Phase 8 · Extending Claude Code
Day 20 · How a Skill Is Built
Day 20

How a Skill Is Built

~60 min

Goal — Understand the anatomy of a skill by reading a real one.

By the end — A clear mental model of what a SKILL.md contains and how it triggers.

Tip

Map first — Everywhere, or just here? That decides where a skill or MCP goes.

1Understand what a skill is

A skill is a folder with a SKILL.md file at its center. That file tells Claude when to use the skill and how to do the task — a repeatable move you teach it once and reuse.

Ask Claude
Explain what a Claude Code skill is and what a SKILL.md file contains.

2Learn the trigger description

The frontmatter at the top of SKILL.md holds a name and a description. The description is a trigger — it tells Claude the kinds of requests this skill should handle.

Ask Claude
Explain how a skill's description acts as a trigger, using examples of phrases that would match.

3Understand progressive disclosure

A good skill keeps its SKILL.md short and pushes deeper detail into separate reference files that load only when needed. This keeps context lean.

Ask Claude
Explain "progressive disclosure" in a skill — a short main file that points to references loaded on demand.

4Read a real skill

Seeing a real one makes the pattern concrete. Have Claude open one already on your system and walk you through its parts.

Ask Claude
Find a SKILL.md on my system and walk me through its parts — frontmatter, body, and any references.

Recap — A skill is a folder built around SKILL.md — a triggering description up top, steps in the body, and deeper detail in references loaded only when needed.

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