This talk comes from inheriting a complete, widely used open source PHP API library. It’s complete in that it has a full test suite, but problems arise from the approach of mocking JSON responses to assert tests on. The responses have to be maintained, and that can inevitably lead to false positives if the underlying API changes without the maintainer being notified. You can be clever and use a git submodule to bring in a standardised response that can be used by the same library in other languages, but then you need someone to maintain that across various departments of engineering.