What Is My User Agent? - Browser Detection Tool 2025

A user agent string is a text identifier that browsers send with every HTTP request, containing information about the browser type, version, operating system, and device. It's crucial for web developers because it allows servers to deliver optimized content based on browser capabilities, enables analytics tracking, helps with debugging compatibility issues, and allows for progressive enhancement strategies. Without user agents, websites couldn't adapt to different browsers and devices effectively.

Yes, user agents can be modified through browser developer tools (F12 > Network conditions), browser extensions, command-line flags, or by using privacy-focused browsers. This is commonly done for testing websites across different browsers, accessing mobile versions of sites on desktop, bypassing outdated browser restrictions, or enhancing privacy. However, some websites may detect spoofed user agents through JavaScript feature detection or behavioral analysis, and changing your user agent might break some site functionality.

User agent strings evolved over decades of web history, accumulating compatibility tokens to ensure older websites continue functioning. They contain Mozilla/5.0 for Netscape compatibility, multiple browser names for sites checking specific strings, version numbers for feature detection, and platform details for OS-specific code. This "user agent string append-only history" ensures backward compatibility but creates complexity. Modern approaches like Client Hints aim to simplify this, but legacy support remains necessary.

User agent detection is generally reliable for basic browser identification but has limitations. Browsers can spoof user agents, new browsers might not be recognized, and minor versions might be misidentified. Best practice is to combine user agent detection with feature detection using JavaScript. Use user agents for analytics and general optimization, but rely on feature detection for critical functionality. Modern APIs like Navigator.userAgentData provide more structured information.

User agent sniffing reads the UA string to guess browser capabilities, while feature detection tests if specific features exist. Feature detection is more reliable - instead of assuming Chrome 90 supports a feature, you test if window.feature exists. User agent sniffing is useful for analytics, serving different content to mobile devices, and working around known browser bugs. Best practice: use feature detection for functionality, user agents for optimization and analytics.

Legitimate bots identify themselves clearly in user agents: Googlebot for Google, bingbot for Bing, Slackbot for Slack, etc. They often include contact information or documentation URLs. However, malicious bots may use fake user agents to appear as regular browsers. To verify legitimate bots, check if the IP address matches the claimed bot's official IP ranges, perform reverse DNS lookups, and monitor behavior patterns. Most legitimate bots also respect robots.txt directives.