What is an undetectable browser?
"Undetectable browser" is what people search for when they want a browser that websites can't identify as automated, or can't link to their other sessions. In practice it means an anti-detect or stealth browser: a real browser engine with the automation tells removed and a controllable device fingerprint. (It is also the brand name of one commercial anti-detect browser; this page is about the general idea.)
The honest answer is that no browser is undetectable. Detection isn't a single test that you pass or fail. A site combines dozens of weak signals: how the browser was launched, how it is being controlled, what hardware and fonts it reports, whether its TLS handshake matches its user agent, the reputation of the IP address, and how the session behaves. A browser only controls some of those.
What a browser can do is avoid standing out. The two properties that matter are commonness and coherence. Commonness means presenting a device that lots of real people have: a mainstream Chrome version, a common screen size, a normal GPU. Coherence means every signal agrees: the platform matches the fonts, the GPU string matches how canvas actually renders, the worker reports the same values as the page, and the TLS handshake belongs to the Chrome version in the user agent.
Most "undetectable" claims fail on coherence, not on any single value. A spoofed user agent over a mismatched network handshake, a Windows platform with a macOS GPU string, or a navigator.webdriver set to false by a detectable JavaScript override: each looks fine on its own and contradicts something else. Randomising values on every visit has the same problem, because real devices don't change between page loads.
That is why engine-level browsers exist. Changing values inside the browser engine, rather than overriding them from JavaScript, keeps them native in every frame and worker, and lets the network layer follow the same identity. Clearcote does this for Chromium, deriving the whole device from one seed; see how detection works for the full breakdown.
To see how a browser actually looks, test it rather than trusting a label. The Clearcote fingerprint audit runs 272 coherence checks on whatever browser opens it, with an explanation for each result.