The number that stands out in the WPP whistleblower filings isn’t the $100 million the complaint is seeking, or even the $1 billion GroupM was allegedly generating annually from undisclosed trading income. It’s 97.4% — the share of GroupM’s proprietary inventory that its own largest clients weren’t allegedly using. Google, its biggest U.S. client at $2.3 billion in annual billings, was using less than 1%.
Those figures came from GroupM’s own internal documents, now in the public court record as a result of the lawsuit. They are the most detailed illustration yet of a straightforward but uncomfortable reality: the clients funding the model weren’t the ones benefiting from it. Whether that changes (and how) is now one of the more pressing questions in the agency business.
In fairness, it always has been for some marketers. To them, the idea of agencies buying inventory for themselves and reselling it to clients — often at a markup and without full disclosure — has never sat comfortably alongside the idea of an agency acting in their best interest. What’s changed is that marketers are no longer willing to live with that ambiguity.
“There are some clients we work with who want to understand, report and have a percentage when it comes to principal media — think how much of total media was bought and how much of that was inventory,” said Tatjana Slykova, chief operating officer and CFO at Abintus Consulting. “Before, they wanted only to see it on the plan. Now, they want to manage it. They want to see it before their agencies put that proprietary media into a live campaign.”
The result is a new wave of contracts that are far more specific. They spell out when an agency can act as a principal, in which channels and under what conditions, eliminating ambiguity about whether media is being brokered on a client’s behalf or resold from inventory the agency already owns. They abandon the pretense of total conflict avoidance. Instead, they try to define which conflicts are acceptable and which are prohibited, drawing explicit lines under undisclosed rebates, kickbacks and incentive structures that reward agency margin over client outcomes.
“Advertisers we work with are focusing more on first understanding how media principal works and then how they can put in a contractual framework to protect themselves against anything happening without their approval,” said Ruben Schreurs, CEO of media management firm Ebiquity.
The shift in advertiser expectations has been building for some time, and the GroupM documents suggest it was being felt inside agencies too. The same internal document reveals that WPP’s own legal team had begun imposing volume and margin caps on proprietary trading deals and was taking a more active role in defining how those arrangements were structured. That kind of internal constraint rarely emerges in a vacuum. It tends to reflect the same advertiser concerns Slykova describes — marketers demanding more oversight, asking harder questions, and making clear that the latitude agencies had previously enjoyed was narrowing.
That awareness, though, is distributed unevenly across the market. The most sophisticated global advertisers have been here for a while — contracts already tightened, penalties already embedded. Behind them is a growing tier of marketers only now realizing how much of their spend has been flowing through principal inventory without their knowledge. Further back still, a long tail of smaller clients who haven’t yet begun to ask the question.
And even where awareness exists, it doesn’t automatically convert into action. The only real moment of leverage is the pitch. Outside of a formal review, renegotiating terms is near impossible. And even clients who do secure the right clauses often discover that a contract without governance behind it is barely worth the paper it’s written on.
“What we found about principal media is it all starts with the contract,” said Bill Duggan, group evp at the Association of National Advertisers. “You need protection in your media agency contract and guidelines for principal media, otherwise, if you don’t have those guidelines, the agency can buy principal media.”
But having the right contract is only part of the problem. Even where advertisers have negotiated caps and approval rights into their agreements, enforcing them in practice is a different matter entirely. One senior marketer who has worked closely with major advertisers on principal media arrangements described a pattern that will be familiar to anyone who has tried to hold an agency to contractual limits.
“The minute you say 10% you assume it’s going to be 10% capped across the board in every market, on every business unit,” they said. “In reality, agencies are so desperate to pile into this that they’ll play fast and loose with the contractual obligations. They might zero out in a whole bunch of markets and then put 50% of one market’s budget through proprietary media where they make the most money.”
The approval process is where it gets murkier still. Contracts may require explicit sign-off on principal media buys, but in practice that sign-off often reaches someone without the context to meaningfully evaluate what they’re approving — a brand manager in a business unit rather than a senior marketing decision-maker who understands the implications. The spirit of the contract, in other words, doesn’t always survive contact with the operational reality of how media gets bought.
A recent Forrester report underscored how uneven that evolution remains. Marketers familiar with principal media described using tighter contracts and more explicit governance to protect their interests. But plenty of advertisers still lack the knowledge or leverage to negotiate those terms effectively. For them, the education process is only just beginning — and cases like Foster’s tend to accelerate it in ways that industry guidelines typically don’t.
The result is a market that has effectively split in two. On one side, advertisers who have seen enough — who have pulled back the curtain, done the math, and decided the conflicts are irreconcilable. On the other, those who have reached the opposite conclusion: that principal media is workable, even valuable, provided the terms are honest and the trade-offs are documented upfront rather than discovered afterward. What neither side disputes anymore is that the old arrangements — opaque, undisclosed, and largely uncontested — are no longer viable. The question isn’t whether principal media gets governed. It’s who does the governing, and how seriously.