Skill Development

Skill Development for Codex CLI

This skill provides guidance for creating effective skills for Codex CLI environments.

Compatibility note: Any plugin-oriented examples in this document are legacy references. Prefer the Codex skill locations described below.

Agent Workflow

To create high-quality skills, follow this Hybrid Workflow:

1. Requirement Gathering (Soft Skills): Before writing any files, ask the user for concrete examples of how they intend to use the skill.
   • Agent Question: "Can you give me 3 examples of what you would say to trigger this skill?"
2. Structural Planning (Hard Patterns): Once requirements are clear, plan the file structure using the Standard Skill Structure defined below.
   • Action: Decide if you need scripts/, references/, or just SKILL.md.
3. Implementation (Hybrid): Write the content.
   • Hard Rule: Use correct YAML frontmatter and folder names.
   • Soft Rule: write in the "Imperative/Infinitive" style (see Writing Style Requirements).

Integration with skill-create

Use this skill as the refinement stage after $skill-create generates drafts.

Pipeline:

1. Generate draft skill(s) with $skill-create
2. Refine each draft with $skill-development
3. Run $skill-quality-analyzer for structure and trigger quality
4. Run $skill-tester for behavioral validation
5. Keep only skills that pass governance and quality gates

About Skills

Skills are modular, self-contained packages that extend Codex's capabilities by providing specialized knowledge, workflows, and tools. Think of them as "onboarding guides" for specific domains or tasks—they transform Codex from a general-purpose agent into a specialized agent equipped with procedural knowledge that no model can fully possess.

What Skills Provide

1. Specialized workflows - Multi-step procedures for specific domains
2. Tool integrations - Instructions for working with specific file formats or APIs
3. Domain expertise - Company-specific knowledge, schemas, business logic
4. Bundled resources - Scripts, references, and assets for complex and repetitive tasks

Anatomy of a Skill

Every skill consists of a required SKILL.md file and optional bundled resources:

skill-name/
├── SKILL.md (required)
│   ├── YAML frontmatter metadata (required)
│   │   ├── name: (required)
│   │   └── description: (required)
│   └── Markdown instructions (required)
└── Bundled Resources (optional)
    ├── scripts/          - Executable code (Python/Bash/etc.)
    ├── references/       - Documentation intended to be loaded into context as needed
    └── assets/           - Files used in output (templates, icons, fonts, etc.)

SKILL.md (required)

Metadata Quality: The name and description in YAML frontmatter determine when Codex will use the skill. Be specific about what the skill does and when to use it. Use the third-person (e.g. "This skill should be used when..." instead of "Use this skill when...").

Bundled Resources (optional)

Scripts (scripts/)

Executable code (Python/Bash/etc.) for tasks that require deterministic reliability or are repeatedly rewritten.

• When to include: When the same code is being rewritten repeatedly or deterministic reliability is needed
• Example: scripts/rotate_pdf.py for PDF rotation tasks
• Benefits: Token efficient, deterministic, may be executed without loading into context
• Note: Scripts may still need to be read by Codex for patching or environment-specific adjustments

References (references/)

Documentation and reference material intended to be loaded as needed into context to inform Codex's process and thinking.

• When to include: For documentation that Codex should reference while working
• Examples: references/finance.md for financial schemas, references/mnda.md for company NDA template, references/policies.md for company policies, references/api_docs.md for API specifications
• Use cases: Database schemas, API documentation, domain knowledge, company policies, detailed workflow guides
• Benefits: Keeps SKILL.md lean, loaded only when Codex determines it's needed
• Best practice: If files are large (>10k words), include grep search patterns in SKILL.md
• Avoid duplication: Information should live in either SKILL.md or references files, not both. Prefer references files for detailed information unless it's truly core to the skill—this keeps SKILL.md lean while making information discoverable without hogging the context window. Keep only essential procedural instructions and workflow guidance in SKILL.md; move detailed reference material, schemas, and examples to references files.

