In the field guide I covered what an AGENTS.md is and what belongs in it. This is the hands-on follow-up: we'll build a complete AGENTS.md for a real project, one section at a time, then point an AI coding agent at it and watch the difference it makes. By the end you'll have a working file — and you'll have seen it pay off.
New to AGENTS.md? It's a single Markdown file at the root of your repo that tells AI coding agents how to work in it — build steps, tests, conventions, guardrails. The "why" behind each section is in the field guide.
The project we'll use
We'll write the AGENTS.md for a small but real service: a URL shortener API in Python — FastAPI, SQLite, pytest. A couple of endpoints, a thin data layer, a test suite. Follow along with this, or swap in your own repo — the steps are identical.
Its shape:
