When I added mobile personas to a browser that already had desktop ones working, I assumed the work was mostly string substitution: swap the user agent, set touch points to 5, shrink the viewport, done. That assumption survived about a day.

Mobile fingerprints aren’t a variant of desktop fingerprints. They’re a different set of constraints, and most of the desktop toolkit either doesn’t apply or actively gives you away. Here’s what I ran into, in the order it hurt.

Two months ago I gave an agent a simple job: log into a vendor portal, download last month’s invoice PDF, rename it, drop it in a folder. It worked on my laptop. It failed on the server, silently, in a way that took me a full day to understand — the page loaded, the DOM was there, the login form was there, and the credentials were rejected with a generic error. No CAPTCHA. No block page. Just “something went wrong.”