Assets (assets/)

Files not intended to be loaded into context, but rather used within the output Codex produces.

• When to include: When the skill needs files that will be used in the final output
• Examples: assets/logo.png for brand assets, assets/slides.pptx for PowerPoint templates, assets/frontend-template/ for HTML/React boilerplate, assets/font.ttf for typography
• Use cases: Templates, images, icons, boilerplate code, fonts, sample documents that get copied or modified
• Benefits: Separates output resources from documentation, enables Codex to use files without loading them into context

Progressive Disclosure Design Principle

Skills use a three-level loading system to manage context efficiently:

1. Metadata (name + description) - Always in context (~100 words)
2. SKILL.md body - When skill triggers (<5k words)
3. Bundled resources - As needed by Codex (Unlimited*)

*Unlimited because scripts can be executed without reading into context window.

Skill Creation Process

To create a skill, follow the "Skill Creation Process" in order, skipping steps only if there is a clear reason why they are not applicable.

Step 1: Understanding the Skill with Concrete Examples

Skip this step only when the skill's usage patterns are already clearly understood. It remains valuable even when working with an existing skill.

To create an effective skill, clearly understand concrete examples of how the skill will be used. This understanding can come from either direct user examples or generated examples that are validated with user feedback.

For example, when building an image-editor skill, relevant questions include:

• "What functionality should the image-editor skill support? Editing, rotating, anything else?"
• "Can you give some examples of how this skill would be used?"
• "I can imagine users asking for things like 'Remove the red-eye from this image' or 'Rotate this image'. Are there other ways you imagine this skill being used?"
• "What would a user say that should trigger this skill?"

To avoid overwhelming users, avoid asking too many questions in a single message. Start with the most important questions and follow up as needed for better effectiveness.

Conclude this step when there is a clear sense of the functionality the skill should support.

Step 2: Planning the Reusable Skill Contents

To turn concrete examples into an effective skill, analyze each example by:

1. Considering how to execute on the example from scratch
2. Identifying what scripts, references, and assets would be helpful when executing these workflows repeatedly

Example: When building a pdf-editor skill to handle queries like "Help me rotate this PDF," the analysis shows:

1. Rotating a PDF requires re-writing the same code each time
2. A scripts/rotate_pdf.py script would be helpful to store in the skill

Example: When designing a frontend-webapp-builder skill for queries like "Build me a todo app" or "Build me a dashboard to track my steps," the analysis shows:

1. Writing a frontend webapp requires the same boilerplate HTML/React each time
2. An assets/hello-world/ template containing the boilerplate HTML/React project files would be helpful to store in the skill

Example: When building a big-query skill to handle queries like "How many users have logged in today?" the analysis shows:

1. Querying BigQuery requires re-discovering the table schemas and relationships each time
2. A references/schema.md file documenting the table schemas would be helpful to store in the skill

For Codex CLI skills: When building a hooks-related skill, the analysis shows:

1. Developers repeatedly need to validate hooks.json and test hook scripts
2. scripts/validate-hook-schema.sh and scripts/test-hook.sh utilities would be helpful
3. references/patterns.md for detailed hook patterns to avoid bloating SKILL.md

To establish the skill's contents, analyze each concrete example to create a list of the reusable resources to include: scripts, references, and assets.

Step 3: Create Skill Structure

For Codex CLI, create the skill directory structure:

mkdir -p skill-name/{references,examples,scripts}
touch skill-name/SKILL.md

Note: Unlike the generic skill-creator which uses init_skill.py, Codex CLI skills are created directly in a skill directory and then placed under ~/.codex/skills/, <repo>/.codex/skills/, or <repo>/.agents/skills/.

Step 4: Edit the Skill

When editing the (newly-created or existing) skill, remember that the skill is being created for another instance of Codex to use. Focus on including information that would be beneficial and non-obvious to Codex. Consider what procedural knowledge, domain-specific details, or reusable assets would help another Codex instance execute these tasks more effectively.

