On 30 April 2026, Microsoft will shut down its Prebid video cache. Publishers running Prebid video ads alongside Google Ad Manager will need to have made changes before that date. Without them, video ad calls will return VAST errors and fail to deliver.
This post explains what Prebid video caching is, why it exists, what the shutdown means in practice, what your options are and what THM has done to handle it at the platform level for our clients.
WHAT IS PREBID VIDEO CACHING?
THE PREBID AUCTION AND THE GAM WRAPPER
When a publisher runs header bidding via Prebid, the auction runs client-side before the page calls Google Ad Manager. Prebid collects bids from multiple SSPs simultaneously, selects the highest, and passes it to GAM as a line item. GAM runs its own secondary auction, comparing the Prebid bid against its internal demand stack including AdX. If the Prebid bid wins, GAM serves the ad.
For display ads this is straightforward. The winning creative is a small HTML or JavaScript payload that can be referenced by URL and called on demand.
But video is different.
WHY PREBID VIDEO REQUIRES A CACHE
Video ad creatives are VAST XML files pointing to actual video assets. They are too large and too latency-sensitive to be passed directly through the Prebid-to-GAM handoff. They need to be stored somewhere accessible in the window between the Prebid auction completing and GAM making its final serving decision.
That somewhere is a Prebid video cache server.
For the history of modern header bidding, that cache has been provided first by AppNexus, then by Xandr under AT&T, then by Microsoft Advertising after the 2021 acquisition. Microsoft has absorbed the infrastructure costs as a public service to the open web ad stack. That ends on 30 April.
60%+ Of Prebid-to-GAM video impressions route through Microsoft’s cache
Per Ian Meyers, Director of Engineering at The Trade Desk. That is the share of Prebid video traffic directly affected by this change.
WHAT HAPPENS WHEN THE MICROSOFT PREBID CACHE SHUTS DOWN
THE PRACTICAL EFFECT
WHY IT IS EASY TO MISS
VAST errors in this context will not be obvious as the auctions are still likely to fire. Reporting may show line items delivering. Revenue will drop, but attributing it to a Prebid caching issue rather than a demand or configuration problem takes someone who knows what to look for. Publishers can lose video revenue for days before identifying the root cause.
SCALE OF EXPOSURE
Around four in five US publisher sites that allow ad serving use GAM. Of those, a significant share of Prebid video inventory currently routes through the Microsoft cache. Publishers who set up their Prebid implementations years ago and have not revisited them since are most likely to be affected, because changes to cache configuration are not a routine part of ad ops.
PREBID VIDEO CACHING: THE FIX OPTIONS
SSP-HOSTED CACHING
Several SSPs and exchanges already offer Prebid video caching as a paid option. Magnite and PubMatic both have this capability. This is the most direct replacement for the Microsoft cache.
The requirement for this is that each publisher must reconfigure their GAM and Prebid setup to point to the new cache source. If the Prebid wrapper has not been touched in years, that might take some scratching around.
PREBID LOCAL CACHE
The Prebid.js working group has developed Prebid Local Cache. Rather than storing the VAST file on a remote server, it holds the creative in browser memory using a JavaScript object and redirects GAM to that local storage. Fewer network calls, no external dependency.
The trade-off: because this approach works around GAM rather than integrating with it natively, it is incompatible with Google IMA, the multimedia ad library used by most sites and apps. The result is a potential loss of some of AdX’s demand for publishers who take this route. Prebid is in conversation with Google to resolve this, but there is no confirmed timeline.
SELF-HOSTED PREBID CACHE
For publishers or platforms with the right infrastructure, operating a dedicated Prebid cache server removes the dependency on any third party entirely. The cache sits within your own infrastructure, under your own SLA.
Monthly operating costs at meaningful scale run to tens of thousands of dollars. It also requires the technical capability to build and maintain that infrastructure. Not a realistic option for most publishers to undertake independently.
It is, however, what THM has done.
HOW THM HAS SOLVED PREBID VIDEO CACHING FOR OUR CLIENTS
THM operates its own dedicated Prebid cache server. Our clients’ video ad serving does not route through Microsoft’s infrastructure and is not affected by this change.
We identified this as a material risk to video revenue well ahead of the April deadline and built the solution at the platform level rather than leaving individual clients to navigate the remediation themselves.
WHAT THIS MEANS FOR THM CLIENTS
- Prebid video auctions cache through THM-controlled infrastructure
- No dependency on Microsoft, or any SSP-hosted caching arrangement
- No configuration changes required on the client side
- No exposure to the AdX demand loss associated with the Prebid Local Cache approach
- Full compatibility with Google IMA maintained
This is a consequence of how Tradecore is built. Because we operate the full stack, including the Prebid wrapper, ad server, analytics, and cache infrastructure, we can make platform-level changes that protect clients without requiring them to manage technical remediation on their own.
Publishers running their own Prebid implementations, or working with managed services that have not addressed this, will need to act before 30 April. All three options above are workable with the right technical support but all require some effort.
THE BROADER POINT
The Microsoft cache situation is a useful illustration of a structural pattern in the open web ad stack. Core infrastructure has accumulated dependencies on large players maintaining services that have no obvious long-term commercial rationale. When those rationales change, publishers absorb the consequence, usually without much notice.
AppNexus absorbed the cost because it was building a business. AT&T inherited the obligation. Microsoft carried it forward. Now that Microsoft is winding down its ad tech operations, including the Invest DSP, there is no longer a reason to maintain public infrastructure for the open web stack.
Publishers most exposed are those who built their Prebid setup years ago and have not revisited it since. Based on the conversations I have, that is common and not at all unusual.
If you are running Prebid video and are not certain whether your setup routes through the Microsoft cache, the answer is most likely yes. The deadline is 30 April.
FREQUENTLY ASKED QUESTIONS
What is Prebid video caching?
Prebid video caching is the process of temporarily storing a video ad creative (a VAST XML file) on an intermediate server during the window between the Prebid header bidding auction and Google Ad Manager making its final serving decision. Without a cache, GAM has nowhere to retrieve the video creative from, and the ad fails to deliver.
What is the Microsoft Prebid cache?
The Microsoft Prebid cache is a caching server that Microsoft Advertising has operated as a free public service for the open web since acquiring Xandr in 2021. Before that it was operated by Xandr, and before that by AppNexus. It currently handles more than 60 percent of Prebid-to-GAM video ad impressions. Microsoft is shutting it down on 30 April 2026.
What happens if I do nothing before 30 April?
Video ad calls that currently route through the Microsoft cache will return VAST errors. The Prebid auction runs correctly, but there is no cache available to retrieve the creative from, so the video does not play. You will lose video ad revenue without necessarily seeing an obvious error in your reporting.
What are my options for replacing the Microsoft Prebid cache?
There are three main options: switch to an SSP-hosted cache such as those offered by Magnite or PubMatic; implement Prebid Local Cache, which stores creatives in browser memory but currently carries a significant AdX demand penalty; or operate your own dedicated cache server. Each requires active configuration work.
Does this affect display ads?
No. The caching requirement applies specifically to video ad creatives. Display ads pass through the Prebid-to-GAM handoff differently and are not affected by this change.
How has THM handled this?
THM operates its own dedicated Prebid cache server. Clients on the Tradecore platform are not affected by the Microsoft shutdown and do not need to make any configuration changes.
TALK TO US
If you are unsure whether your video monetisation is exposed to this issue, or if you want a setup where infrastructure changes like this are handled at the platform level, get in touch.