V8 commit c1486295ae5 ships in Chrome 148, routing Math.tanh to platform std::tanh

Until Chrome 147, V8 shipped a static fdlibm port for tanh and returned identical bits on every OS. Commit c1486295ae5 replaced that routine with a direct call to std::tanh, so the function now executes inside glibc on Linux, libsystem_m on macOS, and ucrtbase.dll on Windows. The switch first appeared in V8 14.8.57 and reached stable Chrome 148.

Direct measurements on Chrome 150 confirm the effect. tanh(0.8) returns 0.6640367702678491 under glibc, 0.6640367702678490 under libsystem_m, and 0.6640367702678489 under UCRT. Inputs such as 0.7 and 0.9 each isolate a single platform by one unit in the last place while three-quarters of all inputs remain identical across the three libraries.

Only tanh among the Math.* functions leaks the host libm; the remaining trigonometric and exponential functions continue to use V8's bundled llvm-libc or dbl-64 implementations. CSS sin(), cos(), and atan2() already call the platform library through the layout engine, creating an independent but consistent cross-check. Fingerprinting systems can therefore classify the OS from a single well-chosen call without additional entropy.

Detection tables built from these patterns will propagate to commercial anti-detect browsers within one release cycle. Vendors that fail to mirror the exact host bits will produce contradictions detectable by any site that also records the User-Agent string.