Web debugging via the live page

You have a real headless browser. Use it. The default failure mode is reading source, forming a hypothesis, and asking the user to verify in their devtools. That's slow and wrong: the answer usually lives in runtime state (localStorage, the actual Authorization header the SPA sent, a console error), not in source.

When to reach for the kit

Pattern-match the user's wording to a playbook below. If their description sounds like anything in this list, open the browser first, theorize second.