CLAUDE.md / AGENTS.md.

The instruction file your agent reads at the start of every session before it does anything. It tells the agent who it is, what jobs it does, and how to work: its role, tone, the rules it follows, and the context about your business. Think of it as the onboarding manual you'd hand a new employee on day one, except the agent reads the whole thing every single time.

In Claude Code it's called CLAUDE.md. Everywhere else it's AGENTS.md, same idea, different name. It lives in your .claude folder.

Claude Code has this baked into its rules: before replying to any message, read the CLAUDE.md file first. That's the leverage. Anything you write here loads into every session, so it's where you stop repeating yourself and start getting consistent work. Set up the role, your standing rules, and your business context once, and every agent that touches the folder already knows them.