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.

Rule Detail

Length 3-50 characters

Format Lowercase letters, numbers, hyphens only

Start/end Must be alphanumeric (not hyphen)

Convention Role-based: code-reviewer , test-generator , domain-expert

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:

• Triggering conditions ("Use this agent when...")

• 2-4 <example> blocks showing usage scenarios

• Each example: context, user request, assistant response, commentary

• 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.

model (required)

Value When to use

inherit

Default choice - uses parent session's model

sonnet

Balanced capability/speed

opus

Most capable, for complex reasoning

haiku

Fast/cheap, for simple validation

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

color (required)

Visual identifier in UI. Choose based on agent function:

Color Use for

blue / cyan

Analysis, review, research

green

Success-oriented, generation, creation

yellow

Caution, validation, checking

red

Critical, security, destructive operations

magenta

Creative, design, architecture

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

• Keep under 10,000 characters

• Include skill activation instructions if the agent should load skills

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

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

• Write agent with specific triggering examples

• Use similar phrasing to examples in your test queries

• Verify Claude loads the agent for matching requests

• Test that the agent follows its defined process

• Check output matches defined format

• Test edge cases mentioned in system prompt

Common Mistakes

Mistake Fix

Vague description without examples Add 2-4 <example> blocks with concrete user phrases

model: sonnet when inherit works Use inherit unless agent needs specific capability

Too many tools granted Restrict to minimum needed tools

Generic system prompt Be specific about process, output format, quality standards

No skill activation Add skill loading instructions for knowledge-dependent agents

Multiple agents in one plugin Use one expert agent with skills for different topics