Skill Developer Guide

Purpose

Comprehensive guide for creating and managing skills in Claude Code with auto-activation system, following Anthropic's official best practices including the 500-line rule and progressive disclosure pattern.

When to Use This Skill

Automatically activates when you mention:

•Creating or adding skills
•Modifying skill triggers or rules
•Understanding how skill activation works
•Debugging skill activation issues
•Working with skill-rules.json
•Hook system mechanics
•Claude Code best practices
•Progressive disclosure
•YAML frontmatter
•500-line rule

System Overview

Two-Hook Architecture

1. UserPromptSubmit Hook (Proactive Suggestions)

•File: .claude/hooks/skill-activation-prompt.ts
•Trigger: BEFORE Claude sees user's prompt
•Purpose: Suggest relevant skills based on keywords + intent patterns
•Method: Injects formatted reminder as context (stdout → Claude's input)
•Use Cases: Topic-based skills, implicit work detection

2. Stop Hook - Error Handling Reminder (Gentle Reminders)

•File: .claude/hooks/error-handling-reminder.ts
•Trigger: AFTER Claude finishes responding
•Purpose: Gentle reminder to self-assess error handling in code written
•Method: Analyzes edited files for risky patterns, displays reminder if needed
•Use Cases: Error handling awareness without blocking friction

Philosophy Change (2025-10-27): We moved away from blocking PreToolUse for Sentry/error handling. Instead, use gentle post-response reminders that don't block workflow but maintain code quality awareness.

Configuration File

Location: .claude/skills/skill-rules.json

Defines:

•All skills and their trigger conditions
•Enforcement levels (block, suggest, warn)
•File path patterns (glob)
•Content detection patterns (regex)
•Skip conditions (session tracking, file markers, env vars)

Skill Types

1. Guardrail Skills

Purpose: Enforce critical best practices that prevent errors

Characteristics:

•Type: "guardrail"
•Enforcement: "block"
•Priority: "critical" or "high"
•Block file edits until skill used
•Prevent common mistakes (column names, critical errors)
•Session-aware (don't repeat nag in same session)

Examples:

•database-verification - Verify table/column names before Prisma queries
•frontend-dev-guidelines - Enforce React/TypeScript patterns

When to Use:

•Mistakes that cause runtime errors
•Data integrity concerns
•Critical compatibility issues

2. Domain Skills

Purpose: Provide comprehensive guidance for specific areas

Characteristics:

•Type: "domain"
•Enforcement: "suggest"
•Priority: "high" or "medium"
•Advisory, not mandatory
•Topic or domain-specific
•Comprehensive documentation

Examples:

•backend-dev-guidelines - Node.js/Express/TypeScript patterns
•frontend-dev-guidelines - React/TypeScript best practices
•error-tracking - Sentry integration guidance

When to Use:

•Complex systems requiring deep knowledge
•Best practices documentation
•Architectural patterns
•How-to guides

Quick Start: Creating a New Skill

Step 1: Create Skill File

Location: .claude/skills/{skill-name}/SKILL.md

Template:

markdown

---
name: my-new-skill
description: Brief description including keywords that trigger this skill. Mention topics, file types, and use cases. Be explicit about trigger terms.
---

# My New Skill

## Purpose
What this skill helps with

## When to Use
Specific scenarios and conditions

## Key Information
The actual guidance, documentation, patterns, examples

Best Practices:

•✅ Name: Lowercase, hyphens, gerund form (verb + -ing) preferred
•✅ Description: Include ALL trigger keywords/phrases (max 1024 chars)
•✅ Content: Under 500 lines - use reference files for details
•✅ Examples: Real code examples
•✅ Structure: Clear headings, lists, code blocks

Step 2: Add to skill-rules.json

See SKILL_RULES_REFERENCE.md for complete schema.

Basic Template:

json

{
  "my-new-skill": {
    "type": "domain",
    "enforcement": "suggest",
    "priority": "medium",
    "promptTriggers": {
      "keywords": ["keyword1", "keyword2"],
      "intentPatterns": ["(create|add).*?something"]
    }
  }
}

Step 3: Test Triggers

Test UserPromptSubmit:

bash

echo '{"session_id":"test","prompt":"your test prompt"}' | \
  npx tsx .claude/hooks/skill-activation-prompt.ts

Test PreToolUse:

bash

cat <<'EOF' | npx tsx .claude/hooks/skill-verification-guard.ts
{"session_id":"test","tool_name":"Edit","tool_input":{"file_path":"test.ts"}}
EOF

Step 4: Refine Patterns

Based on testing:

•Add missing keywords
•Refine intent patterns to reduce false positives
•Adjust file path patterns
•Test content patterns against actual files

Step 5: Follow Anthropic Best Practices

✅ Keep SKILL.md under 50