IAB Tech Lab challenges Prebid transaction ID changes

The IAB Technology Laboratory issued a strong statement declaring that recent changes in Prebid's implementation of the Transaction ID (tid) field materially violate the OpenRTB specification. According to Anthony Katsur, Chief Executive Officer at IAB Tech Lab, "The Tech Lab does not endorse this approach, as it risks undermining the integrity and consistency of open technical standards that are critical to interoperability across the programmatic ecosystem."

The statement, published on August 27, 2025, represents a significant escalation in tensions between the standards body and the header bidding wrapper. It marks the first time the IAB Tech Lab has directly contradicted a major implementation decision by Prebid.org, raising questions about the future of technical standards coordination in programmatic advertising.

Technical standards at stake

The controversy centers on transaction identifiers, which serve as unique auction identifiers across all suppliers throughout the entire supply chain. These identifiers document unique impression transactions and enable demand-side platforms to deduplicate auctions and optimize bidding strategies.

According to the OpenRTB specification, transaction IDs must be common across all participants in bid requests. The standard requires that "transaction ID must be common across all participants in this bid request (e.g., potentially multiple exchanges)."

However, Prebid recently modified its implementation to generate unique transaction IDs for each bidder rather than maintaining a consistent identifier across all exchanges. This change eliminates the primary deduplication mechanism that demand-side platforms relied upon to identify duplicate auction opportunities.

The modification affects how buyers identify when multiple bid requests represent the same impression opportunity. Previously, DSPs could use transaction IDs to prevent bidding on duplicate auctions and implement supply path optimization strategies.

Technical implementation details

The changes implement bidder-specific transaction identifiers in several key areas. The ortb2Imp.ext.tid field now generates unique values for each bidder and transaction ID pair. Similarly, ortb2.source.tid creates unique identifiers for each bidder. Both modifications ignore first-party data that publishers may have configured.

GitHub documentation reveals that the auctionId and transactionId properties of bid requests now reference ortb2.source.tid and ortb2Imp.ext.tid respectively. This represents a fundamental shift in how Prebid handles auction identification across the supply chain.

The implementation changes ensure that each bidder receives different transaction identifiers, even when participating in identical auction opportunities. This modification eliminates the cross-exchange visibility that the OpenRTB specification intended to provide.

Publishers using Prebid Server integration face particular challenges. According to technical documentation, the same logic must be implemented server-side since there exists only one location for source.tid and ext.tid in Prebid Server requests.

Industry response patterns

The modification has generated significant discussion across programmatic advertising channels. Trade publication AdTechRadar reported that the change "potentially removes a widely adopted mechanism for supply deduplication," noting concerns about duplicate bids, wasted spend, and higher costs.

Ari Paparo, CEO of Marketecture, described the development as "breaking news" on social media, explaining that "Prebid.org pushed out a breaking change to Prebid.js that makes the unique ID of every auction (TID) no longer work across exchanges."

Publishers expressed mixed reactions to the implementation changes. Some acknowledged concerns about yield compression that could result from buyers using transaction IDs to identify cheaper supply paths. Others questioned the privacy rationale, arguing that established data protection mechanisms already address identifier linking concerns.

Several industry participants noted the timing significance. The changes occur amid ongoing discussions about supply path optimization and transparency initiatives across programmatic advertising platforms.

Privacy concerns driving change

According to discussions in the Prebid community, privacy considerations motivated the implementation modifications. Publishers expressed concerns that unified transaction IDs could enable buyers to stitch bid requests together across different sellers, potentially exposing connections between identifiers that publishers preferred to keep separate.

The privacy rationale centers on preventing buyers from using transaction IDs as linking keys across bid requests and exchanges. This could theoretically allow demand-side platforms to detect relationships between publisher inventory sources or user identifiers that weren't intended to be linkable.

Industry participants noted that Prebid legal counsel and the publisher committee changed the default setting for transaction identifiers to false two years ago. This earlier change was designed to protect publisher contractual obligations and user privacy while allowing publishers to enable the functionality at their discretion.

However, the recent modifications go beyond default settings. They fundamentally alter how transaction IDs function, regardless of publisher preferences or configuration choices.

IAB Tech Lab's collaborative approach

The standards body emphasized its commitment to finding industry solutions rather than unilateral implementations. According to Katsur, "The right path forward is not to bypass or reinterpret the specification, but to work collaboratively on it."

The IAB Tech Lab announced plans to convene a dedicated forum with publishers, buyers, and technology partners. The organization aims to "chart a path forward that addresses the underlying industry concerns while maintaining compliance with OpenRTB."

The collaborative approach acknowledges the tension that transaction IDs have created within the publisher community. According to the statement, "We fully recognize the tension that the tid field has caused within the publisher community. However, making unilateral changes outside the bounds of the specification creates fragmentation and uncertainty for buyers, sellers, and intermediaries alike."

This represents a significant shift in how technical standards are developed and implemented. Rather than allowing individual organizations to interpret specifications independently, the IAB Tech Lab advocates for coordinated industry discussions before implementation changes.

Historical context and precedent

The OpenRTB specification has undergone numerous updates since its initial release. Previous modifications typically followed formal comment periods and industry consultation processes. Major changes required extensive discussion among working group participants before finalization.

IAB Tech Lab has previously clarified video placement definitions and updated specifications to support new advertising formats. These modifications followed established procedures for technical standard evolution.

The organization has also released specifications for emerging technologies, including updates to support Google's Protected Audience API and new protocols for first-party data matching.

The Prebid transaction ID controversy represents the first major instance where the standards body publicly challenged an implementation that diverged from established specifications.

