Skills

In the last chapter you built a library-reviewer custom agent and fanned subagents across the dozen consumers of shared-lib to check for breakage. Along the way you leaned on something the team takes for granted: the audit-event convention — the agreed way to record a significant event through shared-lib’s audit helper. When the approval feature fires, an event gets written a specific way: the same call shape, the same fields, the same naming, whether the code lives in shared-lib or in orders-service.

That convention is a procedure. It has steps, it’s the same every time, and right now you carry it in your head — re-describing it to Copilot whenever the work touches an audited event. A rule won’t fix that cleanly (the convention isn’t always-on context you want hanging over every request), and a prompt file means you have to remember to invoke it. What you want is something Copilot reaches for itself, the moment a task is about recording an audit event, without you saying a word.

That’s a skill.

What a skill is — and isn’t

You’ve now met three ways to give Copilot context, and the distinction between them is the whole point of this chapter:

• A rule is always-on. It’s loaded into every request as background context — your conventions, your stack, your house style. You don’t invoke it; it’s just there.
• A prompt file is an invoked template. You type /scaffold-endpoint, it runs. Useful, but you have to reach for it.
• A skill is a self-contained, model-invoked procedure. It sits on disk with a description of when it applies, and Copilot pulls it in on its own when a task matches that description. You don’t trigger it; the agent decides the moment is right and loads the steps.

The audit-event convention is the perfect candidate. It’s not background context you want on every request, and you don’t want to remember to invoke it by hand — you want Copilot to recognize “this task records an audit event” and apply the team’s procedure automatically, in both repos, the same way every time.

What this chapter does

You’ll package the audit-event convention as a portable SKILL.md, get the wording right so Copilot actually reaches for it, and then see the part that pays off across this whole site — the same skill format works in your other agents too.

• Create the skill — author SKILL.md, its frontmatter, where it lives, and build the audit-event procedure.
• The description does the triggering — why the one description line is what makes Copilot invoke the skill on its own.
• Write once, runs everywhere — Agent Skills is an open standard; the SKILL.md you write here is the same file your Claude Code and Codex agents already read.

Start by writing the file. Create the skill.