What Is Prefetch?

Prefetch is an optional Header Tag Wrapper feature that allows you to define inventory that will be available to a Wrapper configuration before the site content loads (during the execution of the <head> section). This allows bid requests to be made before the page loads, giving Certified Bidding Adapters more time to return bids to the Wrapper. It also increases the rate at which those bids participate in your ad server.

This feature may be useful if you have low Wrapper timeout values, if your pages use infinite scroll, or if you have dynamically loaded ads.

When Prefetch is enabled, all Certified Bidding Adapters are either Prefetched (if on) or not Prefetched (if off), depending on their preferences during the certification process. In all cases, the Wrapper randomizes bidding adapter calls to ensure fair competition.

We do not apply Prefetch to your account by default. Please reach out to your IX Representative if you're interested in using this feature.

Prefetch Workflow

Prefetch is enabled through the Wrapper UI. When it's switched on, it has the following workflow in a given Wrapper configuration:

  1. On page load, the Wrapper accesses a stored list of ads/slots you've indicated will be on the page.
  2. The Wrapper uses this list to generate bid requests for each Certified Bidding Adapter based on the mapping of your inventory setup in the Slot Mapping Table/Load Sheet.
  3. Bid responses are received, and the ads display on your page.
  • Note: It's possible that your page may not render all of the slots defined in the Prefetch list due to the following reasons:
    • Your page layout isn't static
    • AJAX slots were not rendered due to the user not scrolling down far enough
    • User did not perform other triggering behavior
  • The key is to refine your Prefetch implementation to minimize this potential trade-off: finding the right balance of speedy bid request/responses (to ensure the most key-values make it into the ad server) versus over representing inventory that isn’t actually going to be available on the page (which can decrease the win rate).

Two workflows: the first depicts the Wrapper workflow when Prefetch is not enabled, the second depicts the Wrapper workflow when Prefetch is enabled

Verify Prefetch Status

The easiest way to determine if Prefetch is on for a given Wrapper is to open the Wrapper UI and review your Wrapper configuration. Configure Prefetch will be set to either ON or OFF:

Enable Prefetch toggle in the Wrapper UI


  1. Use Requestly or a simple network routing browser plugin.
  2. Block the network call to gpt.js.
  3. Using the console or simple network monitoring tool, determine if bid requests are being fired from the page. If they are, then Prefetch is on, if they are not, then Prefetch is off.

Measure the Impact of Prefetch

We've collaborated with publishers and Certified Bidding Adapters and our findings are conclusive. In cases where new slots are being added to a page, or where the Wrapper timeout is at or below 600ms (which is becoming increasingly common as publishers speed up their web experiences), it is yield-positive for a publisher to enable Prefetch. It also ensures maximum bid participation in the ad server for all bidding adapters. For this reason, there are a number of publishers who use Prefetch today and all Certified Bidding Adapters support this feature. At IX, we continue to innovate on this feature and are working with publishers and our partners to optimize the use of this technology.

If you are interesting in using Prefetch, please reach out to your IX Representative.