Sec-Purpose

The HTTP Sec-Purpose request header indicates when a browser requests a resource for prefetching or prerendering rather than immediate display.

Usage

The Sec-Purpose header allows servers to identify speculative loading requests. Browsers send this header when fetching resources ahead of time in anticipation of future navigation. The server receives the hint and decides whether to fulfill the request, defer side effects, or return a non-success status to prevent the speculative load.

The header appears on requests triggered by <link rel="prefetch">, <link rel="prerender">, or speculation rules API calls. The browser sets the value to prefetch for simple resource prefetching or prefetch;prerender for full page prerendering where the browser renders the page before navigation occurs.

The recommended server-side approach is to allow prefetch and prerender requests to complete but defer problematic side effects. Operations like sign-out, language switching, add-to-cart actions, OTP sending, usage allowance increments, and ad conversion tracking need to be deferred until the user navigates to the page. Client-side JavaScript detects activation through the performance.getEntriesByType("navigation") API by checking for a deliveryType of "navigational-prefetch".

const nav =
  performance.getEntriesByType("navigation")[0];
if (nav.deliveryType === "navigational-prefetch") {
  // trigger deferred actions
}

Returning a non-success HTTP status (outside 200-299 after Redirects) prevents the speculative load entirely. Status codes 204 and 205 prevent prerendering but not prefetching.

The Sec- prefix prevents JavaScript from spoofing the value through fetch() or XMLHttpRequest and avoids CORS preflight conflicts affecting the legacy Purpose header. The header works alongside Sec-Fetch-Dest and Sec-Fetch-Mode to provide complete request context.

No-Vary-Search interaction

Prefetched pages sometimes include URL search parameters (like UTM tracking codes) not affecting the server response. The No-Vary-Search response header tells the browser which search parameters are irrelevant for matching. When a prefetched URL differs from the navigation URL only in ignored parameters, the browser reuses the prefetched response instead of making a new request. Speculation rules support an expects_no_vary_search hint to anticipate this header before the prefetch completes.

Invalidating cached speculations

The Clear-Site-Data header supports "prefetchCache" and "prerenderCache" directives to invalidate cached speculative loads. A state-changing request like a logout or cart update returns these directives so subsequent navigations fetch fresh content.

Clear-Site-Data: "prefetchCache", "prerenderCache"

Values

prefetch

The prefetch value indicates the browser is fetching a resource in anticipation of future need. The user has not navigated to the page.

prefetch;prerender

The prefetch;prerender value indicates the browser intends to fully render the page before navigation. This signals more aggressive preparation including executing JavaScript and loading subresources. The prerender parameter originates from the Speculation Rules API (now part of WHATWG HTML), not from the core Fetch specification which only defines prefetch.

Example

A browser prefetches a linked page. The server logs the request separately from actual page views and defers analytics tracking.

Sec-Purpose: prefetch
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: no-cors

A browser prerenders a search result page expected to receive a click. The server defers side effects until activation.

Sec-Purpose: prefetch;prerender
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate

A server preventing a speculative load by returning a non-success status. The browser discards the speculative response.

HTTP/1.1 503 Service Unavailable

Takeaway

The Sec-Purpose header identifies prefetch and prerender requests, allowing servers to distinguish speculative loading from actual user navigation and defer side effects until page activation.

See also

• Speculation Rules API (MDN)
• Prerender pages in Chrome (Chrome Developers)
• Fetch Standard (WHATWG)
• Pass Sec-Purpose: prefetch with link rel=prefetch (Chrome Status)
• Purpose
• Clear-Site-Data
• Sec-Fetch-Dest
• Sec-Fetch-Mode
• HTTP headers