What is a User Agent?
A User Agent (UA) is a string your browser sends with every HTTP request, telling servers what software is making the request. Servers use it for analytics, conditional CSS delivery, bot detection, and feature gating.
Why does my UA say "Mozilla/5.0"?
Legacy compatibility. Every browser says Mozilla/5.0 — it dates back to the browser wars of the 90s when sites served rich content only to Netscape. All browsers kept it to avoid being blocked.
How do sites detect bots?
Bots usually have strings like Googlebot, Bingbot, or curl. Sophisticated bots spoof real browser UAs but can still be fingerprinted via behavior, headers, and JS execution.
What is the "engine" field?
The rendering engine is what actually draws the webpage. Blink (Chrome, Edge), WebKit (Safari), and Gecko (Firefox) — each behaves slightly differently, which is why CSS sometimes needs vendor prefixes.
Can I trust UA strings?
No — any client can send any UA. Never use it for security decisions. It's useful for analytics and UX optimization, but always validate critical info server-side through other means.