Start with Reusable Skill Contents

To begin implementation, start with the reusable resources identified above: scripts/, references/, and assets/ files. Note that this step may require user input. For example, when implementing a brand-guidelines skill, the user may need to provide brand assets or templates to store in assets/, or documentation to store in references/.

Also, delete any example files and directories not needed for the skill. Create only the directories you actually need (references/, examples/, scripts/).

Update SKILL.md

Writing Style: Write the entire skill using imperative/infinitive form (verb-first instructions), not second person. Use objective, instructional language (e.g., "To accomplish X, do Y" rather than "You should do X" or "If you need to do X"). This maintains consistency and clarity for AI consumption.

Description (Frontmatter): Use third-person format with specific trigger phrases:

---
name: Skill Name
description: This skill should be used when the user asks to "specific phrase 1", "specific phrase 2", "specific phrase 3". Include exact phrases users would say that should trigger this skill. Be concrete and specific.
version: 0.1.0
---

Good description examples:

description: This skill should be used when the user asks to "create a validation rule", "define a skill trigger", "validate tool use instructions", or "design prompt guardrails".

Bad description examples:

description: Use this skill when working with hooks.  # Wrong person, vague
description: Load when user needs hook help.  # Not third person
description: Provides hook guidance.  # No trigger phrases

To complete SKILL.md body, answer the following questions:

1. What is the purpose of the skill, in a few sentences?
2. When should the skill be used? (Include this in frontmatter description with specific triggers)
3. In practice, how should Codex use the skill? All reusable skill contents developed above should be referenced so that Codex knows how to use them.

Keep SKILL.md lean: Target 1,500-2,000 words for the body. Move detailed content to references/:

• Detailed patterns → references/patterns.md
• Advanced techniques → references/advanced.md
• Migration guides → references/migration.md
• API references → references/api-reference.md

Reference resources in SKILL.md:

## Additional Resources

### Reference Files

For detailed patterns and techniques, consult:
- **`references/patterns.md`** - Common patterns
- **`references/advanced.md`** - Advanced use cases

### Example Files

Working examples in `examples/`:
- **`example-script.sh`** - Working example

Step 5: Validate and Test

For Codex CLI skills, validation is different from generic skills:

1. Check structure: Skill directory in ~/.codex/skills/skill-name/, <repo>/.codex/skills/skill-name/, or <repo>/.agents/skills/skill-name/
2. Validate SKILL.md: Has frontmatter with name and description
3. Check trigger phrases: Description includes specific user queries
4. Verify writing style: Body uses imperative/infinitive form, not second person
5. Test progressive disclosure: SKILL.md is lean (~1,500-2,000 words), detailed content in references/
6. Check references: All referenced files exist
7. Validate examples: Examples are complete and correct
8. Test scripts: Scripts are executable and work correctly

Use the skill-reviewer agent:

Ask: "Review my skill and check if it follows best practices"

The skill-reviewer agent will check description quality, content organization, and progressive disclosure.

Step 6: Iterate

After testing the skill, users may request improvements. Often this happens right after using the skill, with fresh context of how the skill performed.

Iteration workflow:

1. Use the skill on real tasks
2. Notice struggles or inefficiencies
3. Identify how SKILL.md or bundled resources should be updated
4. Implement changes and test again

Common improvements:

• Strengthen trigger phrases in description
• Move long sections from SKILL.md to references/
• Add missing examples or scripts
• Clarify ambiguous instructions
• Add edge case handling

Codex CLI Considerations

Skill Locations

Skills for Codex CLI should live in one of these locations:

~/.codex/skills/
└── my-skill/
    ├── SKILL.md
    ├── references/
    ├── examples/
    └── scripts/

<repo>/.agent/skills/
└── my-skill/
    ├── SKILL.md
    ├── references/
    ├── examples/
    └── scripts/

Auto-Discovery

Codex automatically discovers skills:

• Scans skills/ directory
• Finds subdirectories containing SKILL.md
• Loads skill metadata (name + description) always
• Loads SKILL.md body when skill triggers
• Loads references/examples when needed

No Packaging Needed

Skills are plain directories. Install globally via ~/.codex/skills/ or keep project-local under .codex/skills/ / .agents/skills/.

