Skills turn recurring prompts into named, reusable artifacts. Write one markdown file, invoke it as /skillname, and the same prompt runs every time.

What it does

A skill is a markdown file with frontmatter and a body. The frontmatter describes when to trigger it and what tools it needs; the body is the instructions. Claude auto-loads skills that match the current task, and you can also call them explicitly. Skills can live globally, per-user, or per-project.

When to use it

Encoding repeatable workflows (run tests, ship a release, write a blog post).
Sharing team patterns so everyone gets consistent behavior.
Replacing long copy-pasted prompt templates.
Building your own slash commands on top of Claude Code.

Gotchas

Auto-invocation depends on the frontmatter description. Vague descriptions mean skills don't fire when expected.
Skill bodies count against context. Keep them tight - progressive disclosure is your friend.
Loading order matters when multiple skills could match. Test with explicit invocation first.

Official docs: https://code.claude.com/docs/en/skills.md