Hooks

Hooks are scripts the agent runtime runs automatically at points in a session, such as before a tool runs, after a file is edited, or when a session ends. Because the runtime runs them, not the model, they fire every time rather than when the agent remembers to.

In Claude Code, hooks live in .claude/settings.json, keyed to events. The main ones are PreToolUse and PostToolUse around each tool call, plus UserPromptSubmit, SessionStart and Stop. This one runs a formatter after every edit:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          { "type": "command", "command": "npx prettier --write ." }
        ]
      }
    ]
  }
}

Swap the command for a linter, a test run, or a PreToolUse hook that denies a dangerous tool call outright.