Retail media is a $59 billion market in the US and projected to hit $69 billion in 2026. But the infrastructure underneath most retail media networks was never designed for what advertisers expect today: member-level targeting, closed-loop attribution, and measurement that connects digital ad exposure to in-store purchases.

The result is a growing gap between what retail media promises and what the tech stack can actually deliver.

A recent conversation between Kiri Masters, Mark Williamson (AVP Retail Media, Costco), and Jon Flugstad (CBO, MetaRouter) put this problem in sharp focus. Costco has been building its retail media tech stack in public, making architectural choices that break from how most networks have been assembled. The discussion surfaced a set of principles that apply well beyond Costco.

Why the old retail media tech stack doesn't work anymore

Five years ago, the retail media vendor landscape was thin. A handful of ad servers, a few DSP partnerships, and limited identity resolution options meant most retailers ended up with similar stacks. As Flugstad put it during the conversation: "You had Criteo, CitrusAd, PromoteIQ. Then you had LiveRamp and The Trade Desk, and you're doing Meta. That was it. There wasn't that much that was different overall."

That uniformity masked a structural problem. Most of these stacks were monolithic, vendor-dependent, and built around visitor-level tracking rather than true customer identity. They worked well enough when retail media was a line item, but they break down when advertisers start asking harder questions about incrementality, cross-channel attribution, and real business outcomes.

The IAB and MRC released attention measurement guidelines in late 2025, signaling that the industry recognizes the gap. Research from Bain and eMarketer found that 48% of respondents identify measurement and attribution as the top challenge facing retail media networks, and 59% of RMN operators say improving measurement capabilities is their top strategic priority.

The old stack was built for impressions. The market now demands outcomes.

What a composable retail media tech stack looks like

The shift underway is from monolithic vendor platforms to composable, modular architectures where retailers control the data layer and plug in specialized tools at each level. As Masters wrote in her detailed breakdown of Costco's tech stack reveal: "By partnering with established vendors at each layer rather than building proprietary systems, Costco made choices that aligned with their merchant-first approach and need for speed to market." Amazon, Walmart, and Instacart built their advertising ecosystems largely in-house. Costco made a different bet.

Their publicly shared architecture divides into four layers:

Unified data foundation. A private Google Cloud instance consolidating 100% member-identified transaction data. Identity resolution handled by MetaRouter and LiveRamp. Privacy managed by Transcend. Audience segmentation powered by GrowthLoop.

Activation channels. Onsite advertising through a new partnership with Moloco for AI-powered reserved display, alongside Criteo-powered sponsored products. Offsite through The Trade Desk, Google, Yahoo DSP, Epsilon, and StackAdapt. An emerging "in real life" channel including fuel pump video ads across 575 locations and Costco Connection magazine.

Measurement. Clean rooms via Habu, providing closed-loop reporting on new-to-brand and new-to-category metrics that connect digital ad exposure to in-warehouse transactions.

Operations. Self-serve and managed service options, standardized workflows, automated campaign management.

This isn't a framework on a whiteboard. These are real vendor decisions, made public, with real trade-offs. And Williamson was clear that none of it came easy. Costco's data was fragmented across disparate systems, with identifiers that had nothing to do with the actual member. Getting approval to access, organize, and use that data required clearing legal, privacy, and policy at every step.

"That friction forced a lot of clarity in the vision," Williamson said. "It's taken longer, it's cost more money, and it's been way more complicated than it needs to be. But it had to be this way for it to work."

That last line is worth sitting with. The composable approach is harder than buying a monolithic platform. It takes longer. It costs more upfront. But it produces an architecture that belongs to the retailer, not the vendor. And it's modular enough that when a component underperforms or a partner relationship changes, you can swap it without rebuilding from scratch.

Flugstad framed the timing advantage: retailers entering the space now have access to modern tooling that simply didn't exist five years ago. "Mark and the Costco crew have this really interesting opportunity, being a late mover, to basically build modern tooling in the right way. It's a built-in arbitrage opportunity."

Identity resolution is the layer that makes everything else work

Every other capability in a retail media tech stack depends on identity. Targeting requires knowing who the customer is. Measurement requires connecting the person who saw the ad to the person who bought the product. Personalization requires stitching together behavior across channels.

Epsilon's research on retail media growth found that underperforming RMN campaigns are frequently the result of inadequate identity resolution. When retailers can't accurately connect multiple data signals to a single customer, they default to over-targeting the same easily identifiable shoppers, missing new customers entirely.

The technical challenge is compounded by browser restrictions. Safari's Intelligent Tracking Prevention limits cookie persistence to seven days and retains campaign URL tracking data for just 24 hours. That means any identity strategy built on client-side cookies is already degraded for a significant share of web traffic.

