---
name: claude-extensibility
description: "Claude Code extensibility: agents, skills, output styles. Capabilities: create/update/delete agents and skills, YAML frontmatter, system prompts, tool/model selection, resumable agents, CLI-defined agents. Actions: create, edit, delete, optimize, test extensions. Keywords: agent, skill, output-style, SKILL.md, subagent, Task tool, progressive disclosure. Use when: creating agents/skills, editing extensions, configuring tool access, choosing models, testing activation."
---

# Claude Code Extensibility

CRUD operations for agents, skills, and output styles following Anthropic best practices.

## Related Skills

**IMPORTANT:** When creating or editing prompts, use `prompt-enhancer` skill to improve quality.

```
Skill("prompt-enhancer")  → Enhance skill/agent prompt content
```

## Core Principles

- **Simplicity**: Direct tool calls, avoid complex abstractions
- **Focus**: Single, clear responsibility per extension
- **Conciseness**: Target <500 lines, use progressive disclosure
- **Efficiency**: Optimize for token usage and response time

## Extension Types

| Type | Invocation | Purpose | Location |
|------|------------|---------|----------|
| **Agents** | Task tool | Specialized sub-processes | `.claude/agents/` |
| **Skills** | Model-invoked (autonomous) | Domain knowledge | `.claude/skills/{name}/` |
| **Output Styles** | `/output-style` command | Modify main agent behavior | `.claude/output-styles/` |

## Agent Development

**Reference:** `references/agent-development.md` - Full YAML structure, model/tool selection, system prompt patterns, optimization techniques.

### Quick Start: Agent

```yaml
---
name: agent-name
description: Use this agent when [use case]. Use PROACTIVELY for [triggers].\n\nExamples:\n<example>\nContext: [situation]\nuser: [request]\nassistant: [response]\n<commentary>[reasoning]</commentary>\n</example>
tools: Grep, Glob, Read, Bash
model: haiku
permissionMode: default
skills: skill-name
---

# Agent Name

Brief mission statement.

## Core Strategy

### 1. Phase Name
Approach and techniques

<format>
Expected output structure
</format>
```

### YAML Fields

| Field | Required | Description |
|-------|----------|-------------|
| `name` | Yes | Lowercase, hyphens (e.g., `code-reviewer`) |
| `description` | Yes | Single line with `\n` for newlines, include examples |
| `tools` | No | Comma-separated; inherits all if omitted |
| `model` | No | `haiku`, `sonnet`, `opus`, `inherit` (default: sonnet) |
| `permissionMode` | No | `default`, `acceptEdits`, `bypassPermissions`, `plan`, `ignore` |
| `skills` | No | Comma-separated skill names to auto-load |

### Model Selection

| Model | Use When | Target Time |
|-------|----------|-------------|
| `haiku` | Fast tasks, exploration, search | < 3s |
| `sonnet` | Balanced, most use cases | < 10s |
| `opus` | Complex reasoning, architecture | < 30s |
| `inherit` | Match main conversation model | varies |

### Built-in Subagents

| Agent | Model | Tools | Purpose |
|-------|-------|-------|---------|
| `general-purpose` | Sonnet | All | Complex research, multi-step operations |
| `plan` | Sonnet | Read, Glob, Grep, Bash | Research in plan mode |
| `Explore` | Haiku | Read-only | Fast codebase search (quick/medium/very thorough) |

### Agent Locations

| Location | Scope | Priority |
|----------|-------|----------|
| `.claude/agents/` | Project | Highest |
| `~/.claude/agents/` | User (all projects) | Lower |
| Plugin `agents/` | Plugin-specific | Varies |
| `--agents` CLI flag | Session only | Medium |

### CLI-Defined Agents

```bash
claude --agents '{
  "code-reviewer": {
    "description": "Expert code reviewer. Use proactively after code changes.",
    "prompt": "You are a senior code reviewer...",
    "tools": ["Read", "Grep", "Glob", "Bash"],
    "model": "sonnet"
  }
}'
```

### Resumable Agents

Continue previous conversations:
- Each execution gets unique `agentId`
- Transcript stored in `agent-{agentId}.jsonl`
- Resume with previous `agentId` to continue with full context

## Skill Development

**Reference:** `references/skill-development.md` - Full structure, trigger patterns, hook system.

### Quick Start: Skill

```yaml
---
name: skill-name
description: "[What it does]. [Technologies]. Capabilities: [list]. Actions: [verbs]. Keywords: [triggers]. Use when: [scenarios]."
allowed-tools: Read, Grep, Glob
---

# Skill Name

## Purpose
What this skill helps with

## When to Use
Specific scenarios and conditions

## Key Information
Guidance, patterns, examples
```

### YAML Fields

| Field | Required | Description |
|-------|----------|-------------|
| `name` | Yes | Lowercase, hyphens, max 64 chars |
| `description` | Yes | WHAT + WHEN format, max 1024 chars, quoted |
| `allowed-tools` | No | Restrict tool access (security) |

