Agent skill
A folder of instructions, built around a SKILL.md file, that an AI agent reads only when a task calls for it.
Also called SKILL.md.
A skill is a folder with a SKILL.md file inside. The top of the file names the skill and says when to use it. Everything below that is the instructions. Here's the top of one of mine:
---
name: base44-prompt-engineer
description: Craft high-quality, production-grade mega-prompts for the Base44 AI chat...
---The clever bit is that the agent only reads those two lines up front. It opens the rest when your request matches the description. So you can install loads of skills without stuffing every conversation full of them.
Skills follow an open spec, so the same folder works in different agents. Base44 publishes its own set (npx skills add base44/skills) that teaches coding agents its CLI and SDK. Confusingly, Base44 also uses "agent skills" for the instructions inside your app's own agents. Different thing, same name.
My open-source skills on GitHub include the prompt engineer above and one for Modelsheet.
Related:Claude Code and Prompt engineering.
Where I've used it: