It gets injected into context wholesale, every session — every line competes for attention with the rules that actually matter. Official docs and community consensus agree: short, precise, only what Claude can't guess.

A Skill is a folder, not just a Markdown file. Three-level progressive disclosure makes it nearly free to keep around: ~100 tokens of metadata at startup, the body on task match, scripts and references on demand.

Skill won't trigger, CLAUDE.md rules ignored, sessions getting dumber over time — how to diagnose and fix the five most common problems.

Quick reference for the concepts this manual uses most: Skills, the four memory layers, progressive disclosure, Hooks, subagents, and MCP — one sentence each.