In an acceptance test-driven development approach, you will have to define your test cases, based on examples, before implementing your APIs. This will help refine the business expectations about your APIs; share them between developers, testers, and business customers; and later ensure your APIs conform to those expectations.
Here’s an example that will highlight the importance of the Mock and Test phases. If you are a retail company, you will have to return an error if someone under 21 tries to buy alcohol. That’s not something you can formally model in an API contract, but it will instead be reflected in your mocks and your tests.