skill-builder

name	Skill Builder
description	Create new Codex Skills with proper YAML frontmatter, progressive disclosure structure, and complete directory organization. Use when you need to build custom skills for specific workflows, generate skill templates, or understand the Codex Skills specification.

Skill Builder

What This Skill Does

Creates production-ready Codex Skills with proper YAML frontmatter, progressive disclosure architecture, and complete file/folder structure. This skill guides you through building skills that Codex can autonomously discover and use across all surfaces (Codex.ai, Codex, SDK, API).

Prerequisites

• Codex 2.0+ or Codex.ai with Skills support
• Basic understanding of Markdown and YAML
• Text editor or IDE

Quick Start

Creating Your First Skill

# 1. Create skill directory (MUST be at top level, NOT in subdirectories!)
mkdir -p ~/.Codex/skills/my-first-skill

# 2. Create SKILL.md with proper format
cat > ~/.Codex/skills/my-first-skill/SKILL.md << 'EOF'
---
name: "My First Skill"
description: "Brief description of what this skill does and when Codex should use it. Maximum 1024 characters."
---

# My First Skill

## What This Skill Does
[Your instructions here]

## Quick Start
[Basic usage]
EOF

# 3. Verify skill is detected
# Restart Codex or refresh Codex.ai

---

Complete Specification

📋 YAML Frontmatter (REQUIRED)

Every SKILL.md must start with YAML frontmatter containing exactly two required fields:

---
name: "Skill Name"                    # REQUIRED: Max 64 chars
description: "What this skill does    # REQUIRED: Max 1024 chars
and when Codex should use it."       # Include BOTH what & when
---

Field Requirements

name (REQUIRED):

• Type: String
• Max Length: 64 characters
• Format: Human-friendly display name
• Usage: Shown in skill lists, UI, and loaded into Codex's system prompt
• Best Practice: Use Title Case, be concise and descriptive
• Examples:
  • ✅ "API Documentation Generator"
  • ✅ "React Component Builder"
  • ✅ "Database Schema Designer"
  • ❌ "skill-1" (not descriptive)
  • ❌ "This is a very long skill name that exceeds sixty-four characters" (too long)

description (REQUIRED):

• Type: String
• Max Length: 1024 characters
• Format: Plain text or minimal markdown
• Content: MUST include:
  1. What the skill does (functionality)
  2. When Codex should invoke it (trigger conditions)
• Usage: Loaded into Codex's system prompt for autonomous matching
• Best Practice: Front-load key trigger words, be specific about use cases
• Examples:
  • ✅ "Generate OpenAPI 3.0 documentation from Express.js routes. Use when creating API docs, documenting endpoints, or building API specifications."
  • ✅ "Create React functional components with TypeScript, hooks, and tests. Use when scaffolding new components or converting class components."
  • ❌ "A comprehensive guide to API documentation" (no "when" clause)
  • ❌ "Documentation tool" (too vague)

YAML Formatting Rules

---
# ✅ CORRECT: Simple string
name: "API Builder"
description: "Creates REST APIs with Express and TypeScript."

# ✅ CORRECT: Multi-line description
name: "Full-Stack Generator"
description: "Generates full-stack applications with React frontend and Node.js backend. Use when starting new projects or scaffolding applications."

# ✅ CORRECT: Special characters quoted
name: "JSON:API Builder"
description: "Creates JSON:API compliant endpoints: pagination, filtering, relationships."

# ❌ WRONG: Missing quotes with special chars
name: API:Builder  # YAML parse error!

# ❌ WRONG: Extra fields (ignored but discouraged)
name: "My Skill"
description: "My description"
version: "1.0.0"       # NOT part of spec
author: "Me"           # NOT part of spec
tags: ["dev", "api"]   # NOT part of spec
---

Critical: Only name and description are used by Codex. Additional fields are ignored.

