Choose the smallest surface
A bookmarklet may suit a personal, one-off page action, although some pages restrict script execution. An extension makes sense when you need a persistent control or a supported browser API. Before building either, inspect the page structure and decide whether the task can work with only the currently visible content.
Treat the page as changeable
Web pages update their markup and load content after navigation. Ask your assistant to identify a stable selector, describe its fallback, and avoid running the action twice. Prefer a preview of extracted text over automatically sending it elsewhere. Check what the helper does when the expected element is missing.
Give permissions a reason
Every requested permission should map to the action in your brief. A helper that formats selected text should not need unrelated browsing access. Test on a page you control using ordinary, missing, and duplicated elements. Include an undo path where practical, and keep the original text accessible when a transformation fails.