ratacat/creating-agent-skills

Creating Agent Skills

This skill teaches how to create effective Claude Code Skills following Anthropic's official specification.

Core Principles

1. Skills Are Prompts

All prompting best practices apply. Be clear, be direct. Assume Claude is smart - only add context Claude doesn't have.

2. Standard Markdown Format

Use YAML frontmatter + markdown body. No XML tags - use standard markdown headings.

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

# My Skill Name

## Quick Start
Immediate actionable guidance...

## Instructions
Step-by-step procedures...

## Examples
Concrete usage examples...

3. Progressive Disclosure

Keep SKILL.md under 500 lines. Split detailed content into reference files. Load only what's needed.

my-skill/
├── SKILL.md              # Entry point (required)
├── reference.md          # Detailed docs (loaded when needed)
├── examples.md           # Usage examples
└── scripts/              # Utility scripts (executed, not loaded)

4. Effective Descriptions

The description field enables skill discovery. Include both what the skill does AND when to use it. Write in third person.

Good:

description: Extracts text and tables from PDF files, fills forms, merges documents. Use when working with PDF files or when the user mentions PDFs, forms, or document extraction.

Bad:

description: Helps with documents

Skill Structure

Required Frontmatter

| Field | Required | Max Length | Description | |-------|----------|------------|-------------| | name | Yes | 64 chars | Lowercase letters, numbers, hyphens only | | description | Yes | 1024 chars | What it does AND when to use it | | allowed-tools | No | - | Tools Claude can use without asking | | model | No | - | Specific model to use |

Naming Conventions

Use gerund form (verb + -ing) for skill names:

• processing-pdfs
• analyzing-spreadsheets
• generating-commit-messages
• reviewing-code

Avoid: helper, utils, tools, anthropic-*, claude-*

Body Structure

Use standard markdown headings:

# Skill Name

## Quick Start
Fastest path to value...

## Instructions
Core guidance Claude follows...

## Examples
Input/output pairs showing expected behavior...

## Advanced Features
Additional capabilities (link to reference files)...

## Guidelines
Rules and constraints...

What Would You Like To Do?

1. Create new skill - Build from scratch
2. Audit existing skill - Check against best practices
3. Add component - Add workflow/reference/example
4. Get guidance - Understand skill design

Creating a New Skill

Step 1: Choose Type

Simple skill (single file):

• Under 500 lines
• Self-contained guidance
• No complex workflows

Progressive disclosure skill (multiple files):

• SKILL.md as overview
• Reference files for detailed docs
• Scripts for utilities

Step 2: Create SKILL.md

---
name: your-skill-name
description: [What it does]. Use when [trigger conditions].
---

# Your Skill Name

## Quick Start

[Immediate actionable example]

```[language]
[Code example]

Instructions

[Core guidance]

Examples

Example 1: Input: [description] Output:

[result]

Guidelines

• [Constraint 1]
• [Constraint 2]

### Step 3: Add Reference Files (If Needed)

Link from SKILL.md to detailed content:

```markdown
For API reference, see [REFERENCE.md](REFERENCE.md).
For form filling guide, see [FORMS.md](FORMS.md).

Keep references one level deep from SKILL.md.

Step 4: Add Scripts (If Needed)

Scripts execute without loading into context:

## Utility Scripts

Extract fields:
```bash
python scripts/analyze.py input.pdf > fields.json

### Step 5: Test With Real Usage

1. Test with actual tasks, not test scenarios
2. Observe where Claude struggles
3. Refine based on real behavior
4. Test with Haiku, Sonnet, and Opus

## Auditing Existing Skills

Check against this rubric:

- [ ] Valid YAML frontmatter (name + description)
- [ ] Description includes trigger keywords
- [ ] Uses standard markdown headings (not XML tags)
- [ ] SKILL.md under 500 lines
- [ ] References one level deep
- [ ] Examples are concrete, not abstract
- [ ] Consistent terminology
- [ ] No time-sensitive information
- [ ] Scripts handle errors explicitly

## Common Patterns

### Template Pattern

Provide output templates for consistent results:

```markdown
## Report Template

```markdown
# [Analysis Title]

## Executive Summary
[One paragraph overview]

## Key Findings
- Finding 1
- Finding 2

## Recommendations
1. [Action item]
2. [Action item]

### Workflow Pattern

For complex multi-step tasks:

```markdown
## Migration Workflow

Copy this checklist:

• [ ] Step 1: Backup database
• [ ] Step 2: Run migration script
• [ ] Step 3: Validate output
• [ ] Step 4: Update configuration

**Step 1: Backup database**
Run: `./scripts/backup.sh`
...

Conditional Pattern

Guide through decision points:

## Choose Your Approach

**Creating new content?** Follow "Creation workflow" below.
**Editing existing?** Follow "Editing workflow" below.

Anti-Patterns to Avoid

• XML tags in body - Use markdown headings instead
• Vague descriptions - Be specific with trigger keywords
• Deep nesting - Keep references one level from SKILL.md
• Too many options - Provide a default with escape hatch
• Windows paths - Always use forward slashes
• Punting to Claude - Scripts should handle errors
• Time-sensitive info - Use "old patterns" section instead

Reference Files

For detailed guidance, see:

• official-spec.md - Anthropic's official skill specification
• best-practices.md - Skill authoring best practices

Success Criteria

A well-structured skill:

• Has valid YAML frontmatter with descriptive name and description
• Uses standard markdown headings (not XML tags)
• Keeps SKILL.md under 500 lines
• Links to reference files for detailed content
• Includes concrete examples with input/output pairs
• Has been tested with real usage

Sources:

• Agent Skills - Claude Code Docs
• Skill authoring best practices
• GitHub - anthropics/skills