Custom Agent Definitions

Expert knowledge for defining and configuring custom agents in Claude Code.

Core Concepts

Custom Agents allow you to define specialized agent types beyond the built-in ones (Explore, Plan, Bash, etc.). Each custom agent can have its own model, tools, and context configuration.

Agent Definition Schema

Custom agents are defined in .claude/agents/ or via plugin agent directories.

Basic Structure

---
name: my-custom-agent
description: What this agent does
model: sonnet
allowed-tools: Bash, Read, Grep, Glob
---

# Agent System Prompt

Instructions and context for the agent...

Context Forking

The context field controls how the agent's context relates to the parent conversation:

Value	Behavior
fork	Creates an independent context copy - agent sees parent history but changes don't affect parent
(default)	Agent shares context with parent and can see/modify conversation state

Example: Isolated Research Agent

---
name: research-agent
description: Research questions without modifying main context
model: sonnet
context: fork
allowed-tools: WebSearch, WebFetch, Read
---

# Research Agent

You are a research specialist. Search for information and provide findings.
Your research doesn't affect the main conversation context.

When to use context: fork:

• Exploratory research that shouldn't pollute main context
• Parallel investigations with potentially conflicting approaches
• Isolated experiments or testing
• Background tasks that run independently

Agent Field for Delegation

The agent field specifies which agent type to use when delegating via the Agent tool:

---
name: code-review-workflow
description: Comprehensive code review
agent: security-auditor
allowed-tools: Read, Grep, Glob, TodoWrite
---

This allows commands and skills to specify a preferred agent type for delegation.

Disallowed Tools (Restrictions)

The disallowedTools field explicitly prevents an agent from using certain tools:

---
name: read-only-explorer
description: Explore codebase without modifications
model: haiku
allowed-tools: Bash, Read, Grep, Glob
disallowedTools: Write, Edit, NotebookEdit
---

# Read-Only Explorer

Explore and analyze code. Do not make any modifications.

Disallowed Tools vs Allowed Tools:

Field	Purpose	Behavior
allowed-tools	Whitelist of permitted tools	Agent can ONLY use these tools
disallowedTools	Blacklist of forbidden tools	Agent can use all tools EXCEPT these

When to use disallowedTools:

• Creating read-only agents that can explore but not modify
• Restricting dangerous capabilities (Bash execution)
• Sandboxing agents for specific tasks
• Security-sensitive contexts

Complete Example

---
name: security-auditor
description: Security-focused code review agent
model: sonnet
context: fork
allowed-tools: Read, Grep, Glob, WebSearch, TodoWrite
disallowedTools: Bash, Write, Edit
created: 2026-01-20
modified: 2026-01-20
reviewed: 2026-01-20
---

# Security Auditor Agent

You are a security auditor. Analyze code for vulnerabilities.

## Capabilities
- Read and analyze source code
- Search for security patterns
- Research known vulnerabilities
- Track findings in todo list

## Restrictions
- Cannot execute code (no Bash)
- Cannot modify files (no Write/Edit)
- Work in isolated context

## Focus Areas
1. SQL injection vulnerabilities
2. XSS vulnerabilities
3. Authentication/authorization flaws
4. Secrets/credentials in code
5. Insecure dependencies

Defining Agents in Plugins

Plugins can define custom agents in their agents/ directory:

my-plugin/
├── .claude-plugin/
│   └── plugin.json
├── agents/
│   ├── security-auditor.md
│   ├── performance-analyzer.md
│   └── accessibility-checker.md
└── skills/
    └── ...

Each agent file follows the same YAML frontmatter + markdown body structure.

Using Custom Agents

Via Task Tool

Agent tool with subagent_type="security-auditor" for security analysis.

Via Delegation

/delegate Audit auth module for security issues

The delegation system matches tasks to appropriate custom agents.

Best Practices

1. Principle of Least Privilege

Only grant tools the agent actually needs:

# Good: Minimal tools for the task
allowed-tools: Read, Grep, Glob

# Avoid: Overly permissive
allowed-tools: Bash, Read, Write, Edit, Grep, Glob, WebSearch, WebFetch

2. Use Context Forking for Isolation

# Good: Isolated exploratory work
context: fork

3. Combine Allowed and Disallowed

# Explicit whitelist with safety blacklist
allowed-tools: Bash, Read, Grep
disallowedTools: Write, Edit

4. Clear Agent Descriptions

description: |
  Security auditor for identifying vulnerabilities in authentication
  and authorization code. Reports findings without modifying code.

5. Model Selection

Use Case	Model	Model ID
Simple/mechanical tasks	haiku	claude-haiku-4-5
Development workflows	sonnet	claude-sonnet-4-6
Deep reasoning/analysis	opus	claude-opus-4-6

Agent Configuration Fields Reference

Field	Type	Description
name	string	Agent identifier
description	string	What the agent does
model	string	Model to use (haiku, sonnet, opus)
context	string	Context mode: fork or default
permissionMode	string	default, acceptEdits, dontAsk, bypassPermissions, or plan
maxTurns	number	Maximum agentic turns before agent stops
background	bool	Set true to always run as a background task
memory	string	Persistent memory scope: user, project, or local
skills	list	Skill names to preload into agent context at startup
mcpServers	list	MCP server names available to this agent
tools	list	Tools the agent can use (in agents/ dir; use allowed-tools in skills)
disallowedTools	list	Tools the agent cannot use
created	date	Creation date
modified	date	Last modification date
reviewed	date	Last review date

Common Patterns

Read-Only Research Agent

context: fork
allowed-tools: Read, Grep, Glob, WebSearch, WebFetch
disallowedTools: Bash, Write, Edit

Safe Code Executor

allowed-tools: Bash, Read
disallowedTools: Write, Edit

Documentation Writer

allowed-tools: Read, Write, Edit, Grep, Glob
disallowedTools: Bash

Full-Power Developer

allowed-tools: Bash, Read, Write, Edit, Grep, Glob, TodoWrite

Agentic Optimizations

Context	Configuration
Exploratory research	context: fork, model: haiku
Security analysis	context: fork, disallowedTools: Bash, Write, Edit
Quick lookups	model: haiku, minimal tools
Complex implementation	model: sonnet, full tools

Quick Reference

Context Modes

Mode	Isolation	Use Case
(default)	Shared	Normal workflows
fork	Isolated	Research, experiments

Tool Restriction Patterns

Pattern	Fields
Whitelist only	allowed-tools: Tool1, Tool2
Blacklist only	disallowedTools: Tool1, Tool2
Combined	Both fields specified