Subagents and hooks

Two more tools for bigger jobs. A subagent is a helper Claude can hand a task to. A hook is a rule that runs every time, whether Claude remembers it or not.

Subagents. A subagent is a separate Claude with its own context window, its own instructions and, if you like, fewer tools. Claude hands it a job ("search the codebase for every place we format dates") and gets back a short answer instead of pages of search results. Use one to:

• run independent jobs side by side;

• keep noisy output, like a big search or a long log, out of your way;

• review code it didn't write, with fresh eyes.

Each one costs extra tokens, and handing a job to a subagent doesn't make the answer smarter by itself. The fresh eyes are what help a review.

A subagent is a markdown file in .claude/agents/ (or ~/.claude/agents/ for every project). Claude uses it when a task matches its description, or when you ask for it by name.

Hooks. A rule in CLAUDE.md is advice: Claude usually follows it. Anthropic's guide is blunt that a prompt can't guarantee anything. When something must always (or never) happen, use a hook: a shell command Claude Code runs at a set moment, like after every file edit or before every command.

Hooks live in your settings file, .claude/settings.json. This one, from Claude Code's docs, formats every file Claude edits with Prettier:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | xargs npx prettier --write"
          }
        ]
      }
    ]
  }
}

For a plain "never run this command", a deny rule in the same settings file does the job with no script: "permissions": { "deny": ["Bash(npm *)"] }. Use a hook when you need logic or a custom message.

A hook runs real commands on your machine with your permissions, so treat one you didn't write like any other script: read it first.

Your turn. Write a reviewer subagent in your own words: a description that says when to use it, read-only tools, and instructions. Then, after the file, hand it this function to review: function isOpen(hour) { return hour > 7 && hour < 18 }. Moon Café opens at 7:00 and closes at 18:00. Claude plays your subagent here.