 <p>Ideally the web service would use one of the HTTP status codes to tell the client there was a problem, then you could use a response descriptor specifically to handle that and an <code>NSError</code>.</p> <p>You can use a dynamic mping which checks the result type and determines how to process the result. Depending on the flag value a different mapping will be applied to create a different result class with the appropriate details for you to use. Exactly how you handle that is up to you, either with a class type check, or perhaps having your model objects return a 'success' flag, perhaps supported by a protocol.</p>