What you’ll know by the end of this check
- What a Skill is and why it’s different from MCP and subagents
- When to reach for a Skill instead of writing more into CLAUDE.md
- Where the real depth lives (the dedicated Agent Skills course)
The shortest possible answer
A Skill is a named, callable bundle of instructions and references that Claude loads only when relevant. Unlike MCP (which loads tool definitions upfront) and CLAUDE.md (which is always in context), Skills are on-demand.
Claude sees a Skill’s name + short description on every prompt. When the description matches what you’re trying to do, Claude loads the full Skill content into context — and only then.
Why the difference matters
| Mechanism | Loaded when | Cost per session |
|---|---|---|
| CLAUDE.md | Always | High (every prompt carries it) |
| MCP server | Always, when connected | High (tool definitions stay resident) |
| Skill | On-demand, when description matches | Low baseline + full cost only when invoked |
If you have a detailed playbook that only matters for 20% of your work — a release checklist, a specific code review pattern, a data-migration procedure — don’t bloat CLAUDE.md with it. Write it as a Skill.
When to reach for a Skill
- You find yourself pasting the same instructions into multiple prompts.
- You have a multi-step procedure that’s stable but only occasionally needed.
- You want Claude to behave differently for a specific task without polluting the default context.
Where the depth lives
The Claude Code 101 lesson for Skills is intentionally light — it’s an overview. Anthropic has a dedicated course that covers the SKILL.md format, progressive disclosure, skill composition, and distribution:
That course is the next check on this ladder’s sibling checklist (/preflight/claude-skills).
Things to try right now (10 minutes)
- Watch the Skills video at the canonical source.
- Think about one procedure you’ve explained to Claude 3+ times. That’s a Skill candidate.
- When you’re ready to build it, start the dedicated Agent Skills course.
The canonical version
Full official lesson is at anthropic.skilljar.com/claude-code-101/469848.
Deep dive: anthropic.skilljar.com/introduction-to-agent-skills
Ready to verify this check?
You can name the difference between a Skill, an MCP server, and CLAUDE.md. You’ve identified at least one candidate Skill in your own workflow. Mark it cleared.