httpclienthints

hints / ua · opt-in

Sec-CH-UA-WoW64

Sec-CH-UA-WoW64 is a boolean hint reporting whether a 32-bit browser runs on 64-bit Windows, a niche compatibility signal for download pages.

requires Accept-CH UA · opt-in
Example
Sec-CH-UA-WoW64: ?0
JavaScript
getHighEntropyValues(['wow64'])
Available since
Chrome 110 (2023)
Engines
Chrome Edge Opera Firefox Safari
References
MDN · Specification

Sec-CH-UA-WoW64 answers the narrowest question in the whole registry: is this a 32-bit browser running on 64-bit Windows? The name comes from Microsoft’s “Windows 32-bit on Windows 64-bit” compatibility layer, and the old UA string used to expose it as a literal WOW64 token.

Accept-CH: Sec-CH-UA-WoW64

Sec-CH-UA-WoW64: ?1    # 32-bit browser, 64-bit Windows
Sec-CH-UA-WoW64: ?0    # everything else

Why anyone cares

Consider a download page reading Sec-CH-UA-Bitness alone. A 32-bit Chrome reports "32", but if the OS underneath is 64-bit, the machine can run your 64-bit installer, which is almost certainly the better artifact to serve. ?1 here means exactly that: ignore the browser’s bitness, the OS can take x64.

BitnessWoW64Machine can run
"64"?064-bit apps
"32"?164-bit apps (browser is just old)
"32"?032-bit only

That table is the entire use case. It was added late (Chrome 110, 2023) precisely because installer-serving sites kept mis-serving 32-bit builds to capable machines.

In JavaScript

const { wow64 } = await navigator.userAgentData
  .getHighEntropyValues(['wow64']);
// true | false

Should you request it?

Only if you serve Windows binaries. For anything else it is a wasted entry in your Accept-CH list, and on every non-Windows platform the answer is a constant ?0.