Erstwhile Framework uses a custom JSON format for specifying endpoints when consuming external APIs, known as description.json. If you’re working with an Erstwhile-compatible backend, you can set your local environment to grab this description right from the server to build your model layer.
If that isn’t possible, you can still make your own description.json and store it your local /app/configdirectory to use that instead. In these cases, you only need to make a minimal version. Just group the API endpoints into entities, and add those to the authentication section that make sense. You don’t necessarily need to list all of the fields and return values of the endpoints.