We do the same thing. The core of our service is ingesting data and applying transformations to it. There’s so many permutations and complex interactions in here that the only way to ensure a refactor hasn’t broken one of these interactions is to document those edge cases by piping data through the system and reading it back out. We have thousands of these tests and it’s all tidy controlled via docker compose. It takes about 15 minutes to run the test suite. Sure I wish it was faster - but the real unlock is that we can make big sweeping refactors without breaking behaviors of the system. The organizational speed unlock this kind of safety net is well worth a bit of slowness in CI.

We also have mocks/stubs/spies in our unit tests. Those are great for producing hard-to-trigger edge cases. But contract testing? The contract is the data flow. In the end it’s all about using the right tool for the right test. There is no one-size-fits-all.