CLAUDE.md is the rules file your AI reads before doing anything

Every time the AI starts working on your project, it reads a file called CLAUDE.md first. This file contains your ground rules. "Always use this style." "Never touch this file without asking." "Here is how we deploy." Without it, the AI starts fresh every time and follows its own defaults. With it, the AI follows your conventions from the first second.

Without a rules file, every session starts from zero. You end up correcting the same mistakes, re-explaining the same preferences, and watching the AI make choices you would never make. With CLAUDE.md, the AI already knows how your project works before you say a word. It is the difference between onboarding someone new every morning versus working with someone who already knows the house rules.

CLAUDE.md is your project's culture document for an audience of one very capable assistant.

Create a plain text file called CLAUDE.md in your project folder. Write down the things you would tell a new teammate on their first day -- not a tutorial, just the rules. What tools and styles do you use? What files should never be changed? How does deployment work? What patterns are intentional even if they look unusual?

Keep it short and direct. Update it every time you catch the AI doing something wrong. If you had to correct it once, that correction belongs in CLAUDE.md so you never have to repeat it.

You can also create a personal rules file that applies across all your projects, not just one. Your project-level rules take priority when there is a conflict.

Always. Every project should have one. Start with five rules and grow it over time. The best CLAUDE.md files are not written all at once -- they are built up from real corrections during real work sessions.