Agent Development for Claude Code Plugins Skill

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.

Canonical agent frontmatter template (MANDATORY shape)

This is the template every new agent MUST follow. Deviating from this shape is the #1 cause of agents that never trigger.

---
name: my-agent                                    # REQUIRED: kebab-case, 3-50 chars, alphanumeric start/end
model: inherit                                    # REQUIRED: always `inherit` unless you have a hard reason
color: blue                                       # RECOMMENDED: one of blue/cyan/green/yellow/magenta/red
tools: Read, Write, Edit, Glob, Grep, Bash        # RECOMMENDED: minimal set; omit for full access
description: |
  One-sentence summary of what the agent does. PROACTIVELY activate for: (1) concrete trigger, (2) concrete trigger, ..., (N) concrete trigger. Provides: comma-separated capability nouns.

  <example>
  Context: Realistic situation where the agent should fire
  user: "A realistic user quote — the kind of thing someone would actually type"
  assistant: "Short 1-2 sentence response. Mention loading a specific skill if relevant."
  <commentary>Triggers for specific-keyword-1, specific-keyword-2, specific-keyword-3</commentary>
  </example>

  <example>
  Context: Another realistic situation covering a different capability
  user: "..."
  assistant: "..."
  <commentary>Triggers for ...</commentary>
  </example>

  <example>
  Context: A debugging / troubleshooting scenario
  user: "..."
  assistant: "..."
  <commentary>Triggers for ...</commentary>
  </example>

  <example>
  Context: A "when to pick this vs. that" scenario
  user: "..."
  assistant: "..."
  <commentary>Triggers for ...</commentary>
  </example>
---

You are [role] specializing in [domain]. [Lean orchestrator body — see "Lean Orchestrator Pattern" below.]

Hard rules for the frontmatter

name: is required. Do NOT use the deprecated agent: true flag — that pattern is legacy and results in an unnamed agent that cannot be referenced or routed to reliably. If you find agent: true in an existing file, replace it with name: <kebab-name-from-filename>.
model: inherit is required. Never hard-code a model unless the agent has a documented capability requirement.
description: MUST include the enumerated PROACTIVELY activate for: (1)... (2)... (N)... pattern AND a Provides: ... capability list. A description that only says "Use this agent for help with X" will not route reliably.
description: MUST include at least 4 <example> blocks (5-7 preferred for agents that back multiple skills). Every skill the agent delegates to MUST have at least one example that would route to it.
Use description: | (YAML block scalar) whenever the description spans multiple lines or contains <example> blocks. A folded scalar (>) or implicit flow scalar will mangle the examples.
Do NOT put cross-cutting boilerplate (Windows path rules, documentation policy, etc.) inside the YAML description: block. That text is used for routing-match, and boilerplate that appears in many agents poisons the signal. Put it in the markdown body under a clearly named ## Windows file path requirements section (or similar) instead.

Deprecated / broken patterns to migrate

| Broken pattern | What it does | Fix | |---|---|---| | agent: true (no name:) | Agent cannot be named/routed reliably | Replace with name: <kebab-name> | | description: without <example> blocks | Agent rarely triggers | Add 4-6 <example> blocks covering each skill | | description: with "Use this agent for X" prose only | Vague routing, poor trigger | Rewrite with PROACTIVELY activate for: (1)... enumeration | | Windows boilerplate inside YAML description: | Pollutes routing signal | Move to ## Windows file path requirements in body | | model: missing or hard-coded (e.g. model: sonnet) | Fails to inherit session model | Set model: inherit | | Single <example> block with full code in assistant: | Dilutes matching, bloats description | Keep assistant replies to 1-2 sentences; put code in skills |

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.

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

Skill coverage requirement (CRITICAL): When the agent delegates to skills, every skill MUST have at least one <example> block that would route to it. Count skills, count examples, and verify full coverage. If the plugin has 9 skills and only 7 trigger examples, 2 skills will have reduced activation reliability. Add examples until every skill has explicit coverage. When there are more skills than the 7-example limit allows, combine related skills into shared examples that mention both domains.

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
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

| Metric | Target | Hard Maximum | |--------|--------|-------------| | Word count | 1,500-2,500 words | 3,000 words | | Character count | ~10,000-15,000 chars | 20,000 chars |

What Belongs in the Agent Body

| Section | Required | Purpose | |---------|----------|---------| | Role identity | Yes | "You are [role] specializing in [domain]" | | Skill activation rules | Yes | Topic-to-skill mapping table | | High-level process | Yes | Design/workflow steps | | Output format | Yes | What to include in responses | | Brief service summaries | Optional | 2-3 sentences per area to help decide which skill to load | | Edge cases / troubleshooting tips | Optional | Quick reference only |

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:

| Element | Guideline | |---------|-----------| | Intro text | 1-2 sentences on when to trigger | | Example blocks | 3-7 blocks covering diverse scenarios | | Total description | Should fit naturally — focus on quality trigger examples over length |

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.

Preventing Trigger Phrase Overlap Between Skills

When a plugin has multiple skills, their trigger phrases and description terms must not create ambiguity. If two skills both claim the same keyword (e.g., both "programmatic-development" and "tmdl-mastery" claim "TMDL"), the agent cannot reliably route requests.

Disambiguation rules:

Audit trigger terms across all skills — list every trigger phrase from every skill description side by side. Flag any term that appears in more than one skill.
Assign exclusive ownership — each ambiguous term must belong to exactly one skill. The other skill should use a more specific phrase (e.g., "TMDL file editing" vs. "programmatic deployment using TMDL").
Add disambiguation hints to the agent's skill activation table — for terms that could route to multiple skills, add a clarifying note: "TMDL editing/syntax → tmdl-mastery; TMDL in deployment pipelines → programmatic-development".
Test with ambiguous queries — after writing descriptions, mentally test phrases like "help me with TMDL" and verify the routing is unambiguous.

Validation Checklist

Before finalizing an agent:

[ ] Name: 3-50 chars, lowercase, hyphens, starts/ends alphanumeric
[ ] Description: includes triggering conditions and 2-4 <example> blocks
[ ] Every skill has trigger coverage: count skills and verify each has at least one example that routes to it
[ ] No trigger phrase overlap: no ambiguous keyword claimed by multiple skills without disambiguation
[ ] 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 | | Skills without trigger examples | Every skill must have at least one example that routes to it — count and verify | | Trigger phrase overlap between skills | Audit all skill descriptions for shared keywords; assign exclusive ownership or add disambiguation | | 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 | | Example blocks with full code/JSON | Keep examples concise (1-2 sentence responses); code belongs in skills | | Same cross-cutting block in every skill | Put platform guidelines in agent body or one shared reference, not each SKILL.md |

Agent Skills: Agent Development for Claude Code Plugins

Install this agent skill to your local

Skill Files