<arch>

A string indicating the underlying platform architecture, such as: "x86", "ARM", "[arm64-v8a, armeabi-v7a, armeabi]".

A server requests the Sec-CH-UA-Arch header by including the Accept-CH in a response to some request from the client, using the name of the desired header as a token:

HTTP/1.1 200 OK
Accept-CH: Sec-CH-UA-Arch

The client may choose to provide the hint, and add the Sec-CH-UA-Arch header to subsequent requests. For example, on a Windows X86 based computer, the client might add the header as shown:

GET /GET /my/page HTTP/1.1
Host: example.site

Sec-CH-UA: " Not A;Brand";v="99", "Chromium";v="96", "Google Chrome";v="96"
Sec-CH-UA-Mobile: ?0
Sec-CH-UA-Platform: "Windows"
Sec-CH-UA-Arch: "x86"

Note above that the low entropy headers are added to the request even though not specified in the server response.

• Client hints
• User-Agent Client Hints API
• Improving user privacy and developer experience with User-Agent Client Hints (web.dev)
• Accept-CH
• HTTP Caching > Varying responses and Vary