Agent Development for Claude Code Plugins

Overview

Agents are autonomous subprocesses that handle complex, multi-step tasks independently. Master agent structure, triggering conditions, and system prompt design to create 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

Important - Field Name Difference: Agents use tools to restrict tool access. Skills use allowed-tools for the same purpose. Don't confuse these when switching between component types.

Note on Official Documentation: The color field documented in this skill is supported by Claude Code and is generated by the built-in /agents command, but it is not yet reflected in the official sub-agents documentation. See anthropics/claude-code#8501 for tracking. The plugins-reference.md may show an older agent format using a capabilities field; for Claude Code plugins, prefer the structure documented in this skill which uses tools for tool restrictions.

Quick Start

Minimal working agent (copy-paste ready):

---
name: my-reviewer
description: Use this agent when the user asks to review code. Examples:

<example>
Context: User wrote new code
user: "Review my changes"
assistant: "I'll use the my-reviewer agent to analyze the code."
<commentary>
Code review request triggers the agent.
</commentary>
</example>

model: inherit
color: blue
---

You are a code reviewer. Analyze code for issues and provide feedback.

**Process:**
1. Read the code
2. Identify issues
3. Provide recommendations

**Output:** Summary with file:line references for each finding.

For complete format with all options, see Agent File Structure.

When to Use Agents vs Commands vs Skills

Component	Best For	Triggering	Example Use Case
Agents	Autonomous multi-step tasks	Proactive or description-matched	Code review after implementation
Commands	User-initiated actions	Explicit /command invocation	/deploy production
Skills	Knowledge and guidance	Model-invoked based on context	Domain expertise for PDF processing

See also: For command development, load the command-development skill. For skill development, load the skill-development skill.

Choose Agents When

• Task requires autonomous, multi-step execution
• Proactive triggering after certain events is desired
• Specialized subprocess with focused tools needed
• Work should happen in the background or as a subagent

Choose Commands When

• User should explicitly trigger the action
• Task has clear start/end with specific inputs
• Action should not happen automatically
• Workflow requires user confirmation at each step

For command development guidance, see the command-development skill.

Choose Skills When

• Providing knowledge or procedural guidance
• Extending Claude's domain expertise
• No autonomous execution needed
• Information should be available contextually on-demand

For skill development guidance, see the skill-development skill.

Agent File Structure

Complete Format

---
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)

When to choose:

• haiku - Fast, simple tasks; quick analysis; cost-sensitive operations
• sonnet - Balanced performance; most use cases (default recommendation)
• opus - Complex reasoning; detailed analysis; highest capability needed

Recommendation: Use inherit (recommended default) unless the agent specifically needs:

• haiku for fast, cost-sensitive operations
• opus for complex reasoning requiring maximum capability

color (required)

Visual identifier for agent in UI.

Note: This field is supported by Claude Code but not yet in official documentation. See the Overview note for details.

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: Comma-separated tool names

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 entirely

Important: Agents use tools while Skills use allowed-tools. The field names differ between component types. For skill tool restrictions, see the skill-development skill.

System Prompt Design

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

Structure

Standard template:

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

---
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	Comma-separated tool names	Read, Grep

Note: Agents use tools to restrict tool access. Skills use allowed-tools for the same purpose. The field names differ between component types.

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 - Four system prompt patterns (Analysis, Generation, Validation, Orchestration) with complete templates and common pitfalls
• references/triggering-examples.md - Example block anatomy, four example types, template library, and debugging guide
• references/agent-creation-system-prompt.md - The exact prompt used by Claude Code's agent generation feature with usage patterns

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/:

• create-agent-skeleton.sh - Generate new agent file from template
• 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 agent file using skeleton: ./skills/agent-development/scripts/create-agent-skeleton.sh agent-name agents/
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 ./skills/agent-development/scripts/validate-agent.sh agents/your-agent.md
8. Test triggering with real scenarios
9. Document agent in plugin README

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