Skill Creator

Skill Creator

This skill provides guidance for creating effective Claude Code skills following the official documentation.

What is a Skill?

A Skill is a markdown file that teaches Claude how to do something specific. Skills transform Claude from a general-purpose agent into a specialized agent with procedural knowledge, domain expertise, and bundled resources.

Skill Structure

skill-name/
├── SKILL.md                 # Required: frontmatter + instructions
├── reference.md             # Optional: detailed documentation
├── examples.md              # Optional: usage examples
├── templates/               # Optional: template files
└── scripts/                 # Optional: utility scripts
    ├── helper.py
    └── validate.sh

Where Skills Live

Higher priority locations override lower ones when names conflict.

SKILL.md Format

Frontmatter Fields (Complete Reference)

---
name: skill-name
description: What this skill does and when to use it
allowed-tools:
  - Read
  - Grep
  - Glob
model: claude-sonnet-4-20250514
context: fork
agent: Plan
hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/validate.sh"
          once: true
user-invocable: true
disable-model-invocation: false
---

Field Reference

allowed-tools Configuration

Format Options

Comma-separated:

allowed-tools: Read, Grep, Glob

YAML array:

allowed-tools:
  - Read
  - Grep
  - Glob

With command filtering:

allowed-tools: Bash(git add:*), Bash(git status:*), Bash(git commit:*)

Common Tools

Read, Write, Edit, Bash, Glob, Grep, WebFetch, WebSearch, Skill, Task

Context Fork (Isolated Execution)

Run skills in separate sub-agent context with own conversation history:

---
name: data-analysis
description: Analyze data and generate reports
context: fork
agent: Plan
---

Hooks Configuration

Only PreToolUse, PostToolUse, and Stop are supported in skill frontmatter.

hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/security-check.sh"
          once: true
  PostToolUse:
    - matcher: "Edit|Write"
      hooks:
        - type: command
          command: "./scripts/run-linter.sh"
  Stop:
    - hooks:
        - type: command
          command: "./scripts/cleanup.sh"

Hook Options

Hook Exit Codes

• Exit 0: Success
• Exit 2: Blocking error (stderr as message)
• Other: Non-blocking error

Invocation Control

Three Invocation Methods

Use user-invocable: false for skills Claude should use but users shouldn't invoke manually.

Use disable-model-invocation: true for skills users invoke manually but Claude shouldn't.

Skills and Subagents

Subagents don't automatically inherit skills. To give a custom subagent access:

File: .claude/agents/code-reviewer.md

---
name: code-reviewer
description: Review code for quality
skills: pr-review, security-check, style-guide
---

Review this code thoroughly...

Core Principles

1. Concise is Key

The context window is a public good. Skills share it with system prompt, conversation history, other skills' metadata, and user requests.

Default assumption: Claude is already smart. Only add context Claude doesn't have.

2. Set Appropriate Degrees of Freedom

3. Progressive Disclosure

Skills use three-level loading:

1. Metadata (always in context) - name + description (~100 words)
2. SKILL.md body (when triggered) - instructions (<5k words)
3. Bundled resources (as needed) - scripts, references, assets

Keep SKILL.md under 500 lines. Split content when approaching this limit.

Writing Effective Descriptions

A good description answers:

1. What does this skill do? - List specific capabilities
2. When should Claude use it? - Include trigger keywords

Poor:

description: Helps with documents

Good:

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

File Organization Patterns

Pattern 1: High-level guide with references

# PDF Processing

## Quick start
[code example]

## Advanced features
- **Form filling**: See [FORMS.md](FORMS.md)
- **API reference**: See [REFERENCE.md](REFERENCE.md)

Pattern 2: Domain-specific organization

bigquery-skill/
├── SKILL.md
└── reference/
    ├── finance.md
    ├── sales.md
    └── product.md

Pattern 3: Framework variants

cloud-deploy/
├── SKILL.md
└── references/
    ├── aws.md
    ├── gcp.md
    └── azure.md

Important: Keep references one level deep. Avoid chains (A -> B -> C).

What NOT to Include

• README.md
• INSTALLATION_GUIDE.md
• CHANGELOG.md
• User-facing documentation
• Setup/testing procedures

The skill should only contain information needed for Claude to do the job.

Skill Creation Process

1. Understand the skill - Gather concrete usage examples
2. Plan resources - Identify scripts, references, assets needed
3. Create directory - mkdir -p .claude/skills/skill-name
4. Write SKILL.md - Frontmatter + instructions
5. Add resources - Scripts, references, assets as needed
6. Test - Use the skill on real tasks
7. Iterate - Improve based on actual usage

Complete Examples

Example 1: Simple Single-File Skill

---
name: commit-helper
description: Generate clear commit messages from git diffs. Use when writing commit messages or reviewing staged changes.
allowed-tools: Bash(git diff:*), Bash(git status:*)
---

# Commit Message Generator

## Instructions

1. Run `git diff --staged` to see changes
2. Suggest commit message with:
   - Summary under 50 characters
   - Detailed description
   - Affected components

## Best Practices

- Use present tense imperative
- Explain what and why, not how

Example 2: Multi-File with Scripts

---
name: pdf-processor
description: Extract text, fill forms, merge PDFs. Use when working with PDF files.
allowed-tools: Read, Bash(python:*)
---

# PDF Processing

## Quick Start

```python
import pdfplumber
with pdfplumber.open("doc.pdf") as pdf:
    text = pdf.pages[0].extract_text()

Validation

python scripts/validate.py input.pdf

Resources

• Form filling: See FORMS.md
• API reference: See REFERENCE.md

### Example 3: Security-Focused with Hooks

```yaml
---
name: secure-db-ops
description: Execute database queries with validation and audit logging.
allowed-tools: Bash(psql:*), Bash(mysql:*)
hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/validate-query.sh"
  PostToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/audit-log.sh"
---

# Secure Database Operations

All queries are validated before execution and logged for audit.

Example 4: Forked Context Skill

---
name: code-analysis
description: Analyze code quality and generate detailed reports
context: fork
agent: Plan
---

# Code Analysis

Run comprehensive analysis in isolated context...

Troubleshooting

Skill Not Triggering

Make description more specific with trigger keywords.

Skill Won't Load

1. File must be SKILL.md (case-sensitive)
2. YAML must start with --- on line 1 (no blank lines)
3. Use spaces for indentation (not tabs)
4. Run claude --debug to see errors

Script Permission Issues

chmod +x scripts/*.py scripts/*.sh

Multiple Skills Conflict

Make descriptions distinct with specific trigger terms.

Environment Variables (Hooks Only)

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