Unity-Technologies/UnityCsReference · error · ArgumentException

Incorrect file extension: {prefabAssetPath}. Must be '.prefa

Error message

Incorrect file extension: {prefabAssetPath}. Must be '.prefab'

What it means

Thrown by PrefabStageUtility.OpenPrefab when prefabAssetPath does not end with '.prefab' (case-insensitive). The prefab system only operates on assets with the .prefab extension; any other extension is rejected at the validation boundary.

Source

Thrown at Editor/Mono/SceneManagement/StageManager/PrefabStage/PrefabStageUtility.cs:96

            var prefabStageMode = openedFromInstance != null ? PrefabStage.Mode.InContext : PrefabStage.Mode.InIsolation;
            return OpenPrefab(prefabAssetPath, openedFromInstance, prefabStageMode, changeTypeAnalytics);
        }

        public static PrefabStage OpenPrefab(string prefabAssetPath, GameObject openedFromInstance, PrefabStage.Mode prefabStageMode)
        {
            return OpenPrefab(prefabAssetPath, openedFromInstance, prefabStageMode, StageNavigationManager.Analytics.ChangeType.EnterViaUnknown);
        }

        internal static PrefabStage OpenPrefab(string prefabAssetPath, GameObject openedFromInstance, PrefabStage.Mode prefabStageMode, StageNavigationManager.Analytics.ChangeType changeTypeAnalytics)
        {
            if (string.IsNullOrEmpty(prefabAssetPath))
                throw new ArgumentNullException(nameof(prefabAssetPath));

            if (openedFromInstance != null && !PrefabUtility.IsPartOfPrefabInstance(openedFromInstance))
                throw new ArgumentException("GameObject must be part of a Prefab instance, or null.", nameof(openedFromInstance));

            if (!prefabAssetPath.EndsWith(".prefab", StringComparison.OrdinalIgnoreCase))
                throw new ArgumentException("Incorrect file extension: " + prefabAssetPath + ". Must be '.prefab'", nameof(prefabAssetPath));

            if (AssetDatabase.LoadMainAssetAtPath(prefabAssetPath) == null)
                throw new ArgumentException("Prefab not found at path " + prefabAssetPath, nameof(prefabAssetPath));

            return OpenPrefabMode(prefabAssetPath, openedFromInstance, prefabStageMode, changeTypeAnalytics);
        }

        static PrefabStage GetExistingPrefabStage(string prefabAssetPath, GameObject openedFromInstance, PrefabStage.Mode prefabStageMode)
        {
            var stageHistory = StageNavigationManager.instance.stageHistory;
            for (int i = 1; i < stageHistory.Count; i++)
            {
                var prefabStage = stageHistory[i] as PrefabStage;
                if (prefabStage != null && prefabStage.assetPath == prefabAssetPath)
                {
                    // If PrefabStage.Mode did not match on existing PrefabStage we do not reuse the stage
                    // so we create a new PrefabStage with the correct mode
                    if (prefabStage.mode == prefabStageMode)

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Ensure the path string ends with '.prefab' (case-insensitive).
2. Construct paths using Path.Combine and explicitly append '.prefab'.
3. Validate with prefabAssetPath.EndsWith(".prefab", StringComparison.OrdinalIgnoreCase) before calling.

Example fix

// before
PrefabStageUtility.OpenPrefab("Assets/MyObject", null, mode);

// after
PrefabStageUtility.OpenPrefab("Assets/MyObject.prefab", null, mode);

bool IsValidPrefabPath(string path)
{
    return !string.IsNullOrEmpty(path) && path.EndsWith(".prefab", StringComparison.OrdinalIgnoreCase);
}

When it happens

Trigger: Passing a path with .prefabVariant, .asset, .mat, or no extension. Typo in the path extension. Constructed paths that drop the extension during string manipulation.

Common situations: Path built via Path.GetFileNameWithoutExtension without re-appending '.prefab'. Mixing up prefab asset paths with ScriptableObject '.asset' paths. Cross-platform path separators causing the EndsWith check to miss.

Related errors

• path is null or empty
• Given path is not valid: '{path}'
• Prefab Mode: GameObject must be part of a Prefab instance, o
• Prefab Mode: The 'openedFromInstance' GameObject '{go.name}'
• Cannot save as new prefab using the same path

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/1c7b8c17b2b972b9. Report an issue: GitHub.