The Watch List API provides read access to a user's watchlists for both the default watchlist and any custom lists. Use this API to enable saved and favorites experiences. You can retrieve the default list with userWatchList and fetch one, multiple, or all custom lists with userCustomLists. Both queries return structured results and surface validation or retrieval errors (for example, invalid pagination values or partial/all failures for custom lists). To modify watchlists, use mutations such as addToWatchList to add one or more items by itemID, removeFromWatchList to remove specific items, and removeAllFromWatchList to clear a list. These mutations provide detailed success or failure feedback.