What an agent skill is

A skill is a folder with a SKILL.md file in it. The file starts with a name and a one-line description, followed by instructions written for the agent. Your agent reads only the descriptions up front. When a task matches one, it loads that skill's full instructions, plus any scripts or reference files in the folder. So you can install dozens of skills without filling the context window. Why bother? A skill turns "how we do this here" into something the agent follows every time. A good design skill stops the generic look. A good debugging skill makes it find the cause before it edits anything. You stop re-explaining the same method each session. Skills started with Claude and are now an open format. Codex, Cursor, GitHub Copilot and other agents read the same SKILL.md, so one skill works across tools. Where to start: pick one skill for the thing your agent does worst, install it, and watch whether the results change. Add the next one only when that one earns its place.