Programmatic’s ID Power Struggle, and the Search for Common Ground

Prebid’s TiD changes fractured transparency. Mediation Auction IDs could be the unifying signal programmatic desperately needs.

Programmatic transparency has hit a weird snag. The one signal everyone trusted: the TiD, isn’t what it used to be. Prebid’s changes have scrambled the consistency, DSPs are grumbling about noise, and SSPs are guarding their turf like a cat sitting on a warm laptop. It’s a standoff: buyers need clarity, sellers need protection, and nobody wants to blink first.

Of course, this isn’t the first standoff programmatic has seen. The ecosystem has always been a balancing act between complexity and efficiency. On one side, the open marketplace enables advertisers to reach audiences at scale across thousands of publishers. On the other, every additional layer that you can find in the Lumascape (think mediation, SSPs, exchange, DSPs, DMPs, etc), adds value but also ends up adding more hops, and sometimes more opacity.

In recent years, transparency has become programmatic’s North Star. Initiatives like Supply Path Optimization (SPO), direct publisher integrations, and open standards all reflect a simple truth: when buyers and sellers see how impressions move through the chain, pricing improves, waste drops, and trust grows. 

But transparency comes with tension. DSPs want clarity to sharpen bidding, while publishers and SSPs want to protect yield strategies and control how their inventory is seen. That balance has always been delicate, and the latest changes to how transaction identifiers are handled have made it even harder and more urgent to resolve.

The debate around Prebid’s decision to alter how Transaction IDs (TiDs) are assigned has crystallized the transparency tension. For years, OpenRTB defined the TiD as the common thread stitching an auction together, with source.tid meant to remain consistent across all participants for reconciliation.

That convention shifted in August 2025, when Prebid began issuing bidder-specific TiDs. The change was positioned as a way to protect publisher pricing and reduce leakage, but it fractured the industry’s one shared reference point. Analysts flagged the move as a departure from OpenRTB’s intent, and the IAB Tech Lab publicly challenged it as non-compliant and risky for interoperability.

This moment has forced a larger question: if a single exchange-level TiD is no longer guaranteed, what other signals can preserve a shared view of reality without compromising publisher controls? Increasingly, attention is turning to the Mediation Auction ID (AID), a complementary identifier reshaping how the industry approaches SPO. Together, TiDs and Mediation AIDs mark a turning point: transparency is no longer optional, but its design must work for all sides.

The Industry’s Transparency Tug-of-War

Transparency in programmatic is not just a philosophical preference, it’s an operational requirement. 

For buyers, transparency signals enable more accurate decision-making. A DSP receiving dozens of requests for the same impression needs a way to distinguish unique opportunities from duplicates. Without that clarity, bidding logic becomes inefficient, infrastructure costs skyrocket, and campaign performance suffers. 

For publishers and SSPs, transparency is equally important, though in different ways. Publishers want assurance that their inventory is being represented accurately in the marketplace and that pricing strategies are respected. SSPs, meanwhile, must balance the demands of buyers for cleaner signals with their responsibility to protect publisher value and maintain competitive yield. 

This tension defines the transparency tug-of-war: each side recognizes its importance, but the trade-offs between efficiency, control, and revenue protection create friction. It is within this backdrop that the TiD controversy has unfolded. 

TiDs: The One Identifier Everyone Could Agree On… Until Now

Since their introduction in OpenRTB 2.5, Transaction IDs (TiDs) have served a simple but vital purpose: to act as a unique identifier for an auction. In practice, a TiD functions like a license plate for an impression. It ensures that all participants DSPs, SSPs, and exchanges reference the same transaction consistently across logs and reporting. 

This consistency has long been critical for reconciliation. Buyers could align their bidding data with exchange logs, publishers could confirm how their impressions were auctioned, and intermediaries could troubleshoot discrepancies. In short, TiDs were a signal of trust in a complex and multi-hop supply chain.

Prebid’s Big Little Change: One Auction, Many IDs

Earlier this year, Prebid introduced a major change to how TiDs are generated. Instead of assigning a single TiD per auction, Prebid now creates unique TiDs per bidder, per request.

