Sec-Purpose header
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
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 |
|---|---|
| Forbidden request header | Yes (Sec- prefix) |
| CORS-safelisted request header | 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
| Specification |
|---|
| Fetch # sec-purpose-header |
| Prefetch # sec-purpose-header |
Browser compatibility
| desktop | mobile | |||||||||||
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Sec-Purpose
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Sec-Purpose for <link rel="prefetch"> prefetch
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Sec-Purpose for <script type="speculationrules"> prefetch
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Legend
Tip: you can click/tap on a cell for more information.
- Full support
- Full support
- No support
- No support
- Experimental. Expect behavior to change in the future.
- See implementation notes.
- User must explicitly enable this feature.
See also
Sec-Fetch-Dest,Sec-Fetch-Mode,Sec-Fetch-Site,Sec-Fetch-Userfetch metadata request headers- Prefetch (Glossary)
<link>element with attributerel="prefetch"