Market implications and technical debt

The changes create immediate challenges for demand-side platforms that built optimization strategies around consistent transaction identifiers. These platforms must now reassess their deduplication logic and supply path optimization mechanisms.

Publishers face uncertainty about buyer requirements and expectations. Many implemented transaction ID support specifically because demand partners requested the functionality for transparency and deduplication purposes.

Supply-side platforms must evaluate their own implementations to ensure compatibility with both the OpenRTB specification and the modified Prebid behavior. This creates potential fragmentation in how different technology platforms handle transaction identification.

The technical modifications also affect measurement and attribution systems that relied on consistent transaction identifiers for cross-platform tracking and analysis. These systems must adapt to the new bidder-specific identifier model.

Forward-looking technical considerations

The IAB Tech Lab's call for collaborative solution development suggests potential specification updates to address the underlying concerns. These modifications could formalize privacy protections while maintaining cross-exchange transaction visibility.

Potential solutions might include optional transaction ID scoping, publisher-controlled identifier policies, or enhanced privacy controls within the existing framework. The dedicated forum announced by the IAB Tech Lab will likely explore these technical alternatives.

The controversy highlights broader challenges in balancing transparency, privacy, and technical standards in programmatic advertising. As the industry evolves, coordination between standards bodies and implementation organizations becomes increasingly critical.

Timeline

• June 2022: IAB approves Community Extension for per-impression transaction IDs in OpenRTB
• June 2022: Prebid implements initial transaction ID support following OpenRTB specifications
• March 2024: IAB Tech Lab announces Trusted Server initiative for enhanced publisher control
• September 2024: IAB Tech Lab launches PAIR protocol for first-party data matching
• February 2025: IAB Tech Lab clarifies video placement definitions to improve implementation
• August 27, 2025: Prebid implements bidder-specific transaction IDs in version update
• August 27, 2025: IAB Tech Lab issues statement challenging Prebid's implementation as non-compliant

Summary

Who: IAB Technology Laboratory, led by CEO Anthony Katsur, formally challenged Prebid.org's implementation of transaction ID modifications that affect programmatic advertising standards compliance.

What: The IAB Tech Lab declared that Prebid's recent changes to transaction ID implementation materially violate the OpenRTB specification by creating bidder-specific identifiers instead of maintaining consistent transaction IDs across all auction participants.

When: The statement was published on August 27, 2025, following Prebid's implementation of bidder-specific transaction ID changes in their latest version update.

Where: The announcement affects the global programmatic advertising ecosystem, impacting publishers, demand-side platforms, supply-side platforms, and intermediaries that rely on OpenRTB standards for interoperability.

Why: The conflict arose from Prebid's attempt to address publisher privacy concerns about transaction IDs potentially enabling buyer data stitching across exchanges, while the IAB Tech Lab maintains that unilateral specification changes undermine technical standards and industry interoperability.

PPC Land explains

Transaction ID (TID): A unique identifier assigned to each auction opportunity that enables all participants in the programmatic supply chain to recognize when multiple bid requests represent the same impression. The OpenRTB specification requires these identifiers to remain consistent across all exchanges and suppliers for a single auction, allowing demand-side platforms to implement deduplication strategies and supply path optimization.

OpenRTB: The Real-Time Bidding protocol developed by the IAB Technology Laboratory that standardizes communication between buyers and sellers in programmatic advertising auctions. This specification defines technical requirements for bid requests, responses, and related data structures that enable interoperability across different advertising technology platforms.

IAB Tech Lab: The Interactive Advertising Bureau Technology Laboratory, a non-profit consortium that develops foundational technical standards for digital advertising. Established in 2014, the organization maintains specifications including OpenRTB, ads.txt, and various privacy-enhancing protocols that facilitate programmatic advertising operations.

Prebid: An open-source header bidding wrapper that enables publishers to conduct unified auctions across multiple demand sources simultaneously. Prebid.org operates as both the organization managing this technology and the community of developers, publishers, and advertising technology companies that contribute to its development.

Supply Path Optimization (SPO): The practice by which demand-side platforms analyze and optimize the routes through which they purchase advertising inventory. This process relies on consistent transaction identifiers and other bidstream signals to identify duplicate opportunities, evaluate intermediary fees, and direct spending toward the most efficient supply sources.

Demand-Side Platform (DSP): Technology platforms that enable advertisers and agencies to purchase digital advertising inventory programmatically across multiple publishers and ad exchanges. These platforms use transaction IDs and other bidstream data to optimize campaign performance and prevent duplicate bidding on the same impression opportunities.

Deduplication: The technical process of identifying and eliminating duplicate bid requests that represent the same impression opportunity but arrive through different supply paths. This mechanism prevents advertisers from competing against themselves in auctions and reduces wasted advertising spend across the programmatic ecosystem.

Bidstream: The continuous flow of bid requests and responses that occurs during real-time bidding auctions. This data stream contains technical information about available advertising inventory, including transaction identifiers, placement details, user information, and other signals that enable programmatic buying decisions.

Publisher Privacy: The concept encompassing publishers' concerns about protecting their inventory sources, user data, and business relationships from excessive transparency that could enable unfavorable buyer behaviors. In the context of transaction IDs, these concerns focus on preventing buyers from using consistent identifiers to map publisher relationships or optimize against publisher interests.

Technical Standards Compliance: The adherence to established specifications and protocols that ensure interoperability between different advertising technology systems. The IAB Tech Lab's challenge to Prebid centers on maintaining specification integrity and preventing fragmentation that could undermine the programmatic advertising ecosystem's technical foundations.