There are so many of us doing this, haha. If you work at a big enough company or one with poorly documented APIs, it's just faster to reverse engineer the existing apps/UIs.

Just make sure you don't end up accidentally making one of those APIs semi-official.

I've seen a case where one team developed a temporary hack to automate some process in their product, then allowed another team making a sibling product to use it to test a possible feature; soon after, the possible feature became an actual one, everyone seemingly forgot the API was a throwaway test. Over the years, as both products evolved, that feature got pretty flaky, and in one of many cross-team attempts at fixing it, someone from the original time finally pointed out that the whole thing is still relying on a temporary hack in the original product that was never intended to be productized...

Same here, but I do have access to the codebase. But there's no documentation about all the different APIs. Reading through the code also doesn't always explain what the API does, or what it's for. I work on a separate system that benefits from using these APIs from the main product, there is almost no overlap or communication between engineering departments. It's far from ideal.