diff --git a/src/GameCult.Unity/Assets/Caching/Editor/CultCacheStudioDrawers.cs b/src/GameCult.Unity/Assets/Caching/Editor/CultCacheStudioDrawers.cs
index 8ffbee05..8f008f24 100644
--- a/src/GameCult.Unity/Assets/Caching/Editor/CultCacheStudioDrawers.cs
+++ b/src/GameCult.Unity/Assets/Caching/Editor/CultCacheStudioDrawers.cs
@@ -177,11 +177,20 @@ namespace GameCult.Unity.Caching.Editor
                 GUILayout.MaxHeight(Mathf.Max(textArea.MaxLines, textArea.MinLines, 1) * EditorGUIUtility.singleLineHeight));
         }
 
-        // Every zero-hex GUID that AssetDatabase uses for built-in/generated resources (the ordinary
-        // "all zero" form and the "...f000000000000000" virtual-asset form) has no .meta file and no
-        // Addressables entry, so it cannot be stored as a key.
-        private static bool IsBuiltinResourceGuid(string guid) =>
-            !string.IsNullOrEmpty(guid) && guid.TrimEnd('0', 'f') == string.Empty;
+        // A GUID-string heuristic ("all zero", "...f000...") cannot cover every built-in Unity ships:
+        // "Library/unity default resources" built-ins (verified in 6000.3.24f1, e.g. LegacyRuntime font
+        // and the Cube mesh under GUID 0000000000000000e000000000000000) carry ordinary-looking hex GUIDs
+        // with no all-zero or all-f suffix. Ask AssetDatabase about the object instead: a real project
+        // asset is always either the main asset at its path or one of its sub-assets, and always lives
+        // under Assets/ or Packages/. Built-ins are neither (verified False for both IsMainAsset and
+        // IsSubAsset) and resolve to no path under either root.
+        private static bool IsBuiltinResource(Object asset)
+        {
+            if (asset == null) return false;
+            if (AssetDatabase.IsMainAsset(asset) || AssetDatabase.IsSubAsset(asset)) return false;
+            var path = AssetDatabase.GetAssetPath(asset);
+            return string.IsNullOrEmpty(path) || !(path.StartsWith("Assets/", StringComparison.Ordinal) || path.StartsWith("Packages/", StringComparison.Ordinal));
+        }
 
         // Reads and writes the Addressables key form ruled in docs/addressables-cut.md (operator ruling S):
         // the bare GUID for a main asset, "guid[subAssetName]" for a picked sub-asset. Never rewrites a
@@ -210,19 +219,26 @@ namespace GameCult.Unity.Caching.Editor
                 stale = asset == null;
             }
 
-            if (stale) EditorGUILayout.HelpBox(label + ": stored value \"" + value + "\" does not resolve to an asset.", MessageType.Warning);
+            if (stale)
+            {
+                using (new EditorGUILayout.HorizontalScope())
+                {
+                    EditorGUILayout.HelpBox(label + ": stored value \"" + value + "\" does not resolve to an asset.", MessageType.Warning);
+                    if (GUILayout.Button("Clear", GUILayout.Width(48), GUILayout.Height(EditorGUIUtility.singleLineHeight))) return string.Empty;
+                }
+            }
 
             var next = EditorGUILayout.ObjectField(label, asset, assetType, false);
             if (next == asset) return value;
             if (next == null) return string.Empty;
 
-            var nextGuid = AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(next));
-            if (string.IsNullOrEmpty(nextGuid) || IsBuiltinResourceGuid(nextGuid))
+            if (IsBuiltinResource(next))
             {
                 Debug.LogWarning(label + ": \"" + next.name + "\" is a built-in resource with no Addressables GUID. Not stored.");
                 return value;
             }
 
+            var nextGuid = AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(next));
             return CultAssetGuidKey.Format(nextGuid, AssetDatabase.IsSubAsset(next) ? next.name : null);
         }
 
diff --git a/src/GameCult.Unity/Assets/Caching/README.md b/src/GameCult.Unity/Assets/Caching/README.md
index 6b5097dc..7f1d7ab6 100644
--- a/src/GameCult.Unity/Assets/Caching/README.md
+++ b/src/GameCult.Unity/Assets/Caching/README.md
@@ -167,8 +167,10 @@ public sealed class ItemData
   `AssetReference` keeps and `Addressables.LoadAssetAsync<T>` accepts; optional
   engine asset `Type`, named by the consumer; the Studio uses it when it is a
   `UnityEngine.Object` type, refuses to store a built-in resource's GUID since
-  it has no Addressables entry, and warns rather than silently clearing a
-  stored value that no longer resolves)
+  it has no Addressables entry (checked by asking AssetDatabase whether the
+  picked object is a main or sub-asset under `Assets/` or `Packages/`, not by
+  guessing from the GUID's hex digits), and never silently clears a stored
+  value that no longer resolves — it warns and offers a Clear button instead)
 - `CultInspectorDrawer` (on drawer classes)
 
