Creating Agent Skills

---
name: creating-agent-skills
description: 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
---

# Creating Agent Skills

## 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

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

| Field | Constraints |
|-------|-------------|
| `name` | 1-64 chars, lowercase alphanumeric + hyphens, no leading/trailing/consecutive hyphens, must match parent directory |
| `description` | 1-1024 chars, explains functionality AND use cases |

### Optional Fields

```yaml
---
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
---
```

| Field | Constraints |
|-------|-------------|
| `license` | Short reference (e.g., `MIT`, `Apache-2.0`) |
| `compatibility` | 1-500 chars, environment requirements |
| `allowed-tools` | Space-delimited pre-approved tools (experimental) |
| `metadata` | Arbitrary string key-value pairs |

## Name Validation

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

**Pattern:** `^[a-z0-9]+(-[a-z0-9]+)*$`

## 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.
```

**Include:**
- What the skill does
- When to activate it (keywords agents search for)
- Specific use cases

## Body Content

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

```markdown
---
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:

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

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

## Supporting Files

### scripts/
Executable code agents can invoke:
```python
#!/usr/bin/env python3
# scripts/extract.py
import sys
# Self-contained with clear dependencies
```

### references/
Additional documentation:
- `REFERENCE.md` - Technical details
- `FORMS.md` - Templates
- Domain-specific files

### assets/
Static resources: templates, diagrams, lookup tables.

**Reference with relative paths:** `scripts/extract.py`, `references/REFERENCE.md`

## Complete Example

```markdown
---
name: typescript-expert
description: Expert TypeScript assistance with strict typing and modern patterns. Use for TypeScript projects requiring type safety, generics, or advanced type manipulation.
license: MIT
compatibility: Requires Node.js 18+, TypeScript 5+
---

You are an expert TypeScript developer.

## Guidelines

- Always use strict type checking
- Prefer `unknown` over `any`
- Use type guards for runtime checking
- Leverage template literal types

## Best Practices

- Export types from dedicated `.types.ts` files
- Use `readonly` for immutable data
- Prefer interfaces for objects, types for unions

## Examples

### Type Guard

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

## Validation

Use the validation tool:
```bash
skills-ref validate ./my-skill
```

Checks:
- YAML frontmatter validity
- Name format compliance
- Required fields presence
- Directory name matches `name` field

## 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:

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

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

No content changes required.