---

📂 Directory Structure

Minimal Skill (Required)

~/.Codex/skills/                    # Personal skills location
└── my-skill/                        # Skill directory (MUST be at top level!)
    └── SKILL.md                     # REQUIRED: Main skill file

IMPORTANT: Skills MUST be directly under ~/.Codex/skills/[skill-name]/. Codex does NOT support nested subdirectories or namespaces!

Full-Featured Skill (Recommended)

~/.Codex/skills/
└── my-skill/                        # Top-level skill directory
        ├── SKILL.md                 # REQUIRED: Main skill file
        ├── README.md                # Optional: Human-readable docs
        ├── scripts/                 # Optional: Executable scripts
        │   ├── setup.sh
        │   ├── validate.js
        │   └── deploy.py
        ├── resources/               # Optional: Supporting files
        │   ├── templates/
        │   │   ├── api-template.js
        │   │   └── component.tsx
        │   ├── examples/
        │   │   └── sample-output.json
        │   └── schemas/
        │       └── config-schema.json
        └── docs/                    # Optional: Additional documentation
            ├── ADVANCED.md
            ├── TROUBLESHOOTING.md
            └── API_REFERENCE.md

Skills Locations

Personal Skills (available across all projects):

~/.Codex/skills/
└── [your-skills]/

• Path: ~/.Codex/skills/ or $HOME/.Codex/skills/
• Scope: Available in all projects for this user
• Version Control: NOT committed to git (outside repo)
• Use Case: Personal productivity tools, custom workflows

Project Skills (team-shared, version controlled):

<project-root>/.Codex/skills/
└── [team-skills]/

• Path: .Codex/skills/ in project root
• Scope: Available only in this project
• Version Control: SHOULD be committed to git
• Use Case: Team workflows, project-specific tools, shared knowledge

---

🎯 Progressive Disclosure Architecture

Codex uses a 3-level progressive disclosure system to scale to 100+ skills without context penalty:

Level 1: Metadata (Name + Description)

Loaded: At Codex startup, always Size: ~200 chars per skill Purpose: Enable autonomous skill matching Context: Loaded into system prompt for ALL skills

---
name: "API Builder"                   # 11 chars
description: "Creates REST APIs..."   # ~50 chars
---
# Total: ~61 chars per skill
# 100 skills = ~6KB context (minimal!)

Level 2: SKILL.md Body

Loaded: When skill is triggered/matched Size: ~1-10KB typically Purpose: Main instructions and procedures Context: Only loaded for ACTIVE skills

# API Builder

## What This Skill Does
[Main instructions - loaded only when skill is active]

## Quick Start
[Basic procedures]

## Step-by-Step Guide
[Detailed instructions]

Level 3+: Referenced Files

Loaded: On-demand as Codex navigates Size: Variable (KB to MB) Purpose: Deep reference, examples, schemas Context: Loaded only when Codex accesses specific files

# In SKILL.md
See [Advanced Configuration](docs/ADVANCED.md) for complex scenarios.
See [API Reference](docs/API_REFERENCE.md) for complete documentation.
Use template: `resources/templates/api-template.js`

# Codex will load these files ONLY if needed

Benefit: Install 100+ skills with ~6KB context. Only active skill content (1-10KB) enters context.

