A teachable capability bundled as a unit — instructions and resources for doing one task well, kept in the environment until a context pointer pulls it into the context window for the task at hand. The unit of progressive disclosure in a harness.
Skills are an open standard, defined at agentskills.io — originally developed by Anthropic and since adopted by most major harnesses, so a skill written once works across them. The format is a folder containing:
- A
SKILL.mdfile — metadata (a name and description, at minimum) plus the instructions themselves - Optionally, scripts the agent can run
- Optionally, templates and reference material the instructions point to
Only the name and description sit in context by default. When the agent's task matches, it loads the rest. Until then, the skill takes up almost no room — a sentence or two of tokens, however large its full instructions are.
This distinguishes skills from AGENTS.md, which is loaded into every session regardless of the task. A skill is read when a particular kind of work comes up — releasing, scaffolding a new service, writing a migration — and ignored the rest of the time.
Avoid: "tool" — a tool is what the agent calls; a skill is instructions it reads.
Usage:
"Where should I put the deploy runbook?"
"As a skill — the agent loads it only when the task involves deploys. In AGENTS.md it'd burn tokens on every turn for something we use weekly."