A06: Agent Skills

You have a few prompts you keep retyping, "summarize this in three bullets for a beginner," "explain this error in plain English." A skill saves that once as a capability the agent picks up on its own, whenever your request matches. (Other tools call these custom commands or agents. Antigravity calls them skills.)

A Skill Is a Saved Capability

A skill is a small folder with a SKILL.md file inside. Put it in .agents/skills/ in the folder you work in:

.agents/skills/explain-simply/SKILL.md

SKILL.md starts with a short header, then the instructions. The description is the important part, it tells the agent when to reach for this skill:

---
name: explain-simply
description: Explain code, errors, or ideas to a complete beginner in plain language with one concrete example. Use whenever the user asks to explain or simplify something.
---
Explain the thing to a complete beginner, in plain language.
Give a three-bullet summary, then one concrete example.

Now just ask naturally, "explain this error", and the agent notices your request matches the skill's description and follows it. You do not type a special command; you built a reusable capability and the agent uses it when it fits.

This is different from your AGENTS.md (A05): that is always on for everything you do here. A skill loads only when the agent decides it is relevant, so you can build a small library of specialized skills without cluttering every answer.

flowchart LR A[Retype a long prompt every time] --> B[Slow, inconsistent] C[Save it as a skill once] --> D[Agent applies it automatically, same quality]

One Step Further: MCP (just so you know it exists)

Skills reuse prompts and instructions. If you ever need the AI to use a real external tool, read a database, call a web service, Antigravity supports MCP (Model Context Protocol), a way to plug in extra abilities. That is well beyond this course. For now, just know the word exists so it is not a mystery later.

This Week's Exercise

  1. In the folder where you run agy, make a skill folder: mkdir -p .agents/skills/explain-simply (use your own name).
  2. Inside it, create SKILL.md for a skill that solves a real annoyance for you (a "summarize in three bullets" or "explain simply" skill like above). Write a clear description so the agent knows when to use it.
  3. Make three real requests this week that should trigger it. Run agy inspect to confirm the skill is loaded, and refine the wording until the output is consistently good.
  4. Bring your SKILL.md and an example run to class.

Key Takeaways

  • A skill is a saved, reusable capability: a SKILL.md in a folder under .agents/skills/
  • The description tells the agent when to use the skill, so write it clearly
  • The agent loads a skill automatically when your request matches, no command to type
  • MCP lets the AI use external tools; know it exists, leave it for later