Unity-Technologies/UnityCsReference · error · ArgumentException

Overwriting a folder with an Asset is not allowed: '{path}'

Error message

Overwriting a folder with an Asset is not allowed: '{path}'

What it means

Thrown by PrefabUtility.ValidatePath when the target path for saving a Prefab resolves to an existing directory on disk. Unity prevents writing an asset file over a folder because the filesystem conflict would corrupt the operation. The check uses Directory.Exists(path) after confirming the path is a non-empty, valid .prefab asset path.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:2047

                throw new ArgumentException("Can't save a Prefab instance");

            var root = asset.transform.root.gameObject;
            if (root != asset)
                throw new ArgumentException("GameObject to save Prefab from must be a Prefab root");

            return SavePrefabAsset_Internal(root, out savedSuccessfully);
        }

        internal static void ValidatePath(GameObject instanceRoot, string path)
        {
            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);

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Ensure assetPath ends with a filename and .prefab extension, e.g. "Assets/Prefabs/MyPrefab.prefab" not "Assets/Prefabs".
2. Check Directory.Exists(path) before calling SaveAsPrefabAsset and append a filename if true.
3. Use Path.Combine with an explicit filename component rather than passing a bare folder path.

Example fix

// before
PrefabUtility.SaveAsPrefabAsset(go, "Assets/Prefabs");
// after
PrefabUtility.SaveAsPrefabAsset(go, "Assets/Prefabs/MyPrefab.prefab");

void SavePrefabSafe(GameObject go, string path)
{
    if (Directory.Exists(path))
        throw new ArgumentException("Path is a folder, append a filename: " + path);
    if (!path.EndsWith(".prefab"))
        path = Path.Combine(path, go.name + ".prefab");
    PrefabUtility.SaveAsPrefabAsset(go, path);
}

static bool IsValidPrefabFilePath(string path) =>
    !string.IsNullOrEmpty(path) &&
    path.EndsWith(".prefab") &&
    !Directory.Exists(path) &&
    Paths.IsValidAssetPath(path, ".prefab");

When it happens

Trigger: Calling SaveAsPrefabAsset, SaveAsPrefabAssetAndConnect, or any internal save path where ValidatePath is invoked, with an assetPath string that matches an existing folder name (e.g. "Assets/MyFolder" when MyFolder exists as a directory). Can also occur if the path has no ".prefab" extension but still passes IsValidAssetPath and coincides with a real directory.

Common situations: Developer passes a folder path instead of a full file path (forgets to append "/MyPrefab.prefab"). Path constructed by concatenating a directory variable without a filename. Migrating from older Unity APIs that were more lenient about path formatting.

Related errors

• Saving Prefab to immutable folder 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.
• Invalid build path: '{buildPlayerOptions.locationPathName}'.

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/ba6e0ee1aac87057. Report an issue: GitHub.