How to provide undo for EditorScript changes to a prefab?

I’m writing an Editor Script (or ScriptableWizard) to modify a prefab – specifically add child GameObjects to the prefab. I wish to make this operation undoable but cannot determine how.

I’ve tried Undo.RegisterUndo(), Undo.RegisterCreatedObjectUndo() and Undo.SetSnapshotTarget() + Undo.CreateSnapshot() + Undo.RegisterSnapshot().

Here’s the code:

	[MenuItem("Tools/Add child to prefab", true)]
	static bool validateAddChildToPrefab() {
		GameObject go = Selection.activeObject as GameObject;
		if (null == go) return false;
		return PrefabUtility.GetPrefabType(go) == PrefabType.Prefab || PrefabUtility.GetPrefabType(go) == PrefabType.ModelPrefab;

	[MenuItem("Tools/Add child to prefab")]
	static void addChildToPrefab() {
		GameObject go = (GameObject) Selection.activeObject;
		// register undo -- my question: how to get this to work?
//		Undo.RegisterUndo(go, "Add child to prefab"); // does nothing
//		makeUndoFor(go, "Add child to prefab"); // (see below) does nothing
//		Undo.RegisterCreatedObjectUndo(go, "Add child to prefab"); // error: Undo Created Object may not be performed on persistent or non-default hide flag objects
//		Undo.RegisterSceneUndo("Add child to prefab"); // does nothing (not in scene so wasn't hopeful)

		GameObject clone = (GameObject) PrefabUtility.InstantiatePrefab(go);
		GameObject child = new GameObject("My child");
		child.transform.parent = clone.transform;
		PrefabUtility.ReplacePrefab(clone, go, ReplacePrefabOptions.ReplaceNameBased);
		// Remove the scene instance of the prefab

	private static void makeUndoFor(Object root, string actionName) {
		Object[] objectsToRecord = EditorUtility.CollectDeepHierarchy(new Object[]{ root });
		Debug.Log("Creating UNDO point for "+ objectsToRecord.Length +" objects");
		Undo.SetSnapshotTarget(objectsToRecord, actionName);

p.s. If there’s a better way to add child GameObjects to prefabs (i.e. without instantiating), I’d really like to know! Thanks

you need to put Undo.RegisterCreateObjectUndo() after the InstantiatePrefab and pass it the new object