---

📝 SKILL.md Content Structure

Recommended 4-Level Structure

---
name: "Your Skill Name"
description: "What it does and when to use it"
---

# Your Skill Name

## Level 1: Overview (Always Read First)
Brief 2-3 sentence description of the skill.

## Prerequisites
- Requirement 1
- Requirement 2

## What This Skill Does
1. Primary function
2. Secondary function
3. Key benefit

---

## Level 2: Quick Start (For Fast Onboarding)

### Basic Usage
```bash
# Simplest use case
command --option value

Common Scenarios

1. Scenario 1: How to...
2. Scenario 2: How to...

---

Level 3: Detailed Instructions (For Deep Work)

Step-by-Step Guide

Step 1: Initial Setup

# Commands

Expected output:

Success message

Step 2: Configuration

• Configuration option 1
• Configuration option 2

Step 3: Execution

• Run the main command
• Verify results

Advanced Options

Option 1: Custom Configuration

# Advanced usage

Option 2: Integration

# Integration steps

---

Level 4: Reference (Rarely Needed)

Troubleshooting

Issue: Common Problem

Symptoms: What you see Cause: Why it happens Solution: How to fix

# Fix command

Issue: Another Problem

Solution: Steps to resolve

Complete API Reference

See API_REFERENCE.md

Examples

See examples/

Related Skills

• Related Skill 1
• Related Skill 2

Resources

• External Link 1
• Documentation

---

### 🎨 Content Best Practices

#### Writing Effective Descriptions

**Front-Load Keywords**:
```yaml
# ✅ GOOD: Keywords first
description: "Generate TypeScript interfaces from JSON schema. Use when converting schemas, creating types, or building API clients."

# ❌ BAD: Keywords buried
description: "This skill helps developers who need to work with JSON schemas by providing a way to generate TypeScript interfaces."

Include Trigger Conditions:

# ✅ GOOD: Clear "when" clause
description: "Debug React performance issues using Chrome DevTools. Use when components re-render unnecessarily, investigating slow updates, or optimizing bundle size."

# ❌ BAD: No trigger conditions
description: "Helps with React performance debugging."

Be Specific:

# ✅ GOOD: Specific technologies
description: "Create Express.js REST endpoints with Joi validation, Swagger docs, and Jest tests. Use when building new APIs or adding endpoints."

# ❌ BAD: Too generic
description: "Build API endpoints with proper validation and testing."

Progressive Disclosure Writing

Keep Level 1 Brief (Overview):

## What This Skill Does
Creates production-ready React components with TypeScript, hooks, and tests in 3 steps.

Level 2 for Common Paths (Quick Start):

## Quick Start
```bash
# Most common use case (80% of users)
generate-component MyComponent

**Level 3 for Details** (Step-by-Step):
```markdown
## Step-by-Step Guide

### Creating a Basic Component
1. Run generator
2. Choose template
3. Customize options
[Detailed explanations]

Level 4 for Edge Cases (Reference):

## Advanced Configuration
For complex scenarios like HOCs, render props, or custom hooks, see [ADVANCED.md](docs/ADVANCED.md).

---

🛠️ Adding Scripts and Resources

Scripts Directory

Purpose: Executable scripts that Codex can run Location: scripts/ in skill directory Usage: Referenced from SKILL.md

Example:

# In skill directory
scripts/
├── setup.sh          # Initialization script
├── validate.js       # Validation logic
├── generate.py       # Code generation
└── deploy.sh         # Deployment script

Reference from SKILL.md:

## Setup
Run the setup script:
```bash
./scripts/setup.sh

Validation

Validate your configuration:

node scripts/validate.js config.json

#### Resources Directory

**Purpose**: Templates, examples, schemas, static files
**Location**: `resources/` in skill directory
**Usage**: Referenced or copied by scripts

Example:
```bash
resources/
├── templates/
│   ├── component.tsx.template
│   ├── test.spec.ts.template
│   └── story.stories.tsx.template
├── examples/
│   ├── basic-example/
│   ├── advanced-example/
│   └── integration-example/
└── schemas/
    ├── config.schema.json
    └── output.schema.json

Reference from SKILL.md:

## Templates
Use the component template:
```bash
cp resources/templates/component.tsx.template src/components/MyComponent.tsx

Examples

See working examples in resources/examples/:

• basic-example/ - Simple component
• advanced-example/ - With hooks and context

---

### 🔗 File References and Navigation

Codex can navigate to referenced files automatically. Use these patterns:

#### Markdown Links
```markdown
See [Advanced Configuration](docs/ADVANCED.md) for complex scenarios.
See [Troubleshooting Guide](docs/TROUBLESHOOTING.md) if you encounter errors.