The rationale was straightforward. By introducing bidder-specific identifiers, Prebid sought to:

• Protect publisher pricing strategies by preventing buyers from reverse-engineering supply paths. 
• Reduce the risk of impression leakage that could enable disintermediation. 
• Reinforce user privacy in an era where data signals are under increasing scrutiny. 

But the consequences have been significant. For DSPs, this change effectively breaks the chain of consistency. Without a single TiD traveling across all bidders, DSPs lose the ability to trace a single impression across multiple SSPs. This weakens SPO efforts, complicates log reconciliation, and forces DSPs to operate with less clarity in an already noisy environment. 

The IAB Tech Lab has gone so far as to challenge the change, arguing that bidder-specific TiDs undermine the interoperability that open standards are designed to ensure. Their concern is that such fragmentation erodes one of the few stable signals buyers have relied upon for transparency. 

The debate has become emblematic of a broader industry crossroads: how do we balance publisher protections with buyer efficiency in a way that sustains trust across the ecosystem?

The AdTech Balancing Act: Guardrails vs. Glass Walls

The TiD controversy highlights the industry’s central dilemma: privacy and transparency are both essential, but they often operate in tension.

• Privacy safeguards users and publishers. It ensures that sensitive information is not exposed in ways that could be exploited. 
• Transparency drives efficiency, fairness, and accountability, ensuring that advertisers and DSPs can make informed bidding decisions. 

Neither can be compromised. The challenge lies in designing systems that honor both principles without tipping the scale too far in one direction. 

This is where newer transparency signals, such as the Mediation Auction ID, enter the conversation.

Beyond TiDs: The Rise of Mediation Auction IDs

Where TiD lives at the exchange layer, a Mediation AID is created at the mediation layer, the point in the in-app stack where a platform like AdMob, AppLovin MAX, or ironSource orchestrates a unified auction or a hybrid of real-time bidding and waterfall. Mediation systems decide which connected networks/SSPs may compete and how, then fan requests out and collect responses. 

OpenRTB already acknowledges the presence of an upstream decision maker through the source.fd flag, which indicates whether the final impression decision was taken by the exchange (0) or an upstream source (1). In SDK bidding scenarios, many exchanges explicitly set fd=1 to reflect that the mediation platform made the call: an admission, in effect, that the “auction of record” sits one layer higher.

A Mediation AID captures the identity of that higher-level auction event. When the mediation platform assigns and propagates a per-auction identifier downstream, it gives demand a way to recognize that multiple requests possibly arriving through different SSPs, are actually the same underlying impression opportunity. That one piece of context unlocks a cascade of practical benefits:

• Reliable deduplication and smarter modelling. With a Mediation AID, a DSP can collapse dozens of near-simultaneous requests into a single decision object, reducing noisy spend, stabilizing pacing, and improving first-price strategies. Mediation AIDs are arguably the most relevant ID for deduplication, enabling consolidation of redundant requests tied to the same impression.  
• Clearer price evaluation. Mediation is where publishers express yield preferences- floors, priorities, price experiments. A Mediation AID gives buyers a consistent handle to observe how a given opportunity was valued within the mediation stack, allowing pricing logic to reflect publisher-side intent rather than just network-level heuristics.  
• Lower infrastructure cost for everyone. The ability to recognize “many paths, one impression” has direct systems implications. Instead of spinning up inference and targeting models for each duplicate, a DSP can route computation to one canonical request, trimming bandwidth, CPU, and storage without sacrificing win rate, exactly the kind of efficiency gain the industry has been seeking as duplication grows.

The TiD + Mediation AID Equation: Complementary, Not Competitive

It’s tempting to see Mediation AIDs as a reaction to evolving TiD practices, but they’re better understood as orthogonal. A TiD, when consistent, gives you a shared exchange-level handle; a Mediation AID gives you visibility into the auction of origin one level upstream. In combination, they provide a double-entry system that describes both where an auction was decided (mediation vs. exchange) and how that decision propagated across the hops to demand. 

This duality already exists in the protocol. The source.fd flag distinguishes where the final decision is taken; the schain object expresses the entities in the payment path; ext fields provide room for supply partners to carry additional context such as a Mediation AID. Several exchanges explicitly document both tid requirements and the use of ext to carry vendor-specific context, a natural home for a mediation-level identifier.  

