Writing Claude Code Skills

Writing Claude Code Skills

What is a Skill?

Skills are modular capabilities that extend Claude's functionality. They are model-invoked — Claude autonomously decides when to use them based on context and the skill's description.

Directory Structure

Skills must be stored as directories containing a required SKILL.md file:

~/​.claude/​skills/​my-skill-name/     # Personal (all projects)
.claude/​skills/​my-skill-name/       # Project (team-shared via git)

Multi-file Structure

my-skill/
├── SKILL.md           # Required - main instructions
├── REFERENCE.md       # Optional - detailed reference
├── EXAMPLES.md        # Optional - usage examples
├── scripts/           # Optional - helper scripts
│   └── helper.py
└── templates/         # Optional - templates
    └── template.txt

Required SKILL.md Format

---
name: your-skill-name
description: What this skill does AND when to use it. Include trigger keywords.
---

# Skill Title

## Instructions
Step-by-step guidance for Claude.

## Examples
Concrete usage examples.

Frontmatter Fields

Writing Effective Descriptions

The description determines when Claude activates your skill.

Good (specific, includes triggers, keywords):

description: Extract text and tables from PDF files, fill forms, merge documents. Use when working with PDF files or when the user mentions PDFs, forms, or document extraction.

Bad (vague):

description: Helps with documents

Best Practices

1. One skill = one capability - Keep focused
2. Include trigger keywords in description
3. Use progressive disclosure - Main info in SKILL.md, details in supporting files
4. Document versions for tracking changes
5. Use allowed-tools for security-sensitive skills
6. Test with teammates for activation and clarity
7. Avoid dangerous backtick patterns - See "Dangerous Markdown Patterns" below
8. Run verification before committing - ./​verify.sh

Template for New Skills

---
name: my-new-skill
description: [Action verb] [what it does]. Use when [trigger conditions] or when the user mentions [keywords].
---

# [Skill Title]

## Overview
Brief description of capability.

## Instructions
1. First step
2. Second step
3. Third step

## Examples

### Example 1: [Use case]
[Concrete example]

## Requirements
- List any dependencies
- Required packages or tools

Registering Skills in Settings

After creating a skill, configure permissions in settings.json for Claude to use it.

Required Permissions

For personal skills, add to ~/​.claude/​settings.json:

{
  "permissions": {
    "allow": [
      "Skill(your-skill-name)",
      "Read(~/​.claude/​skills/​**)"
    ]
  }
}

For project skills, add to .claude/​settings.json:

{
  "permissions": {
    "allow": [
      "Skill(your-skill-name)",
      "Read(./​.claude/​skills/​**)"
    ]
  }
}

Notes:

• The Read() permission for the skills directory only needs to be added once (not per-skill)
• Each new skill needs its own Skill(name) entry

Validation Checklist

• SKILL.md exists in skill directory
• YAML frontmatter starts on line 1 with ---
• name field: lowercase, hyphens, no spaces, max 64 chars
• description field: includes what AND when, max 1024 chars
• No tabs in YAML (use spaces only)
• Closing --- before markdown content
• No dangerous backtick patterns (shell chars like !$# in inline code)
• Skill(name) added to permissions.allow in settings.json
• Read(~/​.claude/​skills/​**) added to permissions.allow (once per settings file)
• ./​verify.sh passes with no errors

Adding New Skills to This Project

Skills live in .claude/​skills/ within this repository.

Adding Your Skill

1. Create the skill directory:

   mkdir -p .claude/​skills/<skill-name>
   

2. Write your SKILL.md following the format in this guide

3. Update .claude/​settings.json to add permissions:

   "Skill(<skill-name>)"
   

4. Run verification:

   ./​verify.sh --ui=false
   

Checklist

Before committing:

• Skill follows naming conventions (lowercase, hyphens)
• Description is specific with trigger keywords
• No duplicate of existing skill
• Skill(name) added to .claude/​settings.json
• Tested locally with claude --debug

Skill Verification

Skills can break silently. Use the verification pipeline to catch issues before they cause runtime failures.

Running Verification

./​verify.sh          # Run all checks
bun run verify       # Same via npm script

What Verification Checks

Dangerous Patterns to Avoid

Troubleshooting

Debug with: claude --debug