If it's HTTP you can create the fixtures and serve them with a mock server. I'm a frontend dev, so backend APIs are like 3rd parties to me.

I use a browser extension for scraping actual backend responses, which downloads them with a filename convention the mock server understands. I mostly use it for development, but also for setting up screenshot tests. For example,

  PATCH /select

  'api/user(locked-out).GET.423.json'

screenshot the app and pixel diff it

  PATCH /select

  'api/user.GET.200.json'

screenshot…