What is a CLAUDE.md file and why does every project need one?

CLAUDE.md is a Markdown file at your repo root that holds your project rules, coding standards, “never edit vendor”, “always write MFTF tests”, deploy flow, naming conventions, links to internal docs. Claude Code re-reads it at the start of every session, so your team’s rules become muscle memory for the AI. Write once, AI follows forever.

A good Magento CLAUDE.md includes: tech stack (Magento 2.4.x + Hyvä), “no vendor edits”, “plugin-over-rewrite”, “db_schema not raw SQL”, “MFTF tests required”, deploy commands, and a list of internal modules with their purpose. Sub-folders can have their own CLAUDE.md files that compose with the root one, useful for monorepos.