How is CLAUDE.md different from a README.md?

README.md is for humans landing on the repo — setup, contributors, license. CLAUDE.md is a prompt anchor for Claude Code — it is auto-loaded at session start and sets the rules of engagement. Typical CLAUDE.md sections: project overview, tech stack pinned versions, hard rules (no vendor edits, plugins over preferences, phpcs Magento2, db_schema not raw SQL), test commands, deployment commands, and known gotchas. Treat it as the contract between you and the model: anything not in CLAUDE.md, the model has to guess at. A good Magento CLAUDE.md is 200–400 lines. More than that and the model starts ignoring the bottom half.