Ad buyers lose key tool as Prebid breaks auction tracking system

Demand-side platforms lost a critical tool for detecting auction manipulation on August 27, 2025, when Prebid.org disabled the cross-exchange functionality of transaction IDs. The change eliminates advertisers' ability to identify when they receive multiple bid requests for the same advertising opportunity across different supply-side platforms.

Michael Sullivan from The Trade Desk described the impact during Marketecture Podcast recorded the following day: "There's 50 like I said; there's a bunch of publishers using them. New York Times, CNN, Forbes, Fox News, whoever, you know, BBC, Reuters, and all of like; it's a breaking change."

Transaction IDs function as unique identifiers that allow demand-side platforms to recognize duplicate auction opportunities. When working properly, they enable advertisers to see that a single advertising slot is being offered through multiple supply paths, helping prevent wasteful spending on identical inventory.

"Transaction ID allows you to sort of harmonize back to what square on a page at what point because maybe there's you know ad refresh occurring or something," Sullivan explained. "So you know this transaction refers to these five requests and these SSPs from the first load and this one is for the one 30 seconds later."

The system had reached significant adoption before the change. Sullivan noted that 59% of bid requests included transaction IDs, with major publishers implementing the functionality. The modification renders these existing implementations useless for their intended purpose.

The Scale of Duplicate Bidding

Sullivan provided stark data illustrating why transaction IDs matter to advertisers. He described finding "on one path, one supply path from one SSP on one device. So like one Roku device in a household in a single day, we received 1.7 million bid requests."

The math reveals the magnitude of auction duplication: "If you were to watch all of those 30 second ads back to back, it would be 14,000 hours of content. And if you did that, that would take you until now, August 2025, until April 2027."

This duplication creates multiple problems for advertisers. First, it increases the volume of bid requests that demand-side platforms must process, straining technical infrastructure. Second, it can lead to artificially inflated auction participation that doesn't reflect genuine inventory scarcity.

"There's just more QPS than any DSP can listen to. And so you actually start hurting market liquidity because stuff gets filtered out of the bidstream," Sullivan noted during the podcast discussion.

How Advertisers Used Transaction IDs

Beyond basic deduplication, transaction IDs enabled several sophisticated use cases for demand-side platforms. Sullivan outlined key applications during the podcast discussion.

"The biggest one which is request duplication. So again, if this is a single square on a page and I get one request from index and two from Pubmatic and two from sovrn but 28 from Magnite or whomever, pick whomever, SSP X, then I and they all share the same auction ID, then I know something is up with that path."

The technology also helped detect ID bridging, where supply-side platforms modify user identifiers. "If everyone is sending us a request and they all say, 'Hey, this is, you know, user ID 123, but then SSP X is saying, actually this is ABCDEF.' We're like, 'Wait a minute. Why did this SSP potentially or this partner the supply path may, you know, swapping out the ID here?'"

Sullivan emphasized that transaction IDs provided context beyond simple duplication detection: "It helps us understand where we're competing and what this is in reference to."

The Abrupt Change

The modification arrived without industry consultation, catching stakeholders off-guard. "There was no notice and very light communication," Sullivan said. "I don't feel clear on sort of like what drove that decision. And it may very well be the right decision, but it certainly came to me as a surprise and it's a breaking change."

Chris Kane from Jounce Media described the timing significance: "As of two days ago, a publisher could make this decision of are we going to support transaction ID or not. There was a heated debate about should we or shouldn't we, but now there's no more debate. Can't do it. It's just not available to you. The choice was made for you by pre-bid."

The change fundamentally alters how the system works. Instead of providing consistent identifiers across all supply-side platforms for the same auction, each platform now generates different transaction IDs. This eliminates the cross-reference capability that made deduplication possible.

Industry Standards Violation

The IAB Technology Laboratory condemned the change as violating established specifications. According to their August 27 statement, the modification "materially violates the OpenRTB specification" and "risks undermining the integrity and consistency of open technical standards."

The OpenRTB specification explicitly requires transaction IDs to remain consistent across bidders for the same auction opportunity. Prebid's change breaks this fundamental requirement, creating the first major standards clash between the header bidding wrapper and the industry standards body.