Testing in Codex CLI

Test skills by opening Codex in the target repo and invoking explicit triggers:

# Example explicit invocation
$skill-name <task>

Examples from Existing Skill Packs

Study existing skill packs as examples of best practices:

hook-development skill:

• Excellent trigger phrases: "create a validation rule", "define trigger phrases", etc.
• Lean SKILL.md (1,651 words)
• 3 references/ files for detailed content
• 3 examples/ of working hooks
• 3 scripts/ utilities

agent-development skill:

• Strong triggers: "create an agent", "agent frontmatter", etc.
• Focused SKILL.md (1,438 words)
• References include the AI generation prompt from Codex
• Complete agent examples

settings-management skill:

• Specific triggers: "settings files", "local config", "YAML frontmatter"
• References show real implementations (multi-agent-swarm, ralph-wiggum)
• Working parsing scripts

Each demonstrates progressive disclosure and strong triggering.

Progressive Disclosure in Practice

What Goes in SKILL.md

Include (always loaded when skill triggers):

• Core concepts and overview
• Essential procedures and workflows
• Quick reference tables
• Pointers to references/examples/scripts
• Most common use cases

Keep under 3,000 words, ideally 1,500-2,000 words

What Goes in references/

Move to references/ (loaded as needed):

• Detailed patterns and advanced techniques
• Comprehensive API documentation
• Migration guides
• Edge cases and troubleshooting
• Extensive examples and walkthroughs

Each reference file can be large (2,000-5,000+ words)

What Goes in examples/

Working code examples:

• Complete, runnable scripts
• Configuration files
• Template files
• Real-world usage examples

Users can copy and adapt these directly

What Goes in scripts/

Utility scripts:

• Validation tools
• Testing helpers
• Parsing utilities
• Automation scripts

Should be executable and documented

Writing Style Requirements

Imperative/Infinitive Form

Write using verb-first instructions, not second person:

Correct (imperative):

To create a hook, define the event type.
Configure the MCP server with authentication.
Validate settings before use.

Incorrect (second person):

You should create a hook by defining the event type.
You need to configure the MCP server.
You must validate settings before use.

Third-Person in Description

The frontmatter description must use third person:

Correct:

description: This skill should be used when the user asks to "create X", "configure Y"...

Incorrect:

description: Use this skill when you want to create X...
description: Load this skill when user asks...

Objective, Instructional Language

Focus on what to do, not who should do it:

Correct:

Parse the frontmatter using sed.
Extract fields with grep.
Validate values before use.

Incorrect:

You can parse the frontmatter...
Codex should extract fields...
The user might validate values...

Validation Checklist

Before finalizing a skill:

Structure:

• SKILL.md file exists with valid YAML frontmatter
• Frontmatter has name and description fields
• Markdown body is present and substantial
• Referenced files actually exist

Description Quality:

• Uses third person ("This skill should be used when...")
• Includes specific trigger phrases users would say
• Lists concrete scenarios ("create X", "configure Y")
• Not vague or generic

Content Quality:

• SKILL.md body uses imperative/infinitive form
• Body is focused and lean (1,500-2,000 words ideal, <5k max)
• Detailed content moved to references/
• Examples are complete and working
• Scripts are executable and documented

Progressive Disclosure:

• Core concepts in SKILL.md
• Detailed docs in references/
• Working code in examples/
• Utilities in scripts/
• SKILL.md references these resources

Testing:

• Skill triggers on expected user queries
• Content is helpful for intended tasks
• No duplicated information across files
• References load when needed

Common Mistakes to Avoid

Mistake 1: Weak Trigger Description

❌ Bad:

description: Provides guidance for working with hooks.

