Unity-Technologies/UnityCsReference · error · ArgumentException

Saving Prefab to immutable folder is not allowed: '{path}'

Error message

Saving Prefab to immutable folder is not allowed: '{path}'

What it means

Thrown by ValidatePath when AssetDatabase.TryGetAssetFolderInfo reports the destination directory as an immutable folder (e.g. a built-in package folder or read-only system folder). Unity blocks writes to immutable folders to preserve package integrity and prevent accidental corruption of shipped content.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:2057

        {
            if (String.IsNullOrEmpty(path))
                throw new ArgumentNullException("path is null or empty");

            if (!Paths.IsValidAssetPath(path, ".prefab"))
                throw new ArgumentException("Given path is not valid: '" + path + "'");

            if (Directory.Exists(path))
                throw new ArgumentException("Overwriting a folder with an Asset is not allowed: '" + path + "'");

            string directory = Path.GetDirectoryName(path);

            // We allow relative paths outside the Assets folder so we do not throw if isValidAssetFolder is false
            bool isRootFolder = false;
            bool isImmutableFolder = false;
            bool isValidAssetFolder = AssetDatabase.TryGetAssetFolderInfo(directory, out isRootFolder, out isImmutableFolder);

            if (isValidAssetFolder && isImmutableFolder)
                throw new ArgumentException("Saving Prefab to immutable folder is not allowed: '" + path + "'");

            if (directory.Length > 0 && !Directory.Exists(directory))
                throw new ArgumentException("Given path does not exist: '" + path + "'");

            if (isValidAssetFolder)
            {
                string projectRelativePath = Path.IsPathRooted(path) ? FileUtil.GetProjectRelativePath(path) : path;
                string prefabGUID = AssetDatabase.AssetPathToGUID(projectRelativePath);
                if (!VerifyNestingFromScript(new GameObject[] { instanceRoot }, prefabGUID, PrefabUtility.GetPrefabInstanceHandle(instanceRoot)))
                    throw new ArgumentException("Cyclic nesting detected");
            }
        }

        private static void SaveAsPrefabAssetArgumentCheck(GameObject instanceRoot, string path, bool connectToInstance)
        {
            if (instanceRoot == null)
                throw new ArgumentNullException("Parameter root is null");

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Save the Prefab to a writable location under Assets/ (e.g. "Assets/Prefabs/MyPrefab.prefab").
2. If you need it in a package, use a local embedded package under Packages/ that you own, not a registry-imported immutable one.
3. Verify the target folder's mutability with AssetDatabase.TryGetAssetFolderInfo before saving.

Example fix

// before
PrefabUtility.SaveAsPrefabAsset(go, "Packages/com.unity.something/My.prefab");
// after
PrefabUtility.SaveAsPrefabAsset(go, "Assets/Prefabs/My.prefab");

bool IsWritableAssetFolder(string dir)
{
    bool isRoot, isImmutable;
    if (AssetDatabase.TryGetAssetFolderInfo(dir, out isRoot, out isImmutable))
        return !isImmutable;
    return true; // outside Assets, caller's responsibility
}

static bool IsSafePrefabDestination(string path)
{
    string dir = Path.GetDirectoryName(path);
    bool isRoot, isImmutable;
    bool valid = AssetDatabase.TryGetAssetFolderInfo(dir, out isRoot, out isImmutable);
    return !valid || !isImmutable;
}

When it happens

Trigger: Saving a Prefab into a path under a package marked immutable (e.g. "Packages/com.unity.somepackage/..." or "Library/"). The directory resolves as valid and isImmutable=true via TryGetAssetFolderInfo.

Common situations: Attempting to save a Prefab asset into a read-only imported package's folder. Developer confuses a global-package path (immutable) with an embedded local package path. After upgrading a package, its folder may become immutable.

Related errors

• Can't save an immutable Prefab
• Overwriting a folder with an Asset is not allowed: '{path}'
• Given path does not exist: '{path}'
• Can't connect a Prefab in the StreamingAssets folder to Game
• Scene path "{0}" contains invalid directory separators.

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/c102a11dcad20744. Report an issue: GitHub.