Agent Development

# Agent Development for Claude Code Plugins

## Overview

Agents are autonomous subprocesses that handle complex, multi-step tasks independently. Understanding agent structure, triggering conditions, and system prompt design enables creating powerful autonomous capabilities.

**Key concepts:**
- Agents are FOR autonomous work, commands are FOR user-initiated actions
- Markdown file format with YAML frontmatter
- Triggering via description field with examples
- System prompt defines agent behavior
- Model and color customization

## Agent File Structure

### Complete Format

```markdown
---
name: agent-identifier
description: Use this agent when [triggering conditions]. Examples:

<example>
Context: [Situation description]
user: "[User request]"
assistant: "[How assistant should respond and use this agent]"
<commentary>
[Why this agent should be triggered]
</commentary>
</example>

<example>
[Additional example...]
</example>

model: inherit
color: blue
tools: ["Read", "Write", "Grep"]
---

You are [agent role description]...

**Your Core Responsibilities:**
1. [Responsibility 1]
2. [Responsibility 2]

**Analysis Process:**
[Step-by-step workflow]

**Output Format:**
[What to return]
```

## Frontmatter Fields

### name (required)

Agent identifier used for namespacing and invocation.

**Format:** lowercase, numbers, hyphens only
**Length:** 3-50 characters
**Pattern:** Must start and end with alphanumeric

**Good examples:**
- `code-reviewer`
- `test-generator`
- `api-docs-writer`
- `security-analyzer`

**Bad examples:**
- `helper` (too generic)
- `-agent-` (starts/ends with hyphen)
- `my_agent` (underscores not allowed)
- `ag` (too short, < 3 chars)

### description (required)

Defines when Claude should trigger this agent. **This is the most critical field.**

**Must include:**
1. Triggering conditions ("Use this agent when...")
2. Multiple `<example>` blocks showing usage
3. Context, user request, and assistant response in each example
4. `<commentary>` explaining why agent triggers

**Format:**
```
Use this agent when [conditions]. Examples:

<example>
Context: [Scenario description]
user: "[What user says]"
assistant: "[How Claude should respond]"
<commentary>
[Why this agent is appropriate]
</commentary>
</example>

[More examples...]
```

**Best practices:**
- Include 2-4 concrete examples
- Show proactive and reactive triggering
- Cover different phrasings of same intent
- Explain reasoning in commentary
- Be specific about when NOT to use the agent

### model (required)

Which model the agent should use.

**Options:**
- `inherit` - Use same model as parent (recommended)
- `sonnet` - Claude Sonnet (balanced)
- `opus` - Claude Opus (most capable, expensive)
- `haiku` - Claude Haiku (fast, cheap)

**Recommendation:** Use `inherit` unless agent needs specific model capabilities.

### color (required)

Visual identifier for agent in UI.

**Options:** `blue`, `cyan`, `green`, `yellow`, `magenta`, `red`

**Guidelines:**
- Choose distinct colors for different agents in same plugin
- Use consistent colors for similar agent types
- Blue/cyan: Analysis, review
- Green: Success-oriented tasks
- Yellow: Caution, validation
- Red: Critical, security
- Magenta: Creative, generation

### tools (optional)

Restrict agent to specific tools.

**Format:** Array of tool names

```yaml
tools: ["Read", "Write", "Grep", "Bash"]
```

**Default:** If omitted, agent has access to all tools

**Best practice:** Limit tools to minimum needed (principle of least privilege)

**Common tool sets:**
- Read-only analysis: `["Read", "Grep", "Glob"]`
- Code generation: `["Read", "Write", "Grep"]`
- Testing: `["Read", "Bash", "Grep"]`
- Full access: Omit field or use `["*"]`

## System Prompt Design

The markdown body becomes the agent's system prompt. Write in second person, addressing the agent directly.

### Structure

**Standard template:**
```markdown
You are [role] specializing in [domain].

**Your Core Responsibilities:**
1. [Primary responsibility]
2. [Secondary responsibility]
3. [Additional responsibilities...]

**Analysis Process:**
1. [Step one]
2. [Step two]
3. [Step three]
[...]

**Quality Standards:**
- [Standard 1]
- [Standard 2]

**Output Format:**
Provide results in this format:
- [What to include]
- [How to structure]

**Edge Cases:**
Handle these situations:
- [Edge case 1]: [How to handle]
- [Edge case 2]: [How to handle]
```

### Best Practices

✅ **DO:**
- Write in second person ("You are...", "You will...")
- Be specific about responsibilities
- Provide step-by-step process
- Define output format
- Include quality standards
- Address edge cases
- Keep under 10,000 characters

❌ **DON'T:**
- Write in first person ("I am...", "I will...")
- Be vague or generic
- Omit process steps
- Leave output format undefined
- Skip quality guidance
- Ignore error cases

## Creating Agents

### Method 1: AI-Assisted Generation

Use this prompt pattern (extracted from Claude Code):

```
Create an agent configuration based on this request: "[YOUR DESCRIPTION]"

Requirements:
1. Extract core intent and responsibilities
2. Design expert persona for the domain
3. Create comprehensive system prompt with:
   - Clear behavioral boundaries
   - Specific methodologies
   - Edge case handling
   - Output format
4. Create identifier (lowercase, hyphens, 3-50 chars)
5. Write description with triggering conditions
6. Include 2-3 <example> blocks showing when to use

Return JSON with:
{
  "identifier": "agent-name",
  "whenToUse": "Use this agent when... Examples: <example>...</example>",
  "systemPrompt": "You are..."
}
```

Then convert to agent file format with frontmatter.