### Description Format (WHAT + WHEN)

**Structure:**
```
"[Core purpose]. [Technologies/Stack]. Capabilities: [list]. Actions: [verbs]. Keywords: [triggers]. Use when: [scenarios]."
```

**Good example:**
```yaml
description: "Extract text and tables from PDF files, fill forms, merge documents. Formats: .pdf. Tools: pypdf, pdfplumber. Capabilities: text extraction, form filling, document merging. Actions: extract, fill, merge PDFs. Keywords: PDF, form, document, pypdf, pdfplumber. Use when: working with PDF files, extracting data from documents, filling PDF forms."
```

**Bad examples:**
```yaml
description: Helps with documents  # Too vague
description: PDF skill  # Missing WHEN triggers
```

### Tool Access Control

Restrict Claude's tools with `allowed-tools`:

```yaml
---
name: safe-reader
description: "Read-only file access. Use when viewing code without modifications."
allowed-tools: Read, Grep, Glob
---
```

### Skill Locations

| Location | Scope |
|----------|-------|
| `.claude/skills/{name}/SKILL.md` | Project (shared via git) |
| `~/.claude/skills/{name}/SKILL.md` | User (all projects) |
| Plugin `skills/` | Plugin-bundled |

### Skill Structure

```
my-skill/
├── SKILL.md (required)
├── references/ (optional - detailed docs)
├── scripts/ (optional - utilities)
└── templates/ (optional - templates)
```

## Output Styles

Modify Claude Code's main agent behavior.

### Quick Start: Output Style

```markdown
---
name: My Custom Style
description: Brief description of behavior
keep-coding-instructions: true
---

# Custom Style Instructions

You are an interactive CLI tool that helps users...

## Specific Behaviors
[Define assistant behavior...]
```

### YAML Fields

| Field | Purpose | Default |
|-------|---------|---------|
| `name` | Display name | Filename |
| `description` | UI description | None |
| `keep-coding-instructions` | Retain coding instructions | `false` |

### Built-in Styles

- **Default**: Standard software engineering
- **Explanatory**: Educational insights between tasks
- **Learning**: Collaborative with `TODO(human)` markers

### Output Style Locations

- User: `~/.claude/output-styles/`
- Project: `.claude/output-styles/`

### Usage

```bash
/output-style              # Access menu
/output-style explanatory  # Switch directly
```

## Testing

### Key Question: Does it activate when expected?

**Agent Testing:**
```bash
Task(
  subagent_type="agent-name",
  description="Test task",
  prompt="Detailed test prompt"
)
```

**Skill Testing:**
- Test prompts that SHOULD trigger
- Test prompts that should NOT trigger
- Debug with: `claude --debug`

## Common Workflows

### Create Agent

1. Create `.claude/agents/{name}.md`
2. Write YAML frontmatter (name, description, tools, model)
3. Write system prompt (<500 lines)
4. Test with Task tool
5. Optimize based on performance

### Create Skill

1. Create `.claude/skills/{name}/SKILL.md`
2. Write YAML frontmatter with WHAT + WHEN description
3. Write content (<500 lines)
4. **Use `Skill("prompt-enhancer")` to improve prompt**
5. Add reference files for detailed content
6. Test: Does it activate when expected?

### Optimize Extension

1. Measure baseline (lines, token usage, response time)
2. Move details to reference files
3. **Use `Skill("prompt-enhancer")` to improve prompts**
4. Remove second-person voice
5. Use code blocks over prose
6. Add XML structure
7. Test and verify improvements

## Best Practices

### Anthropic Guidelines

✅ **500-line rule**: Keep SKILL.md and agent prompts under 500 lines
✅ **Progressive disclosure**: Use reference files for detailed content
✅ **Proactive language**: Include "use PROACTIVELY" in descriptions
✅ **WHAT + WHEN descriptions**: Both capability and triggers
✅ **Test first**: Build 3+ evaluations before extensive documentation
✅ **Least privilege**: Limit tools to necessary set

### Anti-Patterns

❌ Vague descriptions without triggers
❌ Over 500 lines without references
❌ Second-person voice ("you should...")
❌ All tools when subset suffices
❌ No examples in agent descriptions

## Quick Reference

**Agent Model Selection:**
- Haiku: Fast, simple tasks (< 3s)
- Sonnet: Balanced, most use cases (< 10s)
- Opus: Complex reasoning (< 30s)
- Inherit: Match main conversation

**File Locations:**
- Agents: `.claude/agents/*.md`
- Skills: `.claude/skills/{name}/SKILL.md`
- Output Styles: `.claude/output-styles/*.md`

**Management Commands:**
- `/agents` - Interactive agent management
- `/output-style` - Switch output styles

---

**Status**: Production Ready | **Lines**: ~200 | **Progressive Disclosure**: ✅