Impact on Auction Quality

Sullivan argued that transaction IDs served broader market health beyond individual advertiser benefits. "When advertisers continue to run budgets in the open internet and performance degrades? Eventually, they stop running them out here and performance degrades because the buyer's algorithm is not grabbing a clean signal that of what they actually want to buy."

The removal affects advertisers' ability to optimize their bidding strategies. Without transaction IDs, demand-side platforms lose visibility into supply chain mechanics that influence auction dynamics and pricing.

"Performance degrades for two reasons," Sullivan explained. "One is that spend goes to the publishers that create the most auction duplication, not the publishers that make the best ad product. And two, which is super in the weeds, is there's just more QPS than any DSP can listen to."

Publisher Resistance Context

The change occurs amid publisher concerns about transaction ID adoption. Kane noted that "publishers are scared of the trade desk" and worried about revenue impacts from increased buyer transparency.

Some publishers benefited from auction duplication as it increased their effective yield by creating additional bidding opportunities. Transaction IDs threatened this advantage by enabling advertisers to identify and potentially reduce spending on duplicated inventory.

"Publishers who are doing a lot of request duplication well" might lose revenue, Sullivan acknowledged, "but I also think like publishers who are not who are high-quality publishers are currently getting under-monetized as a result of it."

Alternative Detection Methods

Despite losing transaction IDs, demand-side platforms retain other tools for identifying suspicious auction patterns. Sullivan noted that sophisticated buyers already employ multiple detection methods.

"From a Sincera perspective like you know we never say oh we went to this site or property or watched this CTV and saw something once and therefore that's the truth like we have to see it over and over and over again," he explained. "So like there's definitely like magnitude and repeatability of some of these different pieces."

However, transaction IDs provided a more definitive and immediate identification method compared to statistical analysis approaches that require observing patterns over time.

Market Implications

The elimination of transaction ID functionality represents a step backward for auction transparency advocates. Kane characterized it as "a real step backwards that publishers can't make that choice right now" regarding transaction ID implementation.

For demand-side platforms, the change removes a tool that had become integrated into bidding algorithms and optimization strategies. Advertisers who had begun incorporating transaction ID data into their programmatic operations must now rely on less precise alternative methods.

"Trade desk doesn't need to self-preference to win. Trade just needs a clean auction to win," Sullivan stated, arguing that transparency benefits all market participants rather than advantaging specific platforms.

According to PPC Land's reporting, the controversy reflects fundamental questions about programmatic advertising's future structure and transparency requirements. The standards clash demonstrates ongoing tensions between technical standardization and commercial interests in the programmatic supply chain.

The publication notes that Brian O'Kelley predicted the development could accelerate direct supply relationships between demand-side platforms and publishers, potentially bypassing traditional programmatic intermediaries.

Technical Implementation Challenges

Publishers who invested engineering resources in transaction ID support based on OpenRTB specifications now face technical debt from the standards change. The modification particularly affects publishers using Prebid Server integration, where implementation options remain limited.

The breaking change nature means existing implementations cease functioning as intended, requiring publishers to evaluate whether continued transaction ID support provides any remaining value under the new limitations.

Future Outlook

The incident raises questions about governance processes in open-source advertising technology development. With the IAB Tech Lab directly challenging a major implementation decision, the industry faces uncertainty about how similar conflicts will be resolved.

Demand-side platforms must now adapt their auction analysis and optimization systems to function without reliable cross-exchange transaction tracking. This may accelerate development of alternative transparency tools and supply chain monitoring capabilities.

Timeline

• June 2022: IAB approves Community Extension for per-impression transaction IDs in OpenRTB specification
• June 2022: Prebid implements initial transaction ID support following OpenRTB specifications
• June 9, 2025: Trade Desk launches Deal Desk platform for advertising deal management
• August 12, 2025: Michael Sullivan posts LinkedIn question about transaction ID implementation concerns
• August 27, 2025: Prebid.org releases breaking change eliminating cross-exchange transaction ID functionality
• August 27, 2025: IAB Technology Laboratory declares Prebid changes violate OpenRTB specification
• August 28, 2025: Industry debate recorded on Architecture Podcast with Sullivan and Kane
• August 29, 2025: Brian O'Kelley posts LinkedIn analysis predicting DSP direct supply relationships

PPC Land explains

Transaction ID: A unique identifier assigned to individual advertising impressions within programmatic auctions that enables tracking across multiple supply-side platforms. When functioning properly, transaction IDs allow demand-side platforms to recognize when different bid requests represent the same advertising opportunity, preventing duplicate bidding and enabling more efficient auction participation. The system was designed to provide transparency in the complex web of programmatic advertising intermediaries.

