Purpose

When a browser speculatively loads a resource through prefetch or prerender, the Purpose request header signals the speculative nature of the request to the server.

Table of Contents

Usage
Values
- prefetch
Example
See also

Usage

The Purpose header signals to the server when a request originates from a prefetch or prerender action. Browsers send the header on requests triggered by <link rel="prefetch">, <link rel="prerender">, or the speculation rules API. Servers receiving this header distinguish speculative loads from actual user navigation, allowing them to skip analytics tracking, defer expensive personalization, or serve lighter responses for speculative requests.

The header has a long history across browser engines. Firefox introduced X-Moz: prefetch. Safari used X-Purpose: preview for its Top Sites page previews. Chrome adopted Purpose: prefetch as part of its NoState Prefetch implementation.

The Purpose header is being replaced by Sec-Purpose, which uses the Sec- prefix to prevent JavaScript from spoofing the value through fetch() or XMLHttpRequest. Chrome sent both Purpose and Sec-Purpose during a transition period. Chrome stopped sending Purpose, completing the migration to Sec-Purpose.

The Purpose header also conflicts with the CORS specification. Because Purpose is not a CORS safelisted header, prefetch requests to cross-origin resources trigger unnecessary preflight checks. The Sec- prefix on Sec-Purpose avoids this because Sec- prefixed headers are excluded from CORS processing.

Note

Older Firefox versions sent X-Moz: prefetch before Sec-Purpose replaced it. Current Firefox sends Sec-Purpose only. Server-side code detecting prefetch requests across browsers needs to check for Purpose and Sec-Purpose, with X-Moz relevant only for requests from older Firefox versions.

Values

prefetch

The value prefetch indicates the browser is fetching a resource speculatively. This is the only defined value. Both prefetch and prerender requests send this same value in the Purpose header.

Example

A browser prefetching a page using the legacy header. Chrome stopped sending Purpose, but older Chromium-based browsers and some other implementations still include the header.

Purpose: prefetch

An older Firefox version prefetching a resource sends the vendor-specific header. Firefox later replaced X-Moz with Sec-Purpose.

X-Moz: prefetch

A Chromium-based browser from the transition period sends both header names on the same prefetch request. Checking multiple header names ensures prefetch requests are identified regardless of browser version.

Purpose: prefetch
Sec-Purpose: prefetch