Definition
An agent skill is a reusable package that teaches an agent how to perform a class of work. It commonly contains instructions, procedures, examples, and references; some formats also bundle scripts, schemas, templates, or other resources.
Current skill systems use progressive disclosure: the harness first exposes a compact description, then loads the full package when the work calls for it. This saves context, but it also means metadata influences which behavior becomes active. Descriptions, dependencies, and executable resources are therefore part of the security surface.
A skill should be treated like maintained software. Give it a clear trigger and scope, version it, record provenance, test representative and adversarial cases, and re-evaluate it when models or tools change. A long instruction file that nobody can reliably select or verify is not yet a dependable capability.
Distinguish it from nearby terms
A skill packages reusable know-how. A tool exposes an executable capability or external boundary. A prompt supplies instructions for the current context and may invoke either. A skill can include tools or code, but the concepts are not interchangeable.
Check your understanding
A research skill tells an agent to prefer primary sources but bundles an unreviewed script that uploads notes elsewhere. Reviewing only the prose misses part of the skill.