User-Agent parser

What a User-Agent string is

Every browser sends a User-Agent header with each request, identifying itself - in principle. In practice the format is a historical accident: modern browsers all claim to be "Mozilla/5.0" and list several engines they are compatible with, a legacy of decades of sites checking for specific browser names and every new browser needing to pass those same checks.

What this reads out of it

The browser and its version, the operating system, whether it looks like a mobile, tablet or desktop device, and whether it matches a known search-engine or social-media crawler pattern instead of a browser at all.

Why it can get it wrong

This is pattern matching against known formats, not a lookup against an authoritative registry - because there is not one. An unusual, very new, or deliberately spoofed User-Agent may not be identified correctly, and this says so rather than guessing with false confidence.

Nothing is sent anywhere extra

Parsing happens entirely in your browser. Using the "Use my browser's User-Agent" button just reads a value your browser already sends with every request - it does not disclose anything additional.