Relative File Paths

Use the template located at `resources/templates/api-template.js`
See examples in `resources/examples/basic-usage/`

Inline File Content

## Example Configuration
See `resources/examples/config.json`:
```json
{
  "option": "value"
}

**Best Practice**: Keep SKILL.md lean (~2-5KB). Move lengthy content to separate files and reference them. Codex will load only what's needed.

---

### ✅ Validation Checklist

Before publishing a skill, verify:

**YAML Frontmatter**:
- [ ] Starts with `---`
- [ ] Contains `name` field (max 64 chars)
- [ ] Contains `description` field (max 1024 chars)
- [ ] Description includes "what" and "when"
- [ ] Ends with `---`
- [ ] No YAML syntax errors

**File Structure**:
- [ ] SKILL.md exists in skill directory
- [ ] Directory is DIRECTLY in `~/.Codex/skills/[skill-name]/` or `.Codex/skills/[skill-name]/`
- [ ] Uses clear, descriptive directory name
- [ ] **NO nested subdirectories** (Codex requires top-level structure)

**Content Quality**:
- [ ] Level 1 (Overview) is brief and clear
- [ ] Level 2 (Quick Start) shows common use case
- [ ] Level 3 (Details) provides step-by-step guide
- [ ] Level 4 (Reference) links to advanced content
- [ ] Examples are concrete and runnable
- [ ] Troubleshooting section addresses common issues

**Progressive Disclosure**:
- [ ] Core instructions in SKILL.md (~2-5KB)
- [ ] Advanced content in separate docs/
- [ ] Large resources in resources/ directory
- [ ] Clear navigation between levels

**Testing**:
- [ ] Skill appears in Codex's skill list
- [ ] Description triggers on relevant queries
- [ ] Instructions are clear and actionable
- [ ] Scripts execute successfully (if included)
- [ ] Examples work as documented

---

## Skill Builder Templates

### Template 1: Basic Skill (Minimal)

```markdown
---
name: "My Basic Skill"
description: "One sentence what. One sentence when to use."
---

# My Basic Skill

## What This Skill Does
[2-3 sentences describing functionality]

## Quick Start
```bash
# Single command to get started

Step-by-Step Guide

Step 1: Setup

[Instructions]

Step 2: Usage

[Instructions]

Step 3: Verify

[Instructions]

Troubleshooting

• Issue: Problem description
  • Solution: Fix description

### Template 2: Intermediate Skill (With Scripts)

```markdown
---
name: "My Intermediate Skill"
description: "Detailed what with key features. When to use with specific triggers: scaffolding, generating, building."
---

# My Intermediate Skill

## Prerequisites
- Requirement 1
- Requirement 2

## What This Skill Does
1. Primary function
2. Secondary function
3. Integration capability

## Quick Start
```bash
./scripts/setup.sh
./scripts/generate.sh my-project

Configuration

Edit config.json:

{
  "option1": "value1",
  "option2": "value2"
}

Step-by-Step Guide

Basic Usage

[Steps for 80% use case]

Advanced Usage

[Steps for complex scenarios]

Available Scripts

• scripts/setup.sh - Initial setup
• scripts/generate.sh - Code generation
• scripts/validate.sh - Validation

Resources

• Templates: resources/templates/
• Examples: resources/examples/

Troubleshooting

[Common issues and solutions]

### Template 3: Advanced Skill (Full-Featured)

```markdown
---
name: "My Advanced Skill"
description: "Comprehensive what with all features and integrations. Use when [trigger 1], [trigger 2], or [trigger 3]. Supports [technology stack]."
---

