The .claude/rules/ directory holds path-scoped instructions that only activate when Claude works in matching files. It keeps CLAUDE.md from ballooning.

What it does

Each rule file declares a path pattern and a body of instructions. When Claude touches a file that matches, those instructions join the context for that turn. Rules for frontend components don't waste tokens when Claude is editing the backend, and vice versa.

When to use it

Language-specific style rules (TypeScript strict mode, Python typing).
Framework conventions (Next.js app router patterns, Django app structure).
Per-directory guidance for monorepos with diverse stacks.
Anything you'd otherwise wrap in "if editing X, do Y" instructions.

Gotchas

Overly broad patterns load rules for every file. Be specific.
Rules stack on top of CLAUDE.md - conflicting rules resolve with the narrowest scope winning.
Rule files are committed artifacts. Treat them like code, review changes.

Official docs: https://code.claude.com/docs/en/memory.md#organize-rules-with-claude-rules