What is an agent skill?

An agent skill is a structured instruction file (commonly SKILL.md) that teaches an AI agent a repeatable procedure — when to trigger, what steps to follow, and what quality gates to enforce. Unlike a one-off prompt, a skill persists across sessions and fires automatically when its trigger conditions match. Well-written skills read like contracts: explicit checklists, hard gates ('do not commit until tests pass'), and named failure modes. They're the primary mechanism for turning a general-purpose model into a specialist that follows your playbook.