Creating Agent Skills Skill for Codex

Creating Agent Skills

Use when creating Agent Skills packages (SKILL.md format) for Codex CLI, GitHub Copilot, or Amp - provides the agentskills.io specification with frontmatter constraints, directory structure, and validation rules

Overview

Agent Skills is an open standard for portable AI agent capabilities. One SKILL.md file works across Codex CLI, GitHub Copilot, and Amp.

Official Spec: https://agentskills.io/specification

Installation Directories

| Tool | Location |

| ------------------ | -------------------------------------- |

| Codex CLI | .agents/skills/{skill-name}/SKILL.md |

| GitHub Copilot | .github/skills/{skill-name}/SKILL.md |

| Amp | .agents/skills/{skill-name}/SKILL.md |

Directory Structure

`

my-skill/                 # Must match frontmatter `name`
├── SKILL.md              # Required - main definition
├── scripts/              # Optional - executable code
├── references/           # Optional - additional docs
└── assets/               # Optional - static resources

Frontmatter Specification

Required Fields

---
name: skill-name
description: What it does and when to use it
---

Optional Fields

---
name: pdf-processing
description: Extracts and processes PDF content. Use for document analysis and text extraction.
license: MIT
compatibility: Requires pdftotext, poppler-utils
allowed-tools: Bash(pdftotext:*) Read Write
metadata:
  category: document-processing
  version: 1.0.0
---

Name Validation

`

✅ Valid: pdf-processing, code-review, data-analysis
❌ Invalid: PDF-Processing (uppercase), -pdf (leading hyphen), pdf--processing (consecutive hyphens)

Description Best Practices

`yaml

# ❌ BAD - Too vague
description: Helps with PDFs

# ❌ BAD - Missing use cases
description: Extracts text from PDFs

# ✅ GOOD - Functionality + use cases
description: Extracts and processes PDF content. Use for document analysis, text extraction, and form data parsing.

Body Content

Markdown instructions after frontmatter. No format restrictions, but recommended sections:

---
name: code-review
description: Reviews code for best practices and security issues. Use when analyzing PRs or conducting audits.
---

## Overview

Brief description of capabilities.

## Process

1. Step-by-step workflow
2. With clear actions

## Guidelines

- Bullet points for rules
- Best practices

## Examples

Code samples showing usage.

Progressive Disclosure

Skills use tiered loading to optimize context:

• Metadata (~100 tokens): name + description load at startup
• Activation (<5000 tokens): Full SKILL.md loads when selected
• On-demand: Supporting files load when referenced

Keep SKILL.md under 500 lines for efficient context usage.

Supporting Files

scripts/

#!/usr/bin/env python3
# scripts/extract.py
import sys
# Self-contained with clear dependencies

Complete Example

Type Guard

function isUser(obj: unknown): obj is User {
  return typeof obj === 'object' && obj !== null && 'id' in obj && 'name' in obj;
}

Validation

Use the validation tool:

skills-ref validate ./my-skill

Quick Checklist

• Frontmatter:
• [ ] name is 1-64 chars, lowercase alphanumeric + hyphens
• [ ] name matches parent directory name
• [ ] description is 1-1024 chars
• [ ] description includes functionality AND use cases
• Content:
• [ ] Under 500 lines for efficient loading
• [ ] Clear instructions agents can follow
• [ ] Examples for complex operations
• Structure:
• [ ] Directory named exactly as name field
• [ ] SKILL.md at directory root
• [ ] Supporting files in appropriate subdirectories

Cross-Tool Compatibility

The SKILL.md format is identical across implementations. To port:

# Codex → Copilot
mv .agents/skills/my-skill .github/skills/my-skill

# Copilot → Codex/Amp
mv .github/skills/my-skill .agents/skills/my-skill