Skills

A skill is a directory with a SKILL.md: frontmatter plus instructions the agent can follow for a specific kind of work. Skills are how Cetas gets domain competence without putting all of it in the prompt — and access to how Cetas itself is configured and used.

/skills

lists what was discovered in this session.

Three tiers of disclosure

Loading everything at startup would be wasteful and noisy, so skills open in stages:

1. Catalog — at discovery, Cetas validates each SKILL.md and keeps only its metadata. The instruction body never enters the context.
2. Activation — when the task calls for it, the agent activates exactly one skill by name. Cetas re-reads and re-validates that file and returns its instructions, listing bundled resources without reading them.
3. Resource read — the agent opens one named resource inside that skill’s directory, on demand.

That last boundary is enforced: a resource path is resolved through canonical real paths and must stay inside the skill directory.

Where skills are found

Repository skills come from a checkout, which is untrusted input by definition — the same caution you would apply to any script in a repository applies here. A host can exclude repository skills entirely; Cetas does not invent a trust decision on your behalf.

Skill directories may be symlinks, but the resolved package must stay inside the repository boundary unless the host opts into external packages.

Writing one

---
name: repo-map
description: How to navigate this repository
---

## Layout
...

The frontmatter is validated at discovery: a skill that fails validation is skipped rather than partially loaded, and name collisions are resolved deterministically instead of depending on directory order.

Because bodies are only read on activation, a long skill costs nothing until someone uses it. Write the frontmatter for discovery — the description is what decides whether the agent ever opens the file.