# How do sites detect anti-detect browsers? > Sites detect anti-detect browsers by contradictions: a page and its workers disagreeing, a GPU name the driver does not match, fonts from another OS. Source: https://mosaicbrowser.com/guides/how-do-sites-detect-anti-detect-browsers ## How do sites detect anti-detect browsers? Sites detect anti-detect browsers mostly by contradictions rather than by any single value: a page and its workers reporting different machines, a GPU name the driver's behaviour does not match, fonts from a different operating system, a Chromium build claiming to be Chrome, or a timezone that disagrees with the IP. ![Caught by a contradiction, not a value, beside a screenshot: a Mosaic leak test: every surface a site reads, checked from inside the profile page](https://mosaicbrowser.com/covers/how-do-sites-detect-anti-detect-browsers.webp) A site rarely catches an anti-detect browser by its values. An Apple M4 Max, 16 threads and a Berlin timezone are all ordinary. It catches the browser when two of its values could not both be true. Every way sites detect anti-detect browsers below is a version of that comparison, and we found most of them by failing them on our own engine before we fixed them. ## Where the page and its workers disagree A page is not one place a script runs. Dedicated workers, shared workers and service workers each have their own copy of `navigator`, their own canvas and their own WebGL, and a page can ask any of them for the same value. The open-source test [CreepJS](https://github.com/abrahamjuliot/creepjs) does exactly that and compares the answers. That is where tools that change values with injected JavaScript fall over. A script added to the page can redefine `navigator.hardwareConcurrency` in the window. It cannot reach a service worker, and a worker started from a `blob:` URL often starts before the script does. So the window says 8 threads and the worker says 16, and the disagreement is worth more to a detector than the real number would have been. We measured our own engine against this before it was finished. A stock Chrome encoded the same canvas to identical bytes in the window and in a worker. Ours matched stock in the window and differed in the worker, because our canvas noise reached the worker's encoder and not the page's. We fixed it in the engine, and CreepJS's worker check then read "3 worker realms agree with the window across 45 fields". That is why Mosaic changes nothing with page scripts: a value moved in C++ inside the browser is the same in every realm, because there is only one copy of it. ## A GPU name the driver does not back up Changing the WebGL renderer string is easy. Changing what the GPU actually does is not. A page can read the extension list, the shader precision formats, how long a draw call takes and the exact pixels it produces, and none of that moves when the name does. We measured how strict this is on [Pixelscan](https://pixelscan.net/fingerprint-check), one profile patched in place, on an Apple M4 Max Mac. Claiming the host's own chip passed with "fingerprint consistent, no masking". Claiming an Apple M2 Pro, M1, M4 Pro or plain M4 failed with "masking detected", every round. Not the family, not the generation: the exact chip. So Mosaic draws macOS profiles with the host's own GPU and varies everything that can honestly vary. Mosaic is a desktop app Email yourself the link and open it on your computer. [How it works: Its own Chromium](https://mosaicbrowser.com/features/engine) ## Fonts from a different operating system A browser claiming Windows has Windows fonts, measured by how wide the text renders. Fonts and the way the operating system rasterises them come from the real computer, and no browser switch reaches them. On our macOS build machine, a profile claiming Linux read "Masking detected" on Pixelscan with 1 font detected against 159; one claiming Windows never finished the scan. That is why Mosaic, which will make a Windows or Linux profile on a Mac, shows a caution in the editor when you do: on the same Mac, IPHey scored a macOS claim 100/100 and a Windows claim 80/100. Any browser that lets one computer claim another desktop operating system has to answer this contradiction somehow, and it is worth asking how. The detail of what Pixelscan compares is in [Pixelscan's inconsistent fingerprint warning](https://mosaicbrowser.com/guides/pixelscan-inconsistent-fingerprint). ## A Chromium build claiming to be Chrome Most anti-detect browsers are Chromium, and most claim to be Google Chrome. The two differ in ways a page can count: - `speechSynthesis.getVoices()` returns 199 voices in branded Chrome on a Mac and 180 in an unbranded build, because 19 `Google …` voices ship only with Google's build. - An unbranded Chromium runs field-trial settings that Google's Chrome does not. Measured against Chrome of the same version, that was 20 API differences, such as `navigator.runAdAuction` missing and `window.ModelContext` present. - Without Google's signatures, the first Widevine DRM request carries a per-install device certificate: 1,704 bytes where branded Chrome sends 2. The certificate is stable per install, so it ties every profile on the machine together. A useful control: Google's own unbranded Chrome for Testing, with no patches at all, earned the same IPHey warning as our build did. Some of what detectors flag is "this is Chromium", not "this is spoofed". Our engine now restores the voices, turns the field-trial config off and leaves Widevine off unless it is switched on. ## The network disagreeing with the page The TLS handshake has its own fingerprint, JA3 or JA4, which Cloudflare exposes to