Speculation-Rules header

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

See full compatibility

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The HTTP Speculation-Rules response header provides one or more URLs pointing to text resources containing speculation rule JSON definitions. When the response is an HTML document, these rules will be added to the document's speculation rule set. See the Speculation Rules API for more information.

The resource file containing the speculation rules JSON can have any valid name and extension, but it will be requested with a destination type of speculationrules, and must be served with an application/speculationrules+json MIME type.

Note: This mechanism provides an alternative to specifying the JSON definition inside an inline <script type="speculationrules"> element. Specifying an HTTP header is useful in cases where developers are not able to directly modify the document itself.

Header type	Response header

Syntax

http

Speculation-Rules: <url-list>

Directives

<url-list>: A comma-separated list of URLs pointing to text resources containing speculation rule JSON definitions. The JSON contained in the text files must follow the same rules as that contained inside inline <script type="speculationrules"> elements. See Speculation rules JSON representation for the syntax reference.

Examples

Speculation-Rules field with a single file

The following response contains one file reference:

http

Speculation-Rules: "/rules/prefetch.json"

Speculation-Rules field with multiple files

The following response contains multiple file reference as a comma-separated list:

http

Speculation-Rules: "/rules/prefetch.json","/rules/prerender.json"

Note: The URL values must be contained in quotes.

Specifications

Specification
HTML # the-speculation-rules-header

Browser compatibility

	desktop					mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`Speculation-Rules` Experimental	Chrome – Full support Chrome 121 (Release date: 2024-01-23) footnote Full support	Edge – Full support Edge 121 (Release date: 2024-01-25) footnote Full support	Firefox – No support Firefox footnote No support	Opera – Full support Opera 107 (Release date: 2024-02-07) footnote Full support	Safari – No support Safari 26.2 (Release date: 2025-12-12) disabled disabled From version 26.2 users must explicitly set the `SpeculationRules prefetch` preference.	Chrome Android – Full support Chrome Android 121 (Release date: 2024-01-23) footnote Full support	Firefox for Android – No support Firefox for Android footnote No support	Opera Android – Full support Opera Android 81 (Release date: 2024-03-14) footnote Full support	Safari on iOS – No support Safari on iOS 26.2 (Release date: 2025-12-12) disabled disabled From version 26.2 users must explicitly set the `SpeculationRules prefetch` preference.	Samsung Internet – Full support Samsung Internet 25 (Release date: 2024-04-24) footnote Full support	WebView Android – Full support WebView Android 121 (Release date: 2024-01-23) footnote Full support	WebView on iOS – No support WebView on iOS footnote No support

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.
: User must explicitly enable this feature.

Speculation-Rules header

Legend