Demand-Side Platform (DSP): Technology platforms that enable advertisers and agencies to purchase digital advertising inventory programmatically across multiple ad exchanges and supply sources. DSPs like The Trade Desk use algorithms and data to optimize bidding strategies, targeting, and campaign performance in real-time auctions. These platforms rely on transparency tools like transaction IDs to make informed purchasing decisions and avoid wasteful spending on duplicate inventory.

Prebid: An open-source header bidding wrapper that enables publishers to offer their advertising inventory to multiple demand sources simultaneously before making calls to their ad server. Prebid.org maintains the technology that powers header bidding implementations across thousands of websites, making it a critical infrastructure component in programmatic advertising. The organization's decision to modify transaction ID functionality affects the entire programmatic ecosystem.

Supply-Side Platform (SSP): Technology platforms that help publishers manage, sell, and optimize their available advertising inventory. SSPs connect publishers to multiple ad exchanges, demand-side platforms, and other buying sources to maximize revenue from their advertising space. In the transaction ID context, SSPs are where duplicate bid requests often originate as publishers work with multiple partners to monetize the same inventory.

Request Duplication: The practice where the same advertising opportunity appears multiple times through different supply paths, creating multiple bid requests for identical inventory. This can occur when publishers work with several SSPs or when supply chain intermediaries create additional auction opportunities. Request duplication increases processing costs for advertisers while potentially inflating publisher revenues through artificial scarcity.

OpenRTB Specification: The industry standard protocol that defines how real-time bidding communications should work between buyers and sellers in programmatic advertising. Maintained by the IAB Technology Laboratory, OpenRTB specifications ensure interoperability across different platforms and technologies. The specification explicitly requires transaction IDs to remain consistent across bidders, making Prebid's change a standards violation.

Auction Deduplication: The process of identifying and eliminating duplicate bid opportunities to prevent advertisers from bidding multiple times on the same inventory. This capability relies on consistent identifiers like transaction IDs to recognize when different bid requests represent identical advertising opportunities. Without effective deduplication, advertisers waste resources processing unnecessary bid requests and may inadvertently compete against themselves.

IAB Technology Laboratory: The standards development organization that creates technical specifications for digital advertising, including the OpenRTB protocol that governs real-time bidding communications. The organization issued a rare public statement condemning Prebid's transaction ID changes as violating established standards. This represents the first major public disagreement between the standards body and a key industry implementation.

Header Bidding: An advanced programmatic advertising technique that allows publishers to offer inventory to multiple demand sources simultaneously before calling their primary ad server. This creates more competition for publisher inventory and typically increases revenues compared to traditional waterfall methods. Prebid is the dominant open-source solution enabling header bidding implementations across the web.

Breaking Change: A software modification that eliminates backward compatibility, requiring users to update their implementations or lose functionality. In the context of Prebid's transaction ID modification, the breaking change meant that existing publisher implementations suddenly stopped working as intended. Such changes are typically announced well in advance to allow stakeholders time to adapt their systems and processes.

Summary

Who: Demand-side platforms, advertisers, and The Trade Desk lost access to transaction ID deduplication capabilities when Prebid.org made implementation changes affecting major publishers.

What: Prebid disabled cross-exchange transaction ID functionality, eliminating advertisers' ability to detect duplicate bid requests for the same advertising opportunities across multiple supply-side platforms.

When: The breaking change was implemented on August 27, 2025, without advance notice or industry consultation.

Where: The modification affects programmatic advertising auctions globally, impacting advertiser spend optimization and supply chain transparency.

Why: Privacy considerations reportedly motivated the change, though it removes valuable transparency tools that helped advertisers detect auction manipulation and wasteful spending on duplicate inventory.