How to Write Effective CLAUDE.md Files

CLAUDE.md files give AI coding tools persistent context about your project. Most teams either skip them entirely or write generic ones that don't help. Here's how to make them useful.

What belongs in a CLAUDE.md

Three things: architecture decisions, coding conventions, and common patterns. Skip the obvious stuff the AI can infer from the code itself.

Architecture section

Document the non-obvious choices. Why you chose App Router over Pages Router. Why the API layer uses a specific pattern. What the data flow looks like. The AI needs to understand intent, not just structure.

Conventions section

Be specific. Not "use TypeScript" but "prefer type over interface, use explicit return types on exported functions, avoid enums in favor of const objects." The more precise, the fewer corrections you'll make.

Patterns section

Show, don't tell. Include a small example of how a typical component, API route, or test should look. The AI will extrapolate from concrete examples better than abstract descriptions.

What to avoid

Don't duplicate information available in config files. Don't write a novel. Don't include temporary instructions that will become stale. Keep it under 200 lines and update it when patterns change.

The payoff

A good CLAUDE.md means you spend less time correcting AI output and more time reviewing actual logic. For our team, it reduced per-task prompt iterations from five to two on average.