# Custom Agent Definitions

> Define and configure custom agents in Claude Code. Covers context forking, agent field specifications, and disallowedTools restrictions. Use when creating custom agent types, configuring agent isolation, or restricting agent capabilities.

- Skill: `majiayu000/custom-agent-definitions` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/custom-agent-definitions`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/custom-agent-definitions/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/custom-agent-definitions

---


# 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

```yaml
---
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**

```yaml
---
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 Task tool:

```yaml
---
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:

```yaml
---
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

```yaml
---
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

```
Task tool with subagent_type="security-auditor" for security analysis.
```

### Via Delegation

```bash
/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:
```yaml
# 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
```yaml
# Good: Isolated exploratory work
context: fork
```

### 3. Combine Allowed and Disallowed
```yaml
# Explicit whitelist with safety blacklist
allowed-tools: Bash, Read, Grep
disallowedTools: Write, Edit
```

### 4. Clear Agent Descriptions
```yaml
description: |
  Security auditor for identifying vulnerabilities in authentication
  and authorization code. Reports findings without modifying code.
```

### 5. Model Selection
| Use Case | Model |
|----------|-------|
| Quick analysis | haiku |
| Standard work | sonnet |
| Complex reasoning | opus |

## 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 |
| `allowed-tools` | list | Tools the agent can use |
| `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
```yaml
context: fork
allowed-tools: Read, Grep, Glob, WebSearch, WebFetch
disallowedTools: Bash, Write, Edit
```

### Safe Code Executor
```yaml
allowed-tools: Bash, Read
disallowedTools: Write, Edit
```

### Documentation Writer
```yaml
allowed-tools: Read, Write, Edit, Grep, Glob
disallowedTools: Bash
```

### Full-Power Developer
```yaml
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 |