User Agent Parser

The User-Agent HTTP header is sent by browsers and HTTP clients to identify the software making the request. It is a free-form string that typically contains the browser name and version, the rendering engine, the operating system, and sometimes the device type and model. Web servers, analytics platforms, and content delivery networks use User-Agent to adapt responses, track browser usage statistics, and detect bots versus human visitors.

User-Agent strings are notoriously complex and historically inconsistent due to the browser wars of the 1990s and 2000s. Browsers have historically identified themselves as other browsers to receive full-featured content rather than degraded fallbacks. As a result, modern Chrome User-Agent strings contain 'Mozilla/5.0', 'AppleWebKit', 'Chrome', and 'Safari' — four browser names in a single string. This legacy makes reliable parsing difficult and is why parsing User-Agent strings requires regular expression matching against known patterns.

User-Agent sniffing for feature detection is considered poor practice — checking for the existence of specific APIs (feature detection) is far more reliable, since a browser may support a feature regardless of its name or version. However, User-Agent parsing remains useful for server-side analytics, bot detection, serving different assets to mobile versus desktop, and debugging reported issues where the browser matters. The modern Client Hints API (Sec-CH-UA and related headers) provides structured, privacy-preserving browser information as a replacement for the unstructured User-Agent string.