Sec-Purpose

The Sec-Purpose fetch metadata request header indicates the purpose for which the requested resource will be used, when that purpose is something other than immediate use by the user-agent.

The only purpose that is currently defined is prefetch, which indicates that the resource is being requested in anticipation that it will be needed by a page that is likely to be navigated to in the near future, such as a page linked in search results or a link that a user has hovered over. The server can use this knowledge to: adjust the caching expiry for the request, disallow the request, or perhaps to treat it differently when counting page visits.

The header is sent when a page is loaded that has a <link> element with attribute rel="prefetch". Note that if this header is set then a Sec-Fetch-Dest header in the request must be set to empty (any value in the <link> attribute as is ignored) and the Accept header should match the value used for normal navigation requests.

Header type	Fetch Metadata Request Header
Forbidden header name	yes (prefix `Sec-`)
CORS-safelisted request header	no

Syntax

http

Sec-Purpose: prefetch

Directives

The allowed tokens are:

prefetch: The purpose is to prefetch a resource that may be needed in a probable future navigation.

Examples

A prefetch request

Consider the case where a browser loads a file with a <link> element that has the attribute rel="prefetch" and an href attribute containing the address of an image file. The resulting fetch() should result in an HTTP request where Sec-Purpose: prefetch, Sec-Fetch-Dest: empty, and an Accept value that is the same as the browser uses for page navigation.

An example of such a header (on Firefox) is given below:

http

GET /images/some_image.png HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/116.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Sec-Purpose: prefetch
Connection: keep-alive
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: no-cors
Sec-Fetch-Site: same-origin
Pragma: no-cache
Cache-Control: no-cache

Note: At time of writing FireFox incorrectly sets the Accept header as Accept: */* for prefetches. The example has been modified to show what the Accept value should be. This issue can be tracked in Firefox bug 1836334.

Specifications

Specification
Fetch Standard # sec-purpose-header
Prefetch # sec-purpose-header

Browser compatibility

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