The Trade Desk has signed Massarius, an Amsterdam-based publisher house, as the first Dutch publisher to join OpenPath, the demand-side platform's direct inventory access program. Marketing Report reported the partnership on June 29, 2026, describing it as a strategic collaboration intended to make the advertising supply chain more transparent, more efficient, and more objective for both sides of the transaction.

The deal gives advertisers buying through The Trade Desk direct access to Massarius display inventory, bypassing several of the intermediary layers that typically sit between a media buyer and a publisher's ad server. According to Marketing Report, this marks the first OpenPath partnership The Trade Desk has closed with a publisher based in the Netherlands, and the companies frame it as evidence of growing demand for more direct, less intermediated routes into premium inventory on the open internet.

What OpenPath does and what changes for Massarius

OpenPath is The Trade Desk's mechanism for connecting its demand-side platform directly to publisher inventory, cutting out layers of the programmatic supply chain that would otherwise sit between a buyer and a seller. Each additional intermediary in that chain - exchanges, resellers, wrapper technology - typically extracts a fee, and those fees compound. Research cited in an IAS supply-path automation announcement found that 42 to 49 percent of advertising dollars fail to reach publishers because of intermediary fees and technical inefficiencies, a figure that gives context to why direct-access programs like OpenPath have become a competitive battleground among demand-side platforms.

The Trade Desk's own AI system evaluates 20 million advertising opportunities every second, according to the Marketing Report account of the Massarius announcement, and that evaluation engine works in combination with OpenPath to determine what the company describes as the optimal route for each transaction. In practical terms, that means the platform's bidding logic can weigh a Massarius impression alongside every other available opportunity in real time, rather than routing the request through a separate reseller layer first.

For Massarius specifically, the integration follows the completion of Deal Desk training, according to Marketing Report - a step that the source material states allows deals to be configured and automated more efficiently within The Trade Desk's platform. The Trade Desk introduced Deal Desk on June 9, 2025, positioning it as a new pillar of its Kokai buying platform designed to give both sides of a private marketplace transaction shared visibility into deal quality, pacing, and performance before and after activation. Disney was among the first publishers to adopt the tool when it launched; the German supply-side platforms Ströer, Virtual Minds, and YOC integrated a related Price Discovery and Provisioning API in February 2026, automating deal configuration and delivery verification for the German market in a move that parallels what Massarius has now completed for the Netherlands.

The fee question and what the industry has learned about OpenPath's economics

OpenPath is not free to publishers. According to reporting on a competing publisher tool launched by Viant in June 2026, The Trade Desk charges publishers a flat 4.5 percent fee for OpenPath integration, a figure that stands in contrast to Viant's SupplyIQ, which charges nothing. The Trade Desk's chief executive, Jeff Green, described that fee in February 2026 as meant to be nearly breakeven to slightly profitable, according to statements made to investors and referenced in the same coverage.

Whether a flat fee that low is meaningfully different from the layered costs OpenPath is meant to replace depends on what a given publisher's supply chain looked like beforehand. Publishers who previously relied on multiple resold paths to reach The Trade Desk's demand would, in principle, see the 4.5 percent OpenPath fee replace a stack of smaller fees taken by exchanges and resellers along the way. Publishers with simpler, more direct supply chains already in place would see less relative benefit.

The program has produced measurable results for some publishers that adopted it earlier. Hearst Newspapers reported a fourfold improvement in fill rates and a 23 percent revenue increase after implementing OpenPath, while The New York Post documented a 97 percent increase in programmatic display revenue attributable to the integration, according to figures Green cited during a Trade Desk earnings call. Those numbers, however, describe large American media companies with substantial existing programmatic infrastructure and negotiating leverage; Massarius, an 11 to 50-employee publisher house, occupies a different position in the market, and no comparable performance figures for the Dutch integration were included in the June 29 announcement.

OpenPath's growth trajectory through 2025 was, by Green's own account, rapid. The Trade Desk chief executive said OpenPath volume grew by many hundreds of percentage points during 2025, a claim repeated across multiple quarterly earnings calls that year. That growth, though, has coexisted with friction at the largest end of the buy side. Adweek reported on February 20, 2026, that Dentsu and WPP had quietly exited OpenPath, citing what the two holding companies described as insufficient transparency around where ads actually ran and undisclosed fees layered into the program - the same transparency premise OpenPath was originally built to deliver. Publicis followed with its own dispute in March, after an independent audit conducted by FirmDecisions, part of the Ebiquity Group, found issues serious enough that the holding company advised some clients to pause work with The Trade Desk altogether; that dispute was resolved by mid-June 2026, though the underlying tension over fee visibility across the platform's supply chain products has not fully dissipated.

None of that buy-side friction concerns Massarius directly. The Dutch publisher sits on the sell side of the transaction, and the concerns Dentsu, WPP, and Publicis raised were about how The Trade Desk discloses fees to advertisers and agencies buying through the platform, not about how OpenPath compensates the publishers supplying inventory. Still, the timing places this partnership inside a period when OpenPath's core transparency claim has faced its most sustained public scrutiny to date.

Who Massarius is and why a Google Certified Publishing Partner matters here

Massarius is not a household name outside the Dutch and broader Benelux advertising market. According to the company's own description, Massarius makes advanced website monetization methods, including automated trading of online advertisements, accessible to publishers who might otherwise lack the technical resources to compete with larger operators. The company was founded in Amsterdam in 2012 and today operates with offices in the Netherlands, the United Kingdom, and Spain, according to its LinkedIn profile.

For publishers, the company states that its analytical staff manage advertising space trading in real time at what it describes as the optimal price, aiming for transparency across all partners in the value chain. For advertisers, Massarius says it selects the publishers it works with to help ensure brand safety, consistent inventory quality, exclusion of non-human traffic, and adequate viewability rates - the standard set of quality controls that supply-side operators use to differentiate premium inventory from lower-quality programmatic supply.

Massarius describes its broader purpose as helping create what it calls a level playing field, one where advertisers and mid-sized European publishers can compete with large technology platforms and access advertising technology on comparable terms. That framing situates the company's OpenPath deal within a wider pattern across European ad tech: mid-sized publishers seeking direct technical integrations with major demand-side platforms as a way to capture more of the value that would otherwise be extracted by intermediaries between them and advertiser budgets.

The company holds Google Certified Publishing Partner status, according to its LinkedIn overview, and has been ranked number one for innovation in the Adformatie Media Report, a Dutch advertising industry publication. Its specialties, as listed on LinkedIn, span yield management, real-time bidding, video and in-app monetization, machine learning applied to ad space management, and technical work in Python and Google BigQuery - the kind of engineering-adjacent skill set increasingly common among European publisher houses that operate more like technology vendors than traditional media sales teams. The company employs between 11 and 50 people, according to LinkedIn, and has accumulated roughly 2,000 followers on the platform.

Statements from both companies

Phil Duffield, Vice President Northern Europe at The Trade Desk, characterized the partnership as part of the company's broader push into the Dutch market. "Our partnership with Massarius is an important step in our expansion into the Netherlands," Duffield said, according to Marketing Report's account of the announcement. He added that the advertising market continues to evolve, and that both advertisers and publishers are seeking greater transparency and efficiency - a framing consistent with how The Trade Desk has positioned nearly every OpenPath-related announcement over the past two years.

Bert Jan ten Kate, founder and owner of Massarius, emphasized the value the direct connection creates for both sides of the transaction. "This direct access effortlessly creates value for all parties involved," ten Kate said, according to the same source. He also linked the integration to Massarius's broader positioning around sustainable revenue for publishers, describing