Williamson described the problem in terms that have nothing to do with ad tech: "We actually manage that historically as 10 different businesses. Ecommerce, warehouse, same-day, Costco Travel, tires, fuel. Each with its own systems. But it's one member, one Costco. They don't care that structurally speaking, Costco Travel is a different business entity and has a different P&L and a different tech stack. Our job is to connect that together."

That connection is what identity resolution at the infrastructure level provides. It's the difference between operating on visitor-level data and operating on member-level data.

The practical consequence is visible in how Costco now evaluates new technology partners. One of the first questions they ask any vendor that wants to join the Velocity stack: have you integrated with MetaRouter? Because the data and identity layer is the path in and the path out. If a vendor can't connect to it, they can't participate.

Williamson called it plumbing. "It's less about the interface, it's more about the pipes that connect things together. That connective tissue is what makes the stack as modular as we need it to be."

For Costco, the transition from visitor-level to member-level data is what enables everything from personalized onsite display to closed-loop measurement of warehouse sales. And it connects to a bigger thesis that drives the entire retail media strategy: more relevant experiences lead to more conversion, more conversion leads to higher membership renewal rates. Membership is the engine that makes Costco work. Retail media's job, in Williamson's framing, is to make that flywheel spin faster.

The case for transparency in a secretive industry

A LinkedIn survey conducted by Kiri Masters found that 88% of brands say a retailer's tech stack influences their advertising spending decisions. Yet most retail media networks refuse to disclose their infrastructure, even after contracts are signed.

Masters laid out five reasons in her original Forbes column: shame over suboptimal stacks, financial narrative control, a knowledge gap around what media buyers actually want to know, margin protection through opacity, and fear of being copied. In one case she documented, a retailer's communications team delayed announcing a technology partnership for a full year to manage the financial narrative for investors. In another, an executive wanted to share that an RFP had been completed, but the product team forbade them from revealing the decision.

Williamson has been in retail media for 15 years, across Sam's Club, Ahold Delhaize, Epsilon, and CitrusAd. He was candid about the pattern he's seen repeated: "I've been one of the people that's been insecure about my platform. I've been ignorant of my technology partners. And that does put you at a disadvantage when you're trying to have a JBP conversation with a supplier, or you're talking with an agency and you just don't feel confident in your stuff."

Costco went the opposite direction. In January 2026, Williamson walked into an NRF side event and put his entire tech stack on screen. Masters covered the moment: "The reaction in the otherwise subdued room was instant. Nearly everyone raised their phones to photograph the slides." It was the first time a US retail media network had disclosed its full infrastructure publicly.

Williamson's reasoning was rooted in Costco's operating culture: "We are doing partnerships with people that supply goods to Costco that our members love. I feel like it's our obligation to respect them enough to tell them what we're doing and how we're doing it."

The transparency serves a practical function beyond trust. When Costco says no to a capability request, they can point to the stack and explain why. "We can point to and say, this is why it's no, or maybe this is why it's not yet. Because we haven't connected this box with this box, but that's where we're going to go."

Masters put it bluntly in her follow-up analysis: when Amazon and Walmart capture roughly 90% of incremental retail media spending, the smaller networks competing for the remaining share need every advantage available. Transparency may be one of the cheapest and most effective differentiators on the table. As she noted, Costco's experience "points to something I didn't cover then: transparency makes the internal job easier too. It forces clarity. It creates accountability to a public roadmap."

What this means for the next wave of commerce media

The architecture decisions retailers make today will determine whether they can participate in what comes next. Agentic commerce, AI-assisted shopping, and LLM-powered product discovery are already live at retailers like Target, Walmart, and Amazon. The question for every other retailer is whether their tech stack is modular enough to connect to new channels as they emerge.

Williamson's framing of Costco's approach reflects this: "Google today, ChatGPT tomorrow, perhaps. The way that we architect that solution, it has to leave the door open for expansion into wherever people are searching."

Williamson described Costco's posture as deliberate patience rather than hesitation: "Costco doesn't give into peer pressure. There's an incredible amount of patience to let everybody else learn and do what they're going to do. Let's just be really good at what we do."

But patience isn't passivity. The modularity that makes composable stacks harder to build in the short term is what makes them more durable in the long term. A retailer locked into a monolithic vendor platform has to wait for that vendor to build integrations with new channels. A retailer with a composable stack built around a central data and identity layer can plug into new activation channels as they become available. Williamson's team is already asking of every architecture decision: does this leave the door open for whatever comes after search?

Flugstad summed up the underlying principle: "My uncommon belief is that it's not what's in the boxes. It's how the boxes are connected. You could do it right and get it wrong."

For retailers evaluating their retail media tech stack, the question isn't which vendors to choose. It's whether the architecture allows the data to flow where it needs to go, with identity attached, in real time, without leaving the retailer's environment. The retailers that solve for that will be the ones ready for whatever comes next.

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