Why bad: Vague, no specific trigger phrases, not third person

✅ Good:

description: This skill should be used when the user asks to "create a validation rule", "define trigger conditions", or "validate tool instructions". Provides comprehensive guidance with concrete trigger phrases.

Why good: Third person, specific phrases, concrete scenarios

Mistake 2: Too Much in SKILL.md

❌ Bad:

skill-name/
└── SKILL.md  (8,000 words - everything in one file)

Why bad: Bloats context when skill loads, detailed content always loaded

✅ Good:

skill-name/
├── SKILL.md  (1,800 words - core essentials)
└── references/
    ├── patterns.md (2,500 words)
    └── advanced.md (3,700 words)

Why good: Progressive disclosure, detailed content loaded only when needed

Mistake 3: Second Person Writing

❌ Bad:

You should start by reading the configuration file.
You need to validate the input.
You can use the grep tool to search.

Why bad: Second person, not imperative form

✅ Good:

Start by reading the configuration file.
Validate the input before processing.
Use the grep tool to search for patterns.

Why good: Imperative form, direct instructions

Mistake 4: Missing Resource References

❌ Bad:

# SKILL.md

[Core content]

[No mention of references/ or examples/]

Why bad: Codex doesn't know references exist

✅ Good:

# SKILL.md

[Core content]

## Additional Resources

### Reference Files
- **`references/patterns.md`** - Detailed patterns
- **`references/advanced.md`** - Advanced techniques

### Examples
- **`examples/script.sh`** - Working example

Why good: Codex knows where to find additional information

Quick Reference

Minimal Skill

skill-name/
└── SKILL.md

Good for: Simple knowledge, no complex resources needed

Standard Skill (Recommended)

skill-name/
├── SKILL.md
├── references/
│   └── detailed-guide.md
└── examples/
    └── working-example.sh

Good for: Most Codex skills with detailed documentation

Complete Skill

skill-name/
├── SKILL.md
├── references/
│   ├── patterns.md
│   └── advanced.md
├── examples/
│   ├── example1.sh
│   └── example2.json
└── scripts/
    └── validate.sh

Good for: Complex domains with validation utilities

Best Practices Summary

✅ DO:

• Use third-person in description ("This skill should be used when...")
• Include specific trigger phrases ("create X", "configure Y")
• Keep SKILL.md lean (1,500-2,000 words)
• Use progressive disclosure (move details to references/)
• Write in imperative/infinitive form
• Reference supporting files clearly
• Provide working examples
• Create utility scripts for common operations
• Study high-quality local skills as templates

❌ DON'T:

• Use second person anywhere
• Have vague trigger conditions
• Put everything in SKILL.md (>3,000 words without references/)
• Write in second person ("You should...")
• Leave resources unreferenced
• Include broken or incomplete examples
• Skip validation

Additional Resources

Study These Skills

Reference skills that demonstrate best practices:

• ../hook-development/ - Progressive disclosure, utilities
• ../agent-development/ - AI-assisted creation, references
• ../mcp-integration/ - Comprehensive references
• ../skill-quality-analyzer/ - Real-world scoring examples
• ../command-development/ - Clear critical concepts
• ../skill-development/ - Good organization

Reference Files

For complete skill-creator methodology:

• references/skill-creator-original.md - Full original skill-creator content

Implementation Workflow

To create a skill for Codex CLI:

1. Understand use cases: Identify concrete examples of skill usage
2. Plan resources: Determine what scripts/references/examples needed
3. Create structure: mkdir -p skills/skill-name/{references,examples,scripts}
4. Write SKILL.md:
   • Frontmatter with third-person description and trigger phrases
   • Lean body (1,500-2,000 words) in imperative form
   • Reference supporting files
5. Add resources: Create references/, examples/, scripts/ as needed
6. Validate: Check description, writing style, organization
7. Test: Verify skill loads on expected triggers
8. Iterate: Improve based on usage

Focus on strong trigger descriptions, progressive disclosure, and imperative writing style for effective skills that load when needed and provide targeted guidance.