[tpxl]

We did this at my previous company. What our app did was get data from a queue, enrich it with the API, then send it off to another queue. They provided test data and a test environment, but it was horrible, changed unpredictably and data frequently went missing. We recorded all endpoints hit during our integration tests, saved them to files and mocked their API (mostly GETs with no state so backed by a simple rest service with a hashmap).

The API team was an internal team to the company and there was absolutely no communication possible with them, so we kinda had no choice.