Can be easy depending on your setup - you can basically just write high level functional tests matching use cases of your API, but as prompts to a system with some sort of tool access, ideally MCP. You want to see those tests pass, but you want them to pass with the simplest possible prompt (a sort of regularization penalty, if you like). You can mutate the prompts using an LLM if you like to try different/shorter phrasings. The Pareto front of passing tests and prompt size/complexity is (arguably) how good a job you're doing structuring/documenting your API.