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.