How to Write a CLAUDE.md That Actually Changes Model Behaviour
A 2026 playbook for CLAUDE.md files that Claude Code actually follows: the 200-line target, when to use path-scoped rules in .claude/rules/, @import hierarchy and 5-hop max depth, the user-message vs system-prompt gap, the line between CLAUDE.md and auto memory, and when to give up and write a hook instead. Anchored to Claude Code 2.1.x and verified against the official memory docs.