TL;DR

A vendor handed us a sandbox key, a 6-page PDF, and no OpenAPI spec. I used Claude Code to turn ~40 exploratory requests into an inferred schema, a typed client, and a contract test suite in two days. The trick was never letting the agent write types from the docs — only from captured responses. Here's the loop, plus the three times it confidently made things up.

The Problem

We had to integrate a partner's billing API. What we got was:

A sandbox API key