Unity-Technologies/UnityCsReference · error · ArgumentException

Input instance root is using the HideFlags.DontSaveInEditor

Error message

Input instance root is using the HideFlags.DontSaveInEditor flag which is not supported when replacing: Input instance: 

What it means

Thrown by ThrowIfInvalidArgumentsForReplacePrefabInstance when prefabInstanceRoot.hideFlags or its transform's hideFlags has HideFlags.DontSaveInEditor set. Objects marked DontSaveInEditor are not serialized to the scene/Prefab, so replacing their Prefab asset would produce inconsistent state. This guard rejects such objects before the replace.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:2264

                ThrowIfInvalidAssetForReplacePrefabInstance(prefabAssetRoot, mode);

            if (!IsPartOfNonAssetPrefabInstance(prefabInstanceRoot))
                throw new InvalidOperationException(string.Format("Input '{0}' is not a Prefab instance, for plain GameObjects use ConvertToPrefabInstance() instead", prefabInstanceRoot.name));

            if (!IsOutermostPrefabInstanceRoot(prefabInstanceRoot))
                throw new ArgumentException("Input instance is not an outermost Prefab instance root. Input instance: " + prefabInstanceRoot.name, nameof(prefabInstanceRoot));
            if (EditorUtility.IsPersistent(prefabInstanceRoot))
                throw new ArgumentException("Input instance root is from a Prefab asset, this is not supported. Input instance: " + prefabInstanceRoot.name, nameof(prefabInstanceRoot));

            if (PrefabStageUtility.IsGameObjectThePrefabRootInAnyPrefabStage(prefabInstanceRoot))
                throw new InvalidOperationException("Replacing the root Prefab instance in a Variant is not supported since it will break all overrides for existing instances of this Variant, including their positions and rotations." + prefabInstanceRoot.name);
            if (IsAnyPrefabInstanceRoot(prefabInstanceRoot) && EditorSceneManager.IsPreviewSceneObject(prefabInstanceRoot) && prefabInstanceRoot.transform.parent == null) // EditPrefabContentsScope handling
                throw new InvalidOperationException("Replacing the Variant parent is not supported since it will break all overrides for existing instances of this Variant, including their positions and rotations." + prefabInstanceRoot.name);
            if (prefabInstanceRoot.transform.GetType() != prefabAssetRoot.transform.GetType())
                throw new InvalidOperationException(string.Format("Cannot replace the Prefab instance '{0}' with root transform of type {1} with a Prefab asset with root transform of type {2}. Transform types must match.", prefabInstanceRoot.name, prefabInstanceRoot.transform.GetType().Name, prefabAssetRoot.transform.GetType().Name));

            if (prefabInstanceRoot.hideFlags.HasFlag(HideFlags.DontSaveInEditor) || prefabInstanceRoot.transform.hideFlags.HasFlag(HideFlags.DontSaveInEditor))
                throw new ArgumentException("Input instance root is using the HideFlags.DontSaveInEditor flag which is not supported when replacing: Input instance: " + prefabInstanceRoot.name, nameof(prefabInstanceRoot));

            if (mode == InteractionMode.UserAction)
            {
                // Recording undo does not handle missing scripts
                var gameObjectsWithInvalidScript = FindGameObjectsWithInvalidComponent(prefabInstanceRoot);
                if (gameObjectsWithInvalidScript.Count > 0)
                    throw new InvalidOperationException(string.Format($"Cannot replace the Prefab instance when it has a missing script. GameObject '{gameObjectsWithInvalidScript[0].name}' has a missing script. Use InteractionMode.AutomatedAction to force the replace."));
            }
        }

        public static void ReplacePrefabAssetOfPrefabInstances(GameObject[] prefabInstanceRoots, GameObject prefabAssetRoot, InteractionMode mode)
        {
            ReplacePrefabAssetOfPrefabInstances(prefabInstanceRoots, prefabAssetRoot, new PrefabReplacingSettings(), mode);
        }

        public static void ReplacePrefabAssetOfPrefabInstances(GameObject[] prefabInstanceRoots, GameObject prefabAssetRoot, PrefabReplacingSettings settings, InteractionMode mode)
        {
            if (prefabInstanceRoots == null)

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Remove HideFlags.DontSaveInEditor from the instance root (and its transform) before calling ReplacePrefabAssetOfPrefabInstance.
2. Operate on a normal scene instance that is serialized normally.
3. Guard: if (obj.hideFlags.HasFlag(HideFlags.DontSaveInEditor)) return;

Example fix

// before
PrefabUtility.ReplacePrefabAssetOfPrefabInstance(instance, asset, mode);
// instance has HideFlags.DontSaveInEditor

// after
instance.hideFlags = HideFlags.None;
instance.transform.hideFlags = HideFlags.None;
PrefabUtility.ReplacePrefabAssetOfPrefabInstance(instance, asset, mode);

if (prefabInstanceRoot.hideFlags.HasFlag(HideFlags.DontSaveInEditor)
    || prefabInstanceRoot.transform.hideFlags.HasFlag(HideFlags.DontSaveInEditor))
{
    prefabInstanceRoot.hideFlags = HideFlags.None;
    prefabInstanceRoot.transform.hideFlags = HideFlags.None;
}
PrefabUtility.ReplacePrefabAssetOfPrefabInstance(prefabInstanceRoot, prefabAssetRoot, mode);

static bool HasNoDontSaveFlag(GameObject obj)
{
    return obj != null
        && !obj.hideFlags.HasFlag(HideFlags.DontSaveInEditor)
        && !obj.transform.hideFlags.HasFlag(HideFlags.DontSaveInEditor);
}

When it happens

Trigger: Passing an editor-only helper GameObject (created with HideFlags.DontSaveInEditor so it doesn't persist) that happens to be a Prefab instance. Custom editor tooling that creates transient Prefab instances with DontSaveInEditor and then tries to replace them.

Common situations: Gizmo/preview objects created at runtime in editor with DontSaveInEditor. Objects spawned by editor extensions that intentionally avoid serialization but are accidentally flagged as Prefab instances.

Related errors

• Input GameObject is using the HideFlags.DontSaveInEditor fla
• Input '{0}' is not a Prefab instance, for plain GameObjects
• Input instance is not an outermost Prefab instance root. Inp
• Input instance root is from a Prefab asset, this is not supp
• Replacing the root Prefab instance in a Variant is not suppo

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/2c2aa1ac6098879f. Report an issue: GitHub.