Accept-CH
The HTTP Accept-CH response header tells the client which client hint headers the server accepts on subsequent requests.
Usage
Servers send Accept-CH to opt in to receiving client hints. A client hint is a request header providing information about the device, network, or user preferences. The server lists the hint headers the client is expected to include on future requests to the same origin.
The header is delivered over a secure (HTTPS) connection. After receiving the response, the client includes the requested hints on all subsequent same-origin requests for the duration of the session. Servers needing hints on the initial request use Critical-CH alongside Accept-CH to trigger a retry.
Common hints include Device-Memory, Save-Data, viewport dimensions, and user agent data exposed through Sec-CH-UA and related headers.
Values
hint-header-name
A comma-separated list of client hint header names. Each name corresponds to a specific client hint the server wants to receive. Multiple hints appear on a single line, separated by commas.
Example
A server requests the device memory size and the data-saving preference from the client.
Accept-CH: Device-Memory, Save-Data
A server requesting a broader set of client hints, including user agent architecture and viewport width.
Accept-CH: Sec-CH-UA, Sec-CH-UA-Mobile, Device-Memory, Viewport-Width
When combined with Critical-CH, the server signals the hints needed before rendering the first response. The client retries the request with the listed hints attached.
Accept-CH: Sec-CH-UA, Device-Memory
Critical-CH: Sec-CH-UA
Takeaway
The Accept-CH response header enables client hints by listing the hint headers a server accepts, allowing optimized responses based on device and network conditions.