I built a pricing API for LLMs — then realized the real users might not be human

— its cost calculator page calls this API live, gets a model recommendation plus a cost estimate, and returns it inline. Two separate sites, same backend, CORS'd across domains, tested end-to-end. I didn't plan the API to be reusable infrastructure when I built it — it just turned out that "structured, agent-callable, reasoning-included" is a useful shape for more than one problem.

If you're building something similar: design the response for a reader who can't ask a follow-up question. That constraint does more for API design than almost anything else.