# Defining a Subagent — Advanced Agent Workflows and Skills

Source: https://www.geekswithgeeks.com/en/agent-workflows/sub-defining

> Write a subagent definition with a focused role, limited tools and a clear output format.

## A role, tools and a prompt

In Claude Code a subagent is a Markdown file with frontmatter (name, description, optional list of allowed tools) and a system prompt as its body. Giving it **only the tools it needs** is the main safety control: a reviewer that can read files cannot accidentally edit them.

## A read-only reviewer

Saved as `.claude/agents/code-reviewer.md`. The prompt fixes the output format so the main agent can use the result directly.

```markdown
---
name: code-reviewer
description: Reviews changed code for bugs and security issues. Use after finishing a feature.
tools: Read, Grep, Glob
---

You are a careful code reviewer. Read the changed files and report:

- **Bugs**: file, line, why it is wrong
- **Risks**: security or data-loss concerns
- **Verdict**: approve or request changes

Do not edit files. Keep the report under 300 words.
```

## Write the brief like a handoff

State the goal, the relevant paths, what was already tried and what to return. A vague brief like "look into the bug" produces a vague result.

**Quiz:** Why give a reviewer subagent only read tools?

- [ ] Read tools are faster to install
- [x] It cannot accidentally modify files
- [ ] Write tools do not exist
- [ ] The model refuses otherwise

*Answer:* It cannot accidentally modify files. Limiting tools limits the damage a mistake or a prompt injection can do.