See `examples/agent-creation-prompt.md` for complete template.

### Method 2: Manual Creation

1. Choose agent identifier (3-50 chars, lowercase, hyphens)
2. Write description with examples
3. Select model (usually `inherit`)
4. Choose color for visual identification
5. Define tools (if restricting access)
6. Write system prompt with structure above
7. Save as `agents/agent-name.md`

## Validation Rules

### Identifier Validation

```
✅ Valid: code-reviewer, test-gen, api-analyzer-v2
❌ Invalid: ag (too short), -start (starts with hyphen), my_agent (underscore)
```

**Rules:**
- 3-50 characters
- Lowercase letters, numbers, hyphens only
- Must start and end with alphanumeric
- No underscores, spaces, or special characters

### Description Validation

**Length:** 10-5,000 characters
**Must include:** Triggering conditions and examples
**Best:** 200-1,000 characters with 2-4 examples

### System Prompt Validation

**Length:** 20-10,000 characters
**Best:** 500-3,000 characters
**Structure:** Clear responsibilities, process, output format

## Agent Organization

### Plugin Agents Directory

```
plugin-name/
└── agents/
    ├── analyzer.md
    ├── reviewer.md
    └── generator.md
```

All `.md` files in `agents/` are auto-discovered.

### Namespacing

Agents are namespaced automatically:
- Single plugin: `agent-name`
- With subdirectories: `plugin:subdir:agent-name`

## Testing Agents

### Test Triggering

Create test scenarios to verify agent triggers correctly:

1. Write agent with specific triggering examples
2. Use similar phrasing to examples in test
3. Check Claude loads the agent
4. Verify agent provides expected functionality

### Test System Prompt

Ensure system prompt is complete:

1. Give agent typical task
2. Check it follows process steps
3. Verify output format is correct
4. Test edge cases mentioned in prompt
5. Confirm quality standards are met

## Quick Reference

### Minimal Agent

```markdown
---
name: simple-agent
description: Use this agent when... Examples: <example>...</example>
model: inherit
color: blue
---

You are an agent that [does X].

Process:
1. [Step 1]
2. [Step 2]

Output: [What to provide]
```

### Frontmatter Fields Summary

| Field | Required | Format | Example |
|-------|----------|--------|---------|
| name | Yes | lowercase-hyphens | code-reviewer |
| description | Yes | Text + examples | Use when... <example>... |
| model | Yes | inherit/sonnet/opus/haiku | inherit |
| color | Yes | Color name | blue |
| tools | No | Array of tool names | ["Read", "Grep"] |

### Best Practices

**DO:**
- ✅ Include 2-4 concrete examples in description
- ✅ Write specific triggering conditions
- ✅ Use `inherit` for model unless specific need
- ✅ Choose appropriate tools (least privilege)
- ✅ Write clear, structured system prompts
- ✅ Test agent triggering thoroughly

**DON'T:**
- ❌ Use generic descriptions without examples
- ❌ Omit triggering conditions
- ❌ Give all agents same color
- ❌ Grant unnecessary tool access
- ❌ Write vague system prompts
- ❌ Skip testing

## Additional Resources

### Reference Files

For detailed guidance, consult:

- **`references/system-prompt-design.md`** - Complete system prompt patterns
- **`references/triggering-examples.md`** - Example formats and best practices
- **`references/agent-creation-system-prompt.md`** - The exact prompt from Claude Code

### Example Files

Working examples in `examples/`:

- **`agent-creation-prompt.md`** - AI-assisted agent generation template
- **`complete-agent-examples.md`** - Full agent examples for different use cases

### Utility Scripts

Development tools in `scripts/`:

- **`validate-agent.sh`** - Validate agent file structure
- **`test-agent-trigger.sh`** - Test if agent triggers correctly

## Implementation Workflow

To create an agent for a plugin:

1. Define agent purpose and triggering conditions
2. Choose creation method (AI-assisted or manual)
3. Create `agents/agent-name.md` file
4. Write frontmatter with all required fields
5. Write system prompt following best practices
6. Include 2-4 triggering examples in description
7. Validate with `scripts/validate-agent.sh`
8. Test triggering with real scenarios
9. Document agent in plugin README

Focus on clear triggering conditions and comprehensive system prompts for autonomous operation.