Legacy Code Night Shift: Drafting a Living API Contract with a Free Model and a Free Server

Description and use

An automated workflow using a free LLM to draft JSON Schemas and a free server for nightly validation detects API payload changes in a Flask app before they reach production.

Problem

Inherited a Flask app without an OpenAPI spec or contract tests, making it hard to know expected responses before refactoring.

Outcome

Implemented a repeatable, low-cost workflow that automatically drafts and validates API contracts, reducing refactor risk and catching payload changes before production exposure.

Workflow

• Extract routes and view source from Flask app
• Send each route’s source to a free LLM to generate a JSON Schema
• Store generated schemas in a contracts/ folder
• Run a free server nightly to validate each endpoint against its schema and report drift

Evidence

A free model can draft that contract while you sleep, and a free server can validate it every morning.

Source

Keep the temperature at zero. You are not asking for creative descriptions; you want deterministic output that the validator can rely on.

Source