its customers' rules ([Cloudflare docs](https://developers.cloudflare.com/bots/additional-configurations/ja3-ja4-fingerprint/)). A browser whose User-Agent says Chrome and whose handshake looks like an HTTP library is caught before the page loads. Measured side by side on one machine, our profile and Google's Chrome for Testing of the same version produced the same JA4 string, because the engine is Chromium's own network stack. Then the address. A Berlin exit with a Los Angeles timezone and an English-only language list is the first comparison a consistency scanner makes, which is why Mosaic sets timezone, language and geolocation from the proxy's measured exit. ## Automation that announces itself `navigator.webdriver` is true in a browser under automation ([MDN](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/webdriver)), and headless Chrome names itself in its User-Agent. Subtler: Puppeteer and Playwright switch on the DevTools Protocol's `Runtime` domain, and pages can notice. On a fresh profile, enabling `Runtime` alone flipped deviceandbrowserinfo's verdict from `isBot=false` to `isBot=true`. Mosaic's own page control reads and clicks without ever enabling it. ## Values that are identical across every profile The last method is statistical. If every profile from one tool reports the same unusual value, that value marks the tool. We had one: `Notification.permission` read `denied` on all 344 profiles on our build machine, because a setting was forced for every launch, while an unconfigured Chrome says `default`. Nothing leaked in. One value was pinned across every profile, which a site reads as well as any leak. ## How sites detect anti-detect browsers, and what to ask before you pick one Ask how the tool changes values, not how many it changes. A page script cannot reach workers. A foreign operating system cannot bring its fonts. A renamed GPU keeps its driver. Then test it yourself on several detectors, on more than one day, because one verdict from a live site is not evidence. Our own engine is described on the [engine page](https://mosaicbrowser.com/features/engine); what a fingerprint is made of is in the [browser fingerprint glossary entry](https://mosaicbrowser.com/glossary/browser-fingerprint). ## Sources 1. [CreepJS README on GitHub](https://github.com/abrahamjuliot/creepjs), read November 2, 2026 2. [Pixelscan: fingerprint check](https://pixelscan.net/fingerprint-check), read November 2, 2026 3. [Cloudflare docs: JA3/JA4 fingerprint](https://developers.cloudflare.com/bots/additional-configurations/ja3-ja4-fingerprint/), read November 2, 2026 4. [MDN: Navigator.webdriver](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/webdriver), read November 2, 2026 ## Questions ### Can a website tell I am using an anti-detect browser? It can if the browser contradicts itself, for example if a worker reports a different machine from the page, or the claimed operating system does not match the fonts. A browser whose values agree everywhere gives a site nothing to compare. ### Why do JavaScript fingerprint spoofers get caught? Because a script injected into the page cannot reach every place a value is read. Service workers, other frames and the browser's own request headers keep reporting the real machine, and the gap between the two is the signal. ### Is it suspicious to have a unique fingerprint? Most real browsers are unique in some way, so uniqueness alone is normal. A combination that no real device produces is what stands out, such as an Apple GPU on a Windows User-Agent. ### Do detectors flag stock Chromium too? Some do. In our own testing, Google's unpatched Chrome for Testing earned the same IPHey warning as our own build, because it is not the branded Chrome that most people run. ## Related [How do websites detect multiple accounts?](https://mosaicbrowser.com/guides/how-do-websites-detect-multiple-accounts): Websites detect multiple accounts by what two accounts share: cookies, the IP address, the browser fingerprint, payment and phone details, and habits. [Chrome profiles vs an anti-detect browser: which do you need?](https://mosaicbrowser.com/guides/chrome-profiles-vs-anti-detect-browser): Chrome profiles separate cookies for free. An anti-detect browser also gives each profile its own fingerprint and proxy, which matters for client accounts. [Pixelscan says inconsistent fingerprint: what it means](https://mosaicbrowser.com/guides/pixelscan-inconsistent-fingerprint): A Pixelscan inconsistent fingerprint means two values your browser reports cannot both be true of one machine. Five causes we found, each dated. [What is an anti-detect browser?](https://mosaicbrowser.com/guides/what-is-an-anti-detect-browser): An anti-detect browser gives each profile its own fingerprint, proxy and cookies, so sites see separate visitors. What it changes, how, and its limits. [Browser fingerprint: what a site can read about your machine](https://mosaicbrowser.com/glossary/browser-fingerprint): A browser fingerprint is the set of values a site reads about your browser and computer, which together recognize you without cookies. What goes into one. [An anti-detect browser with its own Chromium engine](https://mosaicbrowser.com/features/engine): Mosaic is an anti-detect browser with its own Chromium engine, Tesseract Chrome: stock source plus 64 patches that change fingerprint values in C++.   ## Run this guide on your own accounts Every Mosaic profile is a fingerprint, a proxy exit and a cookie jar that agree with each other, kept on your computer. Free holds 3 profiles. Pro is $49 a month for any number. Mosaic is a desktop app Email yourself the link and open it on your computer.