AGENTS.md is the README for your coding agent: where the code lives, how to build and test it, what style to follow and what never to touch. Codex reads it before it starts working, so a good file is the cheapest way to improve every task that follows. This kit gives you ten battle-tested templates - generic, web (HTML/CSS/JS), Python, Node/TypeScript services, Godot 4, Unity (generic), mobile, data/analytics, and a monorepo root plus a nested package file - each written to be short, concrete and command-first.

A guide explains how Codex discovers and merges AGENTS.md files (global, repository root, nested folders, overrides), how to keep one source of truth when you also use other tools, and a checklist for writing instructions that agents actually follow. A small Python linter (lint_agents_md.py) scores an AGENTS.md for size, runnable commands, vague phrasing and missing safety rules, so you can improve yours quickly.

Codex evolves quickly. Where details could change (file size limits, config keys, fallback filenames), the kit says so and points you to verify against the current official documentation.