Sec-Purpose header

The HTTP 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](https://developer.mozilla.org/en-US/docs/Glossary/Fetch%20Metadata%20Request%20Header)
[Forbidden request header](https://developer.mozilla.org/en-US/docs/Glossary/Forbidden%20request%20header)	Yes (Sec- prefix)
[CORS-safelisted request header](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted%20request%20header)	No

Syntax

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:

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

Browser compatibility

See also

• Sec-Fetch-Dest, Sec-Fetch-Mode, Sec-Fetch-Site, Sec-Fetch-User fetch metadata request headers
• Prefetch (Glossary)
• <link> element with attribute rel="prefetch"