Skills.

Skills are reusable instruction manuals that teach your AI agent how to do a task. They're SOPs for your AI. Install one and your AI just knows how to do that thing, triggered by a single command.

The cleanest way to think about it: a tool gives your agent access to something, a skill tells it how to use that access. A Gmail tool lets your agent into your inbox. A skill is the playbook for how you want your emails written and sent.

The mental model I like is a bookshelf. Claude sees the spine of every skill, the name and a short description, and when a task matches one, it pulls that book off the shelf and follows it. You're downloading expertise.

They matter most when you're building, because they compound. Each skill you write makes the next bit of work faster, since you never re-explain the same process twice. You train the new hire once, then they just know.