User agent
A string the browser sends with every request that declares its name, version, rendering engine, and operating system to the web server.
The user agent is a header the browser attaches to every HTTP request, and a matching value exposed to JavaScript as navigator.userAgent. It declares the browser family, version, rendering engine, and operating system, for example identifying a request as Chrome 148 on Windows 10. Servers historically used it to adapt content to different browsers, and analytics, fingerprinting, and bot-detection systems still read it as the browser's primary statement of what it is.
On its own the user agent is easy to change, which is exactly why nobody trusts it in isolation. Detection systems compare it against client hints, JavaScript engine behavior, supported CSS and API features, and rendering output. A user agent claiming an old Chrome version on a browser that supports brand-new APIs is an immediate inconsistency. For QA and compatibility testing, the same logic applies in reverse: a test against a claimed version is only meaningful if the engine behaves like that version.
Oculr ties the user agent to reality by pinning each profile to one of 20+ Chromium kernel versions, from Chrome 86 to the current majors. The UA string, client hint brands, and feature flags all derive from that pinned version, so the profile claims the version of the engine it is actually running.
