You open a website on your phone and it shows the mobile layout. You open it on a laptop and the menu is wider, the images sharper, the dates in your local format and the text in your language. You did not tell the site any of this. Your browser did, automatically, as part of loading the page.
This guide explains what your browser shares with every website, how sites use it, why they sometimes get it wrong, and what it means for your privacy. You can see your own details as you read with What Is My Browser.
The user-agent string
Every time your browser requests a page, it sends a header called the user agent: a line of text identifying the browser, its version and the operating system. A typical one looks like this:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/129.0.0.0 Safari/537.36
It is full of historical leftovers. "Mozilla/5.0" appears in almost every browser's user agent because, decades ago, websites only sent modern pages to browsers that claimed to be Mozilla, so every browser started claiming it. Chrome mentions Safari and "like Gecko" for similar compatibility reasons. Reading a user agent correctly means knowing which parts actually matter.
Sites use it to choose a mobile or desktop layout, to show download buttons for the right operating system, to work around a known bug in a particular browser version, and in analytics to count visitors by browser and device.
Why your user agent is "frozen" and what client hints are
Detailed user agents made browsers easier to fingerprint and encouraged sites to write fragile browser-specific code. So browsers started reducing them. Chrome-based browsers now send only the major version (129.0.0.0 rather than the full build number) and freeze some details. The most surprising result: Windows 11 still reports itself as "Windows NT 10.0" in the user agent.
The replacement is User-Agent Client Hints. A site can ask for specific details, such as the full browser version, the real platform version or the device model, and Chromium browsers answer through a separate API. What Is My Browser asks for these hints, which is how it can show Windows 11 and the full browser version where the old user agent cannot. Firefox and Safari do not support client hints, so for them the user agent is all there is.
Detection mistakes and why they happen
Browser detection goes wrong more often than you might expect:
- Chrome and Firefox on iPhone are built on Apple's WebKit engine, because Apple requires it, and identify themselves with codes such as CriOS and FxiOS. Simple detectors label them as Safari.
- iPads request desktop websites by default and send a Mac user agent. To most sites an iPad looks like a Mac. Touch support is the giveaway.
- Brave uses Chrome's user agent to avoid standing out. It can only be recognised through a Brave-specific check.
- Samsung Internet includes "Chrome" in its user agent and is often mislabelled.
When a site shows the wrong layout or tells you your browser is unsupported, a detection mistake is a common cause. Updating the browser, or switching off "Request desktop site", often fixes it.
Everything else a site can read without asking
Beyond the user agent, JavaScript on any page can read a long list of details without a permission prompt:
| Category | Examples |
|---|---|
| Display | Screen size, window size, pixel ratio, dark or light mode, reduced-motion preference |
| Language and time | Preferred languages, time zone |
| Hardware | Number of CPU threads, approximate memory, graphics card name |
| Network | Connection type and rough speed (in some browsers), online or offline |
| Privacy settings | Cookies enabled, Do Not Track, Global Privacy Control |
| Features | WebAssembly, service workers, WebGL, clipboard and share support |
Sites use these to adapt: serving sharper images to high-density screens, showing the dark theme you prefer, picking the right language, or turning off animations for people who find motion uncomfortable. The window size matters more than the screen size for layout; our guide on screen resolution versus viewport size explains the difference, and the Screen Resolution Checker measures both.
Fingerprinting: when details add up
Individually, these details are mundane. Millions of people use Chrome on Windows with a 1920 × 1080 screen. Combined, though, a long list of small details can be unusual enough to recognise the same browser across visits, even without cookies. This is called browser fingerprinting.
A fingerprint does not reveal your name or address. It lets a site, or an advertising network present on many sites, say "this is probably the same browser as before". Browsers fight back in different ways: Safari and Firefox limit what they reveal, Brave adds small random variations, and Chromium browsers are moving detailed information behind client hints.
Your IP address is the other big piece of information every site sees. Our guide to what your IP address reveals covers it, and Find My IP shows yours.
Privacy settings that actually matter
- Global Privacy Control (GPC) is a signal that tells sites you do not want your data sold or shared. Some privacy laws require sites to honour it. Brave and Firefox (in some modes) send it, and extensions can add it to other browsers.
- Do Not Track is an older signal that most sites ignore. It is harmless to turn on but rarely changes anything.
- Blocking third-party cookies limits tracking across sites and is the default in Safari and Firefox.
- Private windows start without your cookies and history, which is useful for testing how a site behaves for a new visitor.
Checking these takes seconds in the Privacy section of What Is My Browser.
Getting help faster: copy your details for support
"Which browser are you using?" is usually the first question a help desk asks, and many people are not sure. Instead of guessing or sending screenshots, open What Is My Browser and press Copy for support. It copies your browser and version, operating system, device, display, language, privacy settings, hardware and supported features as plain text, ready to paste into a ticket or email. Developers can reproduce the problem far faster with exact details.
If a site is slow rather than broken, the Ping Tool checks your connection to servers around the world, which helps separate network problems from browser problems.
Frequently asked questions
Does What Is My Browser send my details anywhere?
No. Everything is read and displayed in your browser.
Why does my browser report Windows 10 when I have Windows 11?
The user agent was frozen at "Windows NT 10.0". Chromium browsers share the real version through client hints, which the tool reads.
Why do some values show as Hidden?
Firefox and Safari do not share some details, such as memory and graphics, to make fingerprinting harder.
Can I change my user agent?
Developer tools and extensions can change it, mainly for testing. It can break sites and does not stop fingerprinting by itself.
Is it bad that websites see all this?
It is how the web adapts to different devices. The privacy concern is the combination of details used to track you across sites, which browser privacy settings help reduce.
Your browser introduces you to every website before you click anything. Knowing what it says helps you fix problems faster and make informed privacy choices. See your own details, and copy them for support when you need to, with What Is My Browser.
