreeAsset - test cannot run");
+            }
+
+            // Arrange - Add UIDocument component with both assets assigned
+            var uiDocument = testGameObject.AddComponent<UIDocument>();
+            uiDocument.panelSettings = testPanelSettings;
+            uiDocument.visualTreeAsset = testVisualTreeAsset;
+
+            // Act - This should NOT hang or cause infinite loop
+            // The [Timeout] attribute will fail the test if it takes too long
+            object result = GameObjectSerializer.GetComponentData(uiDocument);
+
+            // Assert
+            Assert.IsNotNull(result, "Should return serialized component data");
+            
+            var resultDict = result as Dictionary<string, object>;
+            Assert.IsNotNull(resultDict, "Result should be a dictionary");
+            Assert.AreEqual("UnityEngine.UIElements.UIDocument", resultDict["typeName"]);
+        }
+
+        /// <summary>
+        /// Test that UIDocument serialization includes expected properties.
+        /// Verifies the structure matches Camera special handling (typeName, instanceID, properties).
+        /// </summary>
+        [Test]
+        [Timeout(10000)]
+        public void GetComponentData_UIDocument_ReturnsExpectedProperties()
+        {
+            // Skip test if we couldn't create the VisualTreeAsset
+            if (testVisualTreeAsset == null)
+            {
+                Assert.Inconclusive("Could not create test VisualTreeAsset - test cannot run");
+            }
+
+            // Arrange
+            var uiDocument = testGameObject.AddComponent<UIDocument>();
+            uiDocument.panelSettings = testPanelSettings;
+            uiDocument.visualTreeAsset = testVisualTreeAsset;
+            uiDocument.sortingOrder = 42;
+
+            // Act
+            var result = GameObjectSerializer.GetComponentData(uiDocument);
+
+            // Assert
+            Assert.IsNotNull(result, "Should return serialized component data");
+            
+            var resultDict = result as Dictionary<string, object>;
+            Assert.IsNotNull(resultDict, "Result should be a dictionary");
+            
+            // Check for expected top-level keys (matches Camera special handling structure)
+            Assert.IsTrue(resultDict.ContainsKey("typeName"), "Should contain typeName");
+            Assert.IsTrue(resultDict.ContainsKey("instanceID"), "Should contain instanceID");
+            Assert.IsTrue(resultDict.ContainsKey("properties"), "Should contain properties");
+            
+            // Verify type name
+            Assert.AreEqual("UnityEngine.UIElements.UIDocument", resultDict["typeName"], 
+                "typeName should be UIDocument");
+            
+            // Verify properties dict contains expected keys
+            var properties = resultDict["properties"] as Dictionary<string, object>;
+            Assert.IsNotNull(properties, "properties should be a dictionary");
+            Assert.IsTrue(properties.ContainsKey("panelSettings"), "Should have panelSettings");
+            Assert.IsTrue(properties.ContainsKey("visualTreeAsset"), "Should have visualTreeAsset");
+            Assert.IsTrue(properties.ContainsKey("sortingOrder"), "Should have sortingOrder");
+            Assert.IsTrue(properties.ContainsKey("enabled"), "Should have enabled");
+            Assert.IsTrue(properties.ContainsKey("_note"), "Should have _note about skipped rootVisualElement");
+            
+            // CRITICAL: Verify rootVisualElement is NOT included (this is the fix for Issue #585)
+            Assert.IsFalse(properties.ContainsKey("rootVisualElement"), 
+                "Should NOT include rootVisualElement - it causes circular reference loops");
+            
+            // Verify asset references use consistent structure (name, instanceID, assetPath)
+            var panelSettingsRef = properties["panelSettings"] as Dictionary<string, object>;
+            Assert.IsNotNull(panelSettingsRef, "panelSettings should be serialized as dictionary");
+            Assert.IsTrue(panelSettingsRef.ContainsKey("name"), "panelSettings should have name");
+            Assert.IsTrue(panelSettingsRef.ContainsKey("instanceID"), "panelSettings should have instanceID");
+        }
+
+        /// <summary>
+        /// Test that UIDocument WITHOUT assets assigned doesn't cause issues.
+        /// This is a baseline test - the bug only occurs with both assets assigned.
+        /// </summary>
+        [Test]
+        public void GetComponentData_UIDocument_WithoutAssets_Succeeds()
+        {
+            // Arrange - Add UIDocument component WITHOUT assets
+            var uiDocument = testGameObject.AddComponent<UIDocument>();
+            // Don't assign panelSettings or visualTreeAsset
+
+            // Act
+            var result = GameObjectSerializer.GetComponentData(uiDocument);
+
+            // Assert
+            Assert.IsNotNull(result, "Should return serialized component data");
+            
+            var resultDict = result as Dictionary<string, object>;
+            Assert.IsNotNull(resultDict, "Result should be a dictionary");
+            Assert.AreEqual("UnityEngine.UIElements.UIDocument", resultDict["typeName"]);
+        }
+
+        /// <summary>
+        /// Test that UIDocument with only panelSettings assigned doesn't cause issues.
+        /// </summary>
+        [Test]
+        public void GetComponentData_UIDocument_WithOnlyPanelSettings_Succeeds()
+        {
+            // Arrange
+            var uiDocument = testGameObject.AddComponent<UIDocument>();
+            uiDocument.panelSettings = testPanelSettings;
+            // Don't assign visualTreeAsset
+
+            // Act
+            var result = GameObjectSerializer.GetComponentData(uiDocument);
+
+            // Assert
+            Assert.IsNotNull(result, "Should return serialized component data");
+        }
+
+        /// <summary>
+        /// Test that UIDocument with only visualTreeAsset assigned doesn't cause issues.
+        /// </summary>
+        [Test]
+        public void GetComponentData_UIDocument_WithOnlyVisualTreeAsset_Succeeds()
+        {
+            // Skip test if we couldn't create the VisualTreeAsset
+            if (testVisualTreeAsset == null)
+            {
+                Assert.Inconclusive("Could not create test VisualTreeAsset - test cannot run");
+            }
+
+            // Arrange
+            var uiDocument = testGameObject.AddComponent<UIDocument>();
+            uiDocument.visualTreeAsset = testVisualTreeAsset;
+            // Don't assign panelSettings
+
+            // Act
+            var result = GameObjectSerializer.GetComponentData(uiDocument);
+
+            // Assert
+            Assert.IsNotNull(result, "Should return serialized component data");
+        }
+    }
+}