The agentskills.io Standard

Skills are the open standard for teaching agents how to do specific tasks. A skill is a folder containing a SKILL.md file with YAML frontmatter and markdown instructions. Both Gemini CLI and Antigravity support the agentskills.io specification. This lesson covers writing skills, organizing them, and using them across projects.

What you'll learn

• Skills follow the agentskills.io open standard: a SKILL.md file with YAML frontmatter in a named folder
• Skills support progressive disclosure: the agent sees a summary first, loads full content only when relevant
• Firebase Skills give Antigravity agents context about Firebase SDKs, APIs, and best practices

Build it

Step 1: Write a skill

Create the directory structure for your skill first.

mkdir -p .gemini/skills/pr-review

Next, define the skill instructions using the standard YAML frontmatter and markdown structure.

# .gemini/skills/pr-review/SKILL.md
---
name: pr-review
description: Reviews pull requests for bugs, security issues, and style problems. Use when asked to review a PR or code changes.
version: 1.0.0
---

## Procedure
1. Read the diff using `git diff origin/main...HEAD`
2. For each changed file, check for:
   - Security vulnerabilities (injection, XSS, hardcoded secrets)
   - Logic errors (off-by-one, null checks, race conditions)
   - Style violations (against the project's GEMINI.md standards)
3. For each issue found, report:
   - File and line number
   - Severity (critical, high, medium, low)
   - Description and suggested fix

## Verification
Run the project's test suite after applying fixes.

Step 2: Use the skill

In Gemini CLI or Antigravity:

/pr-review

Or the agent automatically loads it when the task matches the description.

Step 3: Firebase Skills

Antigravity bundles predefined skills for Firebase development. These provide standard workflows for interacting with the platform.

# Firebase Skills come pre-installed with Antigravity
# They give agents context about:
# - Firebase Authentication
# - Cloud Firestore
# - Firebase Hosting
# - Cloud Functions
# - Security Rules best practices

What goes wrong

Mistake	How you notice it	The fix
Skill description too vague	Agent doesn't load the skill when it should	Be specific about when to use it: "Use when asked to review a PR or code changes" not "For code stuff"
Skill instructions contradict GEMINI.md	Agent behaves inconsistently	Keep skills consistent with project context. GEMINI.md sets general rules; skills provide task-specific procedures
Skill folder name doesn't match frontmatter	/skill-name doesn't work	The folder name is the command name. The frontmatter name field is for metadata

Next: Browser-in-the-Loop Agents