Intent to Extend Experiment: No-Vary-Search support in navigation prefetch cache

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Liviu Tinta

May 29, 2023, 4:47:18 PMMay 29
to blink-dev

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Original I2E

No-Vary-Search Spec

No-Vary-Search hint explainer

Origin Trial


We propose extending the "No-Vary-Search support in navigation prefetch cache" origin trial. We decided to extend the experiment based on positive feedback from the developer experimenting with the header. Discussions with the developer resulted in additional functionality for this experiment in the form of No-Vary-Search hint (see ChromeStatus entry:

We are planning to extend the experiment for 3 more milestones M116 to M118 inclusive. 

Blink component



The developer we are working with would like to experiment with No-Vary-Search hint in the context of No-Vary-Search header to evaluate the added impact of the new functionality.

TAG review

TAG review status

Complete at this time. TAG didn't see any major issue and encouraged us to continue experimenting.


Interoperability and Compatibility

Gecko: No signal. Request for Standards Position:

WebKit: No signal. Informal positive signals from individual engineers at TPAC 2022. Request for Standards Position:

Web developers: Positive. Google Search is interested in experimenting with the header, specifically for the customizing server behavior use case.

Other signals:

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
No. This is a new opt-in feature.


The website owner can debug the feature in DevTools by making sure that, when navigating to a prefetched page by using a URL that matches under No-Vary-Search conditions, the navigation happens from the prefetch cache by looking at the Size column in the Network tab. In case of success, when hovering over the Size column in the Network tab of Dev Tools, they should see: "Served from prefetch cache, resource size: yyyB"

We are also working on a preloading panel ( which shows all ongoing preloads, including both the targeted URL and the cached URL, if they differ due to No-Vary-Search.

Is this feature fully tested by web-platform-tests?

Yes. WPTs landed under the external/wpt/speculation-rules/prefetch/no-vary-search folder.

Tracking bug

Link to entry on the Chrome Platform Status

Yoav Weiss

Jun 2, 2023, 9:05:29 AMJun 2
to Liviu Tinta, blink-dev
LGTM to extend experimentation M116 to M118 inclusive given clear artifacts towards shipping: spec draft, completed TAG review & requests for signals.

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