skill-developer · diet103 — Claude Code skill: what it does, how to install

---
name: skill-developer
description: Create and manage Claude Code skills following Anthropic best practices. Use when creating new skills, modifying skill-rules.json, understanding trigger patterns, working with hooks, debugging skill activation, or implementing progressive disclosure. Covers skill structure, YAML frontmatter, trigger types (keywords, intent patterns, file paths, content patterns), enforcement levels (block, suggest, warn), hook mechanisms (UserPromptSubmit, PreToolUse), session tracking, and the 500-line rule.
---

# 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

### Hook Architecture (five wired hooks)

**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 (regex or optional AI classification)
- **Method**: Injects formatted reminder as context (stdout → Claude's input)
- **Enforcement**: Only `enforcement: "block"` skills are marked MANDATORY; `suggest`/`warn` skills always appear as recommendations

**2. PreToolUse Hook - Skill Verification Guard** (Enforcement)
- **File**: `.claude/hooks/skill-verification-guard.ts`
- **Trigger**: BEFORE Edit/MultiEdit/Write executes
- **Purpose**: Enforce mandatory skill activation (two-try model) and guardrail file/content patterns; optional AI edit analysis
- **Method**: Exit code 2 + stderr blocks the tool; only `enforcement: "block"` skills can block

**3. PostToolUse Hook - Edit Tracker**
- **File**: `.claude/hooks/post-tool-use-tracker.sh`
- **Trigger**: AFTER Edit/MultiEdit/Write completes
- **Purpose**: Track edited files and repos (feeds build checks and the error-handling reminder)

**4. PostToolUse Hook - Skill Activation Tracker**
- **File**: `.claude/hooks/skill-activation-tracker.ts`
- **Trigger**: AFTER the Skill tool runs
- **Purpose**: Clear the activated skill from pending lists so the guard stops blocking

**5. Stop Hook - Session Doc Updater**
- **File**: `.claude/hooks/session-doc-updater.ts`
- **Trigger**: AFTER Claude finishes responding
- **Purpose**: Index dev docs for vector search (Session Intelligence); prunes stale session state

**Optional (shipped but not wired):** `error-handling-reminder.ts` (Stop) — a gentle post-response self-check that analyzes edited files for risky patterns instead of blocking edits.

**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:**
- `frontend-dev-guidelines` - Enforce React/TypeScript patterns (the shipped `block` guardrail)
- A `database-verification` skill verifying table/column names before Prisma queries (hypothetical)

**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](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 500 lines
✅ Use progressive disclosure with reference files
✅ Add table of contents to reference files > 100 lines
✅ Write detailed description with trigger keywords
✅ Test with 3+ real scenarios before documenting
✅ Iterate based on actual usage

---

## Enforcement Levels

### BLOCK (Critical Guardrails)

- Physically prevents Edit/Write tool execution
- Exit code 2 from hook, stderr → Claude
- Claude sees message and must use skill to proceed
- **Use For**: Critical mistakes, data integrity, security issues
- Only `enforcement: "block"` skills can hard-block — in both the UserPromptSubmit mandatory list and the PreToolUse guardrail checks. `suggest`/`warn` matches are always advisory.

**Example:** Database column name verification

### SUGGEST (Recommended)

- Reminder injected before Claude sees prompt
- Claude is aware of relevant skills
- Not enforced, just advisory
- **Use For**: Domain guidance, best practices, how-to guides

**Example:** Frontend development guidelines

### WARN (Optional)

- Low priority suggestions
- Advisory only, minimal enforcement
- **Use For**: Nice-to-have suggestions, informational reminders

**Rarely used** - most skills are either BLOCK or SUGGEST.

---

## Skip Conditions & User Control

### 1. Session Tracking

**Purpose:** Don't nag repeatedly in same session

**How it works:**
- First edit → Hook blocks, updates session state
- Second edit (same session) → Hook allows
- Different session → Blocks again

**State File:** `.claude/hooks/state/skills-used-{session_id}.json`

### 2. File Markers

**Purpose:** Permanent skip for verified files

**Marker:** `// @skip-validation`

**Usage:**
```typescript
// @skip-validation
import { PrismaService } from './prisma';
// This file has been manually verified
```

**NOTE:** Use sparingly - defeats the purpose if overused

### 3. Environment Variables

**Purpose:** Emergency disable, temporary override

**Global disable:**
```bash
export SKIP_MANDATORY_SKILLS=true   # Disables mandatory-skill blocking
export SKIP_PRETOOLUSE_AI=true      # Disables AI edit analysis in the guard
```

**Skill-specific** (via `skipConditions.envOverride` in skill-rules.json):
```bash
export SKIP_FRONTEND_GUIDELINES=true
export SKIP_ERROR_REMINDER=true     # Optional Stop-hook reminder
```

---

## Testing Checklist

When creating a new skill, verify:

- [ ] Skill file created in `.claude/skills/{name}/SKILL.md`
- [ ] Proper frontmatter with name and description
- [ ] Entry added to `skill-rules.json`
- [ ] Keywords tested with real prompts
- [ ] Intent patterns tested with variations
- [ ] File path patterns tested with actual files
- [ ] Content patterns tested against file contents
- [ ] Block message is clear and actionable (if guardrail)
- [ ] Skip conditions configured appropriately
- [ ] Priority level matches importance
- [ ] No false positives in testing
- [ ] No false negatives in testing
- [ ] Performance is acceptable (<100ms or <200ms)
- [ ] JSON syntax validated: `jq . skill-rules.json`
- [ ] **SKILL.md under 500 lines** ⭐
- [ ] Reference files created if needed
- [ ] Table of contents added to files > 100 lines

---

## Reference Files

For detailed information on specific topics, see:

### [TRIGGER_TYPES.md](TRIGGER_TYPES.md)
Complete guide to all trigger types:
- Keyword triggers (explicit topic matching)
- Intent patterns (implicit action detection)
- File path triggers (glob patterns)
- Content patterns (regex in files)
- Best practices and examples for each
- Common pitfalls and testing strategies

### [SKILL_RULES_REFERENCE.md](SKILL_RULES_REFERENCE.md)
Complete skill-rules.json schema:
- Full TypeScript interface definitions
- Field-by-field explanations
- Complete guardrail skill example
- Complete domain skill example
- Validation guide and common errors

### [HOOK_MECHANISMS.md](HOOK_MECHANISMS.md)
Deep dive into hook internals:
- UserPromptSubmit flow (detailed)
- PreToolUse flow (detailed)
- Exit code behavior table (CRITICAL)
- Session state management
- Performance considerations

### [TROUBLESHOOTING.md](TROUBLESHOOTING.md)
Comprehensive debugging guide:
- Skill not triggering (UserPromptSubmit)
- PreToolUse not blocking
- False positives (too many triggers)
- Hook not executing at all
- Performance issues

### [PATTERNS_LIBRARY.md](PATTERNS_LIBRARY.md)
Ready-to-use pattern collection:
- Intent pattern library (regex)
- File path pattern library (glob)
- Content pattern library (regex)
- Organized by use case
- Copy-paste ready

### [ADVANCED.md](ADVANCED.md)
Future enhancements and ideas:
- Dynamic rule updates
- Skill dependencies
- Conditional enforcement
- Skill analytics
- Skill versioning

---

## Quick Reference Summary

### Create New Skill (5 Steps)

1. Create `.claude/skills/{name}/SKILL.md` with frontmatter
2. Add entry to `.claude/skills/skill-rules.json`
3. Test with `npx tsx` commands
4. Refine patterns based on testing
5. Keep SKILL.md under 500 lines

### Trigger Types

- **Keywords**: Explicit topic mentions
- **Intent**: Implicit action detection
- **File Paths**: Location-based activation
- **Content**: Technology-specific detection

See [TRIGGER_TYPES.md](TRIGGER_TYPES.md) for complete details.

### Enforcement

- **BLOCK**: Exit code 2, critical only
- **SUGGEST**: Inject context, most common
- **WARN**: Advisory, rarely used

### Skip Conditions

- **Session tracking**: Automatic (prevents repeated nags)
- **File markers**: `// @skip-validation` (permanent skip)
- **Env vars**: `SKIP_MANDATORY_SKILLS`, per-skill `envOverride` (emergency disable)

### Anthropic Best Practices

✅ **500-line rule**: Keep SKILL.md under 500 lines
✅ **Progressive disclosure**: Use reference files for details
✅ **Table of contents**: Add to reference files > 100 lines
✅ **One level deep**: Don't nest references deeply
✅ **Rich descriptions**: Include all trigger keywords (max 1024 chars)
✅ **Test first**: Build 3+ evaluations before extensive documentation
✅ **Gerund naming**: Prefer verb + -ing (e.g., "processing-pdfs")

### Troubleshoot

Test hooks manually:
```bash
# UserPromptSubmit
echo '{"prompt":"test"}' | npx tsx .claude/hooks/skill-activation-prompt.ts

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

See [TROUBLESHOOTING.md](TROUBLESHOOTING.md) for complete debugging guide.

---

## Related Files

**Configuration:**
- `.claude/skills/skill-rules.json` - Master configuration
- `.claude/hooks/state/` - Session tracking
- `.claude/settings.json` - Hook registration

**Hooks:**
- `.claude/hooks/skill-activation-prompt.ts` - UserPromptSubmit
- `.claude/hooks/skill-verification-guard.ts` - PreToolUse (Edit|MultiEdit|Write)
- `.claude/hooks/post-tool-use-tracker.sh` - PostToolUse (Edit|MultiEdit|Write)
- `.claude/hooks/skill-activation-tracker.ts` - PostToolUse (Skill)
- `.claude/hooks/session-doc-updater.ts` - Stop
- `.claude/hooks/error-handling-reminder.ts` - Stop (optional, not wired by default)

**All Skills:**
- `.claude/skills/*/SKILL.md` - Skill content files

---

**Skill Status**: COMPLETE - Restructured following Anthropic best practices ✅
**Line Count**: < 500 (following 500-line rule) ✅
**Progressive Disclosure**: Reference files for detailed information ✅

**Next**: Create more skills, refine patterns based on usage