custom-agent-definitions

What this skill does

# Custom Agent Definitions

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

## When to Use This Skill

| Use this skill when... | Use agent-teams instead when... |
|---|---|
| Authoring a new `.md` agent definition file in `.claude/agents/` | Spawning multiple already-defined agents that need to coordinate via TeamCreate |
| Configuring a single agent's `model`, `allowed-tools`, or `context: fork` | Setting up a lead/teammate architecture with a shared task list |
| Constraining tool access for a specialised read-only or write-restricted agent | Sequencing parallel work across worktrees (see parallel-agent-dispatch) |
| Writing the system prompt that defines what one agent does | Auditing existing agent definitions for security or completeness (see meta-audit) |

## 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 Agent 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

```
Agent 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 | Model ID |
|----------|-------|----------|
| Simple/mechanical tasks | haiku | claude-haiku-4-5 |
| Development workflows | sonnet | claude-sonnet-4-6 |
| Deep reasoning/analysis | opus | claude-opus-4-7 |

### 6. Report failures loudly
A dispatched agent that hits a wall must say so in its final message — never
a one-word summary like `Terminal.` / `Done.` / `Stopped.` On a blocker, the
agent should commit and push its in-progress work, open a draft PR, and state
exactly what stopped it and which tools were denied. A one-word surrender is
indistinguishable from success to the orchestrator, so the work is silently
cleaned up and lost (issue
[#1422](https://github.com/laurigates/claude-plugins/issues/1422)). See
`parallel-agent-dispatch` → "Loud-failure contract" for the dispatch-prompt
form every brief should carry.

## 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
```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