Save-Data
When a user enables data-saving mode in the browser or operating system, the Save-Data request header is a client hint signaling a preference for reduced data usage.
Usage
The Save-Data header is defined in the WICG Save Data API specification, split out of the Network Information API. Unlike most client hints, this is a low-entropy hint. Browsers send the header without requiring the server to opt in through Accept-CH. When a user enables a data-saving mode in the browser or operating system, the browser attaches Save-Data to outgoing requests automatically.
Servers receiving this header adapt responses to minimize data transfer. Common adaptations include serving lower-resolution images, omitting web fonts, disabling auto-playing media, reducing the number of resources loaded on initial page render, and deferring non-critical scripts.
Combining Save-Data with Device-Memory, Downlink, and ECT gives servers enough context to build an adaptive content delivery strategy covering network conditions and hardware constraints.
Serving different bytes for the same URL puts caching
on the critical path: responses adapted to the hint
need Vary: Save-Data so shared caches keep the
lightweight and full variants apart, and a CDN
without the Vary entry serves the stripped
page to every visitor. CSS offers a header-free
companion through the prefers-reduced-data media
query, letting stylesheets drop background images
under the same user preference the header reports.
Values
on
The on value indicates the user prefers reduced data
usage. This is the only value sent by browsers. When the
data-saving preference is inactive, the browser omits the
header entirely rather than sending an off value.
Example
A browser with data saver mode enabled sends the header on every request. The server detects the preference and responds with a lighter page.
Save-Data: on
A combined request showing Save-Data alongside other network-aware client hints.
Save-Data: on
Device-Memory: 1
Downlink: 0.5
ECT: 2g
The presence of Save-Data: on together with a low
downlink estimate and a slow effective connection type
signals a constrained environment where aggressive
resource optimization applies.
Responses adapted to the hint need
Vary: Save-Data so caches store the light and full
variants separately instead of serving one
population's variant to the other.