Developer Uses Claude AI to Reverse-Engineer Undocumented Vendor API in Two Days

A software developer shared a method for integrating a partner billing API that came with only a sandbox key, a six-page PDF, and no formal specification such as an OpenAPI schema or SDK. Rather than building a client from the sparse documentation, the developer used Claude Code to generate exploratory HTTP requests that probed the real API and captured its actual responses to disk. All type definitions, enums, and nullability decisions were derived strictly from observed HTTP responses, not from the PDF or the AI model's assumptions about typical billing APIs. The process ran in a four-stage loop where the AI proposed probe requests, a harness executed and recorded them, and the agent then inferred a schema solely from the captured JSON files. The approach uncovered significant discrepancies between the documentation and the real API, including 31 fields on an invoice object where the docs listed only six, and an entirely different set of status enum values than what the AI would have guessed.