Claude Code in Action · lesson 5 of 6

Claude Code Hooks

Shell commands that fire on lifecycle events. The harness — not the model — runs them. This is how you build deterministic guardrails Claude can't ignore.

Event types

Event	Fires when
`UserPromptSubmit`	User submits a prompt
`PreToolUse`	Before a tool runs (can block)
`PostToolUse`	After a tool runs
`Stop`	Main agent finishes a turn
`SubagentStop`	A subagent finishes
`Notification`	Display-only notifications
`SessionStart`	New session begins

Configuration

In settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {"type": "command", "command": "pnpm lint --fix"}
        ]
      }
    ],
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {"type": "command", "command": "./scripts/check-bash.sh"}
        ]
      }
    ]
  }
}

Hooks receive a JSON payload over stdin with event details. They can:

Print to stdout — shown to the user.
Exit code 0 — allow.
Exit code 1 (UserPromptSubmit-blocking) or 2 (PreToolUse-blocking) — block the action; stdout is sent to Claude as the reason.

Typical uses

Auto-format / auto-lint on Edit|Write.
Run tests on Stop (catches regressions before turn ends).
Block dangerous bash (rm -rf /, etc.) on PreToolUse.
Notify Slack on Stop when CI builds.
Audit log every tool call.

Hook vs permission

Permission = static yes/no based on the call.
Hook = dynamic — can inspect actual command, run other tools, format reasons.

Use permissions for blanket policies, hooks for context-dependent checks.

CCA-F angle