 CultMesh collaboration should feed the same CultCache mutation surface rather
diff --git a/tests/GameCult.Caching.Tests/CultAssetGuidKeyTests.cs b/tests/GameCult.Caching.Tests/CultAssetGuidKeyTests.cs
index e8f6d055..4838fd86 100644
--- a/tests/GameCult.Caching.Tests/CultAssetGuidKeyTests.cs
+++ b/tests/GameCult.Caching.Tests/CultAssetGuidKeyTests.cs
@@ -70,5 +70,63 @@ namespace GameCult.Caching.Tests
         {
             Assert.That(CultAssetGuidKey.Format("abc123", "Icon_Small"), Is.EqualTo("abc123[Icon_Small]"));
         }
+
+        [Test]
+        public void SubAssetNameWithLeadingAndTrailingSpacesRoundTrips()
+        {
+            // A name.Trim() mutant would strip the spaces on the way out; Addressables sub-asset
+            // names are taken verbatim, so the exact bytes between the brackets must survive.
+            Assert.That(CultAssetGuidKey.TryParse("abc123[ Icon Small ]", out var guid, out var sub), Is.True);
+            Assert.That(guid, Is.EqualTo("abc123"));
+            Assert.That(sub, Is.EqualTo(" Icon Small "));
+        }
+
+        [Test]
+        public void FormatThenTryParseRoundTripsBareGuid()
+        {
+            var key = CultAssetGuidKey.Format("abc123", null);
+            Assert.That(CultAssetGuidKey.TryParse(key, out var guid, out var sub), Is.True);
+            Assert.That(guid, Is.EqualTo("abc123"));
+            Assert.That(sub, Is.Null);
+        }
+
+        [Test]
+        public void FormatThenTryParseRoundTripsSubAsset()
+        {
+            var key = CultAssetGuidKey.Format("abc123", "Icon[2]");
+            Assert.That(CultAssetGuidKey.TryParse(key, out var guid, out var sub), Is.True);
+            Assert.That(guid, Is.EqualTo("abc123"));
+            Assert.That(sub, Is.EqualTo("Icon[2]"));
+        }
+
+        [Test]
+        public void FormatOfNullOrEmptyGuidReturnsEmptyString()
+        {
+            Assert.That(CultAssetGuidKey.Format(null!, null), Is.EqualTo(string.Empty));
+            Assert.That(CultAssetGuidKey.Format(string.Empty, null), Is.EqualTo(string.Empty));
+            Assert.That(CultAssetGuidKey.Format(null!, "Icon"), Is.EqualTo("[Icon]"));
+        }
+
+        [Test]
+        public void BareGuidPreservesCase()
+        {
+            // No lowercasing: a mutant that normalizes case would still pass every other test here
+            // since the fixtures above are already lowercase.
+            Assert.That(CultAssetGuidKey.TryParse("ABC123DEF456", out var guid, out _), Is.True);
+            Assert.That(guid, Is.EqualTo("ABC123DEF456"));
+        }
+
+        [Test]
+        public void FormatOfSubAssetNameContainingBracketsMatchesAddressablesSplit()
+        {
+            // Documents the split ruled for Addressables-shaped keys: "g[Icon[2]]" parses back to
+            // name "Icon[2]", i.e. Format takes everything up to the FIRST '[' as the guid and
+            // everything between it and the LAST ']' as the name, brackets included.
+            var key = CultAssetGuidKey.Format("g", "Icon[2]");
+            Assert.That(key, Is.EqualTo("g[Icon[2]]"));
+            Assert.That(CultAssetGuidKey.TryParse(key, out var guid, out var sub), Is.True);
+            Assert.That(guid, Is.EqualTo("g"));
+            Assert.That(sub, Is.EqualTo("Icon[2]"));
+        }
     }
 }