Agent Development for Claude Code Plugins

Overview

Agents are autonomous subprocesses that handle complex, multi-step tasks independently. Each agent is a markdown file in the agents/ directory with YAML frontmatter defining its configuration and a markdown body serving as its system prompt.

Agent File Format

---
name: agent-identifier
description: |
  Use this agent when [triggering conditions].

  <example>
  Context: [Situation]
  user: "[User request]"
  assistant: "[How to respond and invoke agent]"
  <commentary>
  [Why this agent should trigger]
  </commentary>
  </example>

model: inherit
color: blue
tools:
  - Read
  - Write
  - Grep
---

System prompt body in second person ("You are...")

Frontmatter Fields Reference

name (required)

Agent identifier for namespacing and invocation.

Invalid names: ag (too short), -agent- (starts/ends with hyphen), my_agent (underscores)

description (required - most critical field)

Defines WHEN Claude should trigger this agent. Poor descriptions = agent never triggers.

Must include:

1. Triggering conditions ("Use this agent when...")
2. 2-4 <example> blocks showing usage scenarios
3. Each example: context, user request, assistant response, commentary
4. Both proactive and reactive triggering scenarios

Good description pattern:

description: |
  Use this agent when the user needs help with [domain]. Trigger for:
  - [Scenario 1]
  - [Scenario 2]
  - [Scenario 3]

  <example>
  Context: [Specific situation]
  user: "[What user says]"
  assistant: "[How Claude responds and invokes agent]"
  <commentary>
  [Why this is the right agent for this request]
  </commentary>
  </example>

Common mistake: Vague descriptions without examples. "Helps with code review" will rarely trigger. Include concrete examples with exact user phrases.

Example block rules:

• Keep example blocks concise — assistant response should be 1-2 sentences, not full code
• Limit to 3-7 example blocks total (more dilutes matching quality)
• Do NOT include full JSON schemas, code samples, or CLI output in examples
• Examples show when to trigger and how to respond, not the domain content itself

model (required)

Always use inherit unless the agent specifically needs a different capability level.

color (required)

Visual identifier in UI. Choose based on agent function:

Use distinct colors for different agents within the same plugin.

tools (optional)

Restrict agent to specific tools. Principle of least privilege - only grant what's needed.

# Read-only analysis
tools: ["Read", "Grep", "Glob"]

# Code generation
tools: ["Read", "Write", "Edit", "Grep", "Glob"]

# Full access (omit field entirely)
# tools: (not specified)

Common tool names: Read, Write, Edit, Grep, Glob, Bash, WebSearch, WebFetch, Skill, Agent

MCP tools use format: mcp__server-name__tool-name

System Prompt Design

The markdown body becomes the agent's system prompt. Write in second person ("You are...", "You will...").

Structure Template

You are [role] specializing in [domain].

## Core Responsibilities
1. [Primary responsibility]
2. [Secondary responsibility]

## Process
1. [Step one]
2. [Step two]
3. [Step three]

## Quality Standards
- [Standard 1]
- [Standard 2]

## Output Format
- [What to include]
- [How to structure results]

## Edge Cases
- [Situation]: [How to handle]

Best Practices

DO:

• Write in second person ("You are...", "You will...")
• Be specific about responsibilities and process steps
• Define output format clearly
• Address edge cases
• Include skill activation instructions if the agent should load skills
• Keep the agent body as a lean orchestrator (see below)

DON'T:

• Write in first person ("I am...", "I will...")
• Be vague or generic ("help with stuff")
• Skip process steps
• Leave output format undefined
• Omit quality standards
• Embed domain knowledge that belongs in skills (see below)

Lean Orchestrator Pattern (CRITICAL)

An agent body must be a lean orchestrator, NOT a domain knowledge dump. The agent delegates to skills for detailed knowledge.

Agent Body Size Limits

What Belongs in the Agent Body

What Does NOT Belong in the Agent Body

• Detailed domain knowledge — belongs in skills
• Complete CLI/API references — belongs in skill references/
• Full code examples — belongs in skill examples/
• Duplicated skill content — if it's in a skill, do NOT repeat it in the agent

Anti-Pattern: Content Duplication

NEVER duplicate content between the agent body and skills. This is the most common mistake and causes massive context bloat.

Bad: Agent body contains a full "Plugin.json Schema" section AND the plugin-master skill also contains it. Good: Agent body says "For plugin.json schema details, load plugin-master:plugin-master" and keeps only a 1-sentence summary.

Lean Orchestrator Template

You are [role] specializing in [domain].

## Skill Activation - CRITICAL
[Topic-to-skill mapping table — this is the heart of the agent]

## Core Responsibilities
[2-5 bullet points on what this agent does]

## Process
[5-7 step workflow for handling user requests]

## Quality Standards
[Brief checklist — 5-10 items]

## Output Format
[What to include in responses]

Description Size Limits

Agent descriptions should be concise and effective:

Agent Design Principles (2025)

Agent-First Plugin Design

• Primary plugin interface is ONE expert agent named {domain}-expert
• Plugin named docker-master → agent named docker-expert
• Only 0-2 slash commands for automation workflows
• Users interact conversationally, not through command menus

Single Responsibility

Each agent should have a clear, focused purpose. Don't create "do everything" agents. If a plugin needs multiple capabilities, use one expert agent that loads different skills based on context.

Skill Integration

Expert agents should load relevant skills before answering. Include skill activation instructions in the system prompt:

## Skill Activation
When the user asks about [topic], load `plugin-name:skill-name` before responding.

Validation Checklist

Before finalizing an agent:

• Name: 3-50 chars, lowercase, hyphens, starts/ends alphanumeric
• Description: includes triggering conditions and 2-4 <example> blocks
• Model: set to inherit (unless specific need)
• Color: appropriate for agent function
• Tools: restricted to minimum needed (or omitted for full access)
• System prompt: second person, clear responsibilities, defined process and output
• Frontmatter: valid YAML with all required fields
• File location: agents/agent-name.md

Testing

1. Write agent with specific triggering examples
2. Use similar phrasing to examples in your test queries
3. Verify Claude loads the agent for matching requests
4. Test that the agent follows its defined process
5. Check output matches defined format
6. Test edge cases mentioned in system prompt

Common Mistakes