What is a user agent string?

A user agent (UA) string is a line of text sent by a browser or client application in every HTTP request header. It identifies the browser, version, operating system, and rendering engine. For example: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.0.0 Safari/537.36".

Why do most browsers include "Mozilla/5.0" at the start?

For historical compatibility reasons. In the early web, Netscape Navigator (whose code name was Mozilla) was dominant and many servers only served full-featured pages to "Mozilla" browsers. Other browsers added "Mozilla/5.0" to their UA strings to avoid being served degraded content. This legacy token has persisted in virtually all browsers' UA strings ever since.

Can websites change their behavior based on my user agent?

Yes. Websites use UA strings for browser compatibility detection (to serve different CSS or JavaScript), analytics (to measure which browsers are used), and content negotiation (to serve mobile-optimized pages). This is sometimes called UA sniffing, though modern best practice uses feature detection instead.

UA spoofing means deliberately changing the user agent string your browser sends, making a website think you are using a different browser. This can be useful for testing how a site behaves for different browsers, accessing mobile-only sites from a desktop, or bypassing browser-based restrictions. Most browsers allow changing the UA in developer tools.

What are browser rendering engines?

Rendering engines process HTML, CSS, and JavaScript to draw web pages. The main engines are Blink (used by Chrome, Edge, and Opera), Gecko (Firefox), and WebKit (Safari and browsers on iOS). Because all browsers on iOS are required by Apple to use WebKit, Chrome and Firefox on iOS have different UA strings than their desktop counterparts.

How do bots and crawlers identify themselves in UA strings?

Search engine crawlers use distinctive UA strings. For example, Googlebot: "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)". Web scrapers and automated tools often use default UA strings from libraries like Python requests ("python-requests/2.x") unless configured to spoof a browser UA.

Is User Agent Parser free to use?

Yes. User Agent Parser runs entirely in your browser with no external API calls. All parsing is done client-side with JavaScript regex matching. There is no sign-up, no usage limits, and no data is sent anywhere.

User Agent Parser

About User Agent Parser

Browser user agent checker tools reveal what your browser tells websites about itself. This free tool auto-detects and parses your current UA string, showing browser name, version, engine, OS, and device type. Switch to manual mode to analyze any UA string from logs or testing scenarios. No account or installation needed.

User Agent Parser decodes any browser user agent string into its human-readable components: browser name and version, rendering engine, operating system and version, and device type (desktop, mobile, or tablet). Your current browser's UA string is loaded automatically, or toggle to manual mode and paste any UA string to analyze it. The parser covers Chrome, Firefox, Safari, Edge, Opera, Samsung Browser, and Internet Explorer across Windows, macOS, Linux, iOS, iPadOS, Android, and ChromeOS. User agent strings are sent with every HTTP request and are used by websites for browser compatibility detection, analytics, and content negotiation. For a complete picture of your browser environment including screen resolution, language, and capabilities, see What Is My Browser.

User agent strings have a complicated history rooted in the browser wars of the 1990s. The Mozilla/5.0 token that appears in virtually every modern browser's UA string is a legacy of Netscape Navigator's dominance: when Internet Explorer was released, it included "Mozilla" to ensure servers would serve it the same content as Netscape. Every subsequent browser followed suit to avoid being discriminated against by server-side UA detection logic. The result is a UA string format that is historically entangled and increasingly difficult to parse reliably. Google's Chrome team recognized this problem and introduced User-Agent Client Hints (UA-CH) as a replacement mechanism. UA-CH allows browsers to send structured, opt-in information about the browser and OS through HTTP request headers or JavaScript APIs, rather than a monolithic string. Chrome has been progressively reducing the information in the UA string (a process called UA reduction) since 2022, freezing the OS version and browser version to reduce passive fingerprinting surface. Despite this trend, UA strings remain widely used and UA parsing remains a practical skill for web developers and network administrators. The parser in this tool uses a priority-ordered set of regex patterns that correctly handles the order-dependent nature of UA strings, where tokens like "Chrome" appear in Edge's UA string and must be checked after Edge-specific tokens.

How to use User Agent Parser

Your current browser's UA string is auto-loaded on the page.
Toggle "Parse custom UA string" to analyze any UA string you paste.
View browser name, version, engine, OS, and device type in the result cards.

Frequently Asked Questions

What is a user agent string?: A user agent (UA) string is a line of text sent by a browser or client application in every HTTP request header. It identifies the browser, version, operating system, and rendering engine. For example: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.0.0 Safari/537.36".
Why do most browsers include "Mozilla/5.0" at the start?: For historical compatibility reasons. In the early web, Netscape Navigator (whose code name was Mozilla) was dominant and many servers only served full-featured pages to "Mozilla" browsers. Other browsers added "Mozilla/5.0" to their UA strings to avoid being served degraded content. This legacy token has persisted in virtually all browsers' UA strings ever since.
Can websites change their behavior based on my user agent?: Yes. Websites use UA strings for browser compatibility detection (to serve different CSS or JavaScript), analytics (to measure which browsers are used), and content negotiation (to serve mobile-optimized pages). This is sometimes called UA sniffing, though modern best practice uses feature detection instead.
What is UA spoofing?: UA spoofing means deliberately changing the user agent string your browser sends, making a website think you are using a different browser. This can be useful for testing how a site behaves for different browsers, accessing mobile-only sites from a desktop, or bypassing browser-based restrictions. Most browsers allow changing the UA in developer tools.
What are browser rendering engines?: Rendering engines process HTML, CSS, and JavaScript to draw web pages. The main engines are Blink (used by Chrome, Edge, and Opera), Gecko (Firefox), and WebKit (Safari and browsers on iOS). Because all browsers on iOS are required by Apple to use WebKit, Chrome and Firefox on iOS have different UA strings than their desktop counterparts.
How do bots and crawlers identify themselves in UA strings?: Search engine crawlers use distinctive UA strings. For example, Googlebot: "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)". Web scrapers and automated tools often use default UA strings from libraries like Python requests ("python-requests/2.x") unless configured to spoof a browser UA.
Is User Agent Parser free to use?: Yes. User Agent Parser runs entirely in your browser with no external API calls. All parsing is done client-side with JavaScript regex matching. There is no sign-up, no usage limits, and no data is sent anywhere.

User Agent Parser

About User Agent Parser

How to use User Agent Parser

Frequently Asked Questions

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