/file-watcher

name: file-watcher
description: Configure file watching hooks to auto-react to config changes, env file updates, and dependency modifications. Use to set up reactive workflows.
user-invocable: true

File Watcher

Use Claude Code's `FileChanged` and `CwdChanged` hooks to create reactive workflows that respond to file system changes.

Trigger

Use when:

• Setting up auto-reload for config changes
• Watching for dependency updates
• Monitoring build output
• Creating reactive development workflows

How File Watching Works

Claude Code's `SessionStart` and `CwdChanged` hooks support returning `watchPaths` to register file watchers. The current `cwd-changed.js` script focuses on env injection; to add watch registration, your hook script must output this JSON structure:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "watchPaths": [
      "/absolute/path/to/.env",
      "/absolute/path/to/package.json"
    ]
  }
}

When watched files change, the `FileChanged` hook fires with:

{
  "hook_event_name": "FileChanged",
  "file_path": "/path/to/changed/file",
  "event": "change"
}

Environment Injection

`CwdChanged` and `FileChanged` hooks can write to `CLAUDE_ENV_FILE` to inject environment variables into subsequent Bash commands:

echo "export PROJECT_TYPE=node" >> "$CLAUDE_ENV_FILE"
echo "export TEST_CMD='npm test'" >> "$CLAUDE_ENV_FILE"

Common Watch Patterns

Watch .env for Changes

const envFile = path.join(projectRoot, '.env');
if (fs.existsSync(envFile)) {
  output.hookSpecificOutput = {
    hookEventName: 'SessionStart',
    watchPaths: [envFile]
  };
}

Watch package.json for Dependency Changes

Detect when dependencies change and remind to run `npm install`.

Watch tsconfig.json for Config Changes

Remind to restart TypeScript checks when config changes.

Setup

Add to hooks.json:

{
  "FileChanged": [{
    "matcher": ".env|package.json|tsconfig.json",
    "hooks": [{
      "type": "command",
      "command": "node scripts/file-changed.js"
    }]
  }]
}

Rules

• Use absolute paths for watchPaths (required by Claude Code)
• Matcher uses pipe-separated filenames
• Watcher uses 500ms stability threshold and 200ms poll interval
• Keep file-changed handlers fast (<5s) to avoid blocking
• Use `CLAUDE_ENV_FILE` for injecting env vars, not direct export