How do you test multiple API payloads and edge cases?

I’m building a mobile app and need to reliably test different API payloads and failure scenarios. Ideally, I want to switch between payload variations on the fly and immediately see how the UI reacts. I’ve used Charles and some in-code mocks, but these approaches don’t scale well for dozens of repeatable scenarios.

How do you typically handle this? Mock servers, custom tooling, contract tests, or something else? I’m looking for practical workflows that let you test a wide range of payload variations without adding temporary test logic into the client code.