In short: a TiD (when present) is the cross-participant thread at the exchange layer. A Mediation AID describes the upstream decision event. Together, they restore transparency while honoring the realities of modern in-app monetization. When a shared TiD is present, passing it alongside the Mediation AID creates the richest transparency. Where bidder-specific TiDs are in use, a Mediation AID restores a common reconciliation handle. The approach is neither prescriptive nor exclusionary: both identifiers can coexist and serve different but complementary truths about the same opportunity.

What Makes Mediation AIDs A “Hero Signal”

If transparency is about improving outcomes for both buyers and sellers without overexposing publisher strategy, Mediation AIDs check every box:

• Auction-scoped, not user-scoped 
  They expire with the event, carry no user data, and minimize privacy concerns while staying useful for pricing and reconciliation.
• Reflect real decisioning 
  In SDK bidding, mediation decides the winner, not the exchange. Mediation AIDs align transparency with that reality, operationalizing what OpenRTB’s fd flag signals. 
• Protect publishers without overexposing strategy 
  AIDs can be random and opaque, enough for deduplication and log reconciliation, but not enough to reveal floors or waterfalls. 
• Enable honest multi-hop measurement 
  When an impression travels across multiple SSPs, the AID ensures each path is recognized as part of the same opportunity. 
• Ready to use today 
  They can be implemented immediately via OpenRTB ext fields, no overhaul required. 
• Align incentives on both sides 
  DSPs gain cleaner signals and stronger SPO, while publishers see proof their rules are respected, leading to greater confidence, higher bids, and stronger yield over time.

Two Sides, One Signal: How Mediation AIDs Empower Buyers and Sellers

It’s important to acknowledge the merits on both sides of the TiD debate. Publishers and wrappers have legitimate reasons to guard against path inference that could undercut yield or reveal sensitive mechanics. At the same time, buyers can’t be expected to optimize in the dark; deduplicating requests and aligning modelling to the true unit of competition is foundational, not optional. 

From a buyer’s perspective, Mediation AIDs provide deterministic grouping of requests that belong to one auction, unlocking cleaner SPO, tighter pacing, and more robust first-price strategies. Where duplication is high, the savings in compute and network load are tangible. 

From a publisher and SSP perspective, Mediation AIDs are a way to prove and not just claim that the marketplace is honouring the logic set at mediation. Rather than revealing the logic itself, the signal simply helps the rest of the chain understand that multiple paths point back to the same decision event. Randomized and opaque, AIDs enable deduplication and reconciliation without exposing floors or waterfalls. This balance lets DSPs optimize supply paths and price more confidently, while giving publishers visibility into performance across mediation stacks. 

What makes Mediation AIDs compelling is that they square this circle. They restore a shared view of “what’s the same” without exposing “why it was valued that way.” They make it easier for DSPs to cut waste and for publishers to verify that upstream rules are reflected downstream. And they do it with low privacy risk, using instrumentation the market already understands.

InMobi’s Stance: Anchoring on Transparency as a Long-Term Commitment

For InMobi, the transparency debate is not about choosing sides between DSPs and SSPs but about helping the ecosystem progress toward shared value. We believe that identifiers like the Mediation Auction ID represent the type of practical innovation the industry needs-signals that reduce duplication and inefficiency for buyers while giving publishers confidence that their inventory is being valued and transacted with integrity. 

This perspective reflects InMobi’s broader stance: transparency is not a short-term tactic or a differentiator to be guarded; it is a foundational principle for sustainable programmatic growth. By leaning into transparency signals like Mediation AIDs, TIDs, and GPID, InMobi aims to bridge the needs of both demand and supply, fostering a marketplace that is efficient, trustworthy, and resilient.

A Brief Note On Scope: Mediation AIDs vs. Other Signals

The ecosystem employs several transparency signals, some publisher-defined, some standardized, some community-led. While those have merit, the focus here is intentionally narrow: Mediation AIDs speak directly to the structure of in-app auctions, which is where much of today’s duplication and opacity originates. They’re not a silver bullet for every transparency challenge, but for reconciling what is being auctioned and where that auction was decided they’re unusually high-leverage.