Creating Agent Skills

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

Quick Start

Create a new skill in ~/.claude/skills/:

mkdir -p ~/.claude/skills/my-skill-name

# ~/.claude/skills/my-skill-name/SKILL.md
---
name: my-skill-name
description: Generates weekly status reports from git logs. Use when the user asks for status updates, weekly reports, or standup summaries.
---

# My Skill Name

## Quick Start
Run `git log --since="1 week ago"` and summarize changes by author.

## Instructions
1. Gather commits from the past week
2. Group by author and category (feature, fix, chore)
3. Summarize in bullet points

## Examples
**Input:** "Generate my weekly status"
**Output:**
- **Features:** Added user authentication (3 commits)
- **Fixes:** Resolved login timeout issue
- **Chores:** Updated dependencies

Relationship to Prompt Writing

Skills are prompts packaged for Claude Code. For foundational prompt engineering—including the seven levels of complexity, composable sections, and the Input→Workflow→Output pattern—see the writing-prompts skill.

This skill focuses on packaging prompts as skills:

• YAML frontmatter for discovery
• Progressive disclosure via reference files
• Claude Code tool integration

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: formatting-sql
description: Formats SQL queries with consistent style and indentation. Use when the user has messy SQL, asks to format queries, or mentions SQL style.
---

# Formatting SQL

## Quick Start

Paste your SQL and I'll format it with consistent indentation, uppercase keywords, and aligned clauses.

## Instructions

1. Uppercase all SQL keywords (SELECT, FROM, WHERE, JOIN)
2. Place each major clause on its own line
3. Indent subqueries by 2 spaces
4. Align column lists vertically
5. Add blank lines between CTEs

## Examples

**Input:**
```sql
select u.id,u.name,o.total from users u join orders o on u.id=o.user_id where o.total>100

Output:

SELECT
  u.id,
  u.name,
  o.total
FROM users u
JOIN orders o ON u.id = o.user_id
WHERE o.total > 100

Guidelines

• Preserve existing comments
• Don't change query logic, only formatting

### 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

# Code Review: {filename}

## Summary
{One paragraph describing overall code quality and main concerns}

## Issues Found
- **Line {n}:** {description of issue}
- **Line {n}:** {description of issue}

## Recommendations
1. Extract {function_name} to reduce complexity
2. Add error handling for {edge_case}

Workflow Pattern

For complex multi-step tasks:

## Database Migration Workflow

1. **Backup the database**
   ```bash
   pg_dump -Fc mydb > backup_$(date +%Y%m%d).dump

2. Run migration in a transaction

   psql mydb -c "BEGIN; \i migration.sql; -- verify results before COMMIT"

3. Validate data integrity

   psql mydb -c "SELECT COUNT(*) FROM users WHERE email IS NULL"

   Expected: 0 rows

4. Commit or rollback

   • If validation passes: COMMIT;
   • If validation fails: ROLLBACK; and restore from backup

### Conditional Pattern

Guide through decision points:

```markdown
## 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