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