# My Advanced Skill

## Overview
[Brief 2-3 sentence description]

## Prerequisites
- Technology 1 (version X+)
- Technology 2 (version Y+)
- API keys or credentials

## What This Skill Does
1. **Core Feature**: Description
2. **Integration**: Description
3. **Automation**: Description

---

## Quick Start (60 seconds)

### Installation
```bash
./scripts/install.sh

First Use

./scripts/quickstart.sh

Expected output:

✓ Setup complete
✓ Configuration validated
→ Ready to use

---

Configuration

Basic Configuration

Edit config.json:

{
  "mode": "production",
  "features": ["feature1", "feature2"]
}

Advanced Configuration

See Configuration Guide

---

Step-by-Step Guide

1. Initial Setup

[Detailed steps]

2. Core Workflow

[Main procedures]

3. Integration

[Integration steps]

---

Advanced Features

Feature 1: Custom Templates

./scripts/generate.sh --template custom

Feature 2: Batch Processing

./scripts/batch.sh --input data.json

Feature 3: CI/CD Integration

See CI/CD Guide

---

Scripts Reference

Script	Purpose	Usage
install.sh	Install dependencies	./scripts/install.sh
generate.sh	Generate code	./scripts/generate.sh [name]
validate.sh	Validate output	./scripts/validate.sh
deploy.sh	Deploy to environment	./scripts/deploy.sh [env]

---

Resources

Templates

• resources/templates/basic.template - Basic template
• resources/templates/advanced.template - Advanced template

Examples

• resources/examples/basic/ - Simple example
• resources/examples/advanced/ - Complex example
• resources/examples/integration/ - Integration example

Schemas

• resources/schemas/config.schema.json - Configuration schema
• resources/schemas/output.schema.json - Output validation

---

Troubleshooting

Issue: Installation Failed

Symptoms: Error during install.sh Cause: Missing dependencies Solution:

# Install prerequisites
npm install -g required-package
./scripts/install.sh --force

Issue: Validation Errors

Symptoms: Validation script fails Solution: See Troubleshooting Guide

---

API Reference

Complete API documentation: API_REFERENCE.md

Related Skills

• Related Skill 1
• Related Skill 2

Resources

• Official Documentation
• GitHub Repository
• Community Forum

---

Created: 2025-10-19 Category: Advanced Difficulty: Intermediate Estimated Time: 15-30 minutes

---

## Examples from the Wild

### Example 1: Simple Documentation Skill

```markdown
---
name: "README Generator"
description: "Generate comprehensive README.md files for GitHub repositories. Use when starting new projects, documenting code, or improving existing READMEs."
---

# README Generator

## What This Skill Does
Creates well-structured README.md files with badges, installation, usage, and contribution sections.

## Quick Start
```bash
# Answer a few questions
./scripts/generate-readme.sh

# README.md created with:
# - Project title and description
# - Installation instructions
# - Usage examples
# - Contribution guidelines

Customization

Edit sections in resources/templates/sections/ before generating.

### Example 2: Code Generation Skill

```markdown
---
name: "React Component Generator"
description: "Generate React functional components with TypeScript, hooks, tests, and Storybook stories. Use when creating new components, scaffolding UI, or following component architecture patterns."
---

# React Component Generator

## Prerequisites
- Node.js 18+
- React 18+
- TypeScript 5+

## Quick Start
```bash
./scripts/generate-component.sh MyComponent

# Creates:
# - src/components/MyComponent/MyComponent.tsx
# - src/components/MyComponent/MyComponent.test.tsx
# - src/components/MyComponent/MyComponent.stories.tsx
# - src/components/MyComponent/index.ts

Step-by-Step Guide

1. Run Generator

./scripts/generate-component.sh ComponentName

2. Choose Template

• Basic: Simple functional component
• With State: useState hooks
• With Context: useContext integration
• With API: Data fetching component

3. Customize

Edit generated files in src/components/ComponentName/

Templates

See resources/templates/ for available component templates.

---

## Learn More

### Official Resources
- [Anthropic Agent Skills Documentation](https://docs.Codex.com/en/docs/agents-and-tools/agent-skills)
- [GitHub Skills Repository](https://github.com/anthropics/skills)
- [Codex Documentation](https://docs.Codex.com/en/docs/Codex)

### Community
- [Skills Marketplace](https://github.com/anthropics/skills) - Browse community skills
- [Anthropic Discord](https://discord.gg/anthropic) - Get help from community

### Advanced Topics
- Multi-file skills with complex navigation
- Skills that spawn other skills
- Integration with MCP tools
- Dynamic skill generation

---

**Created**: 2025-10-19
**Version**: 1.0.0
**Maintained By**: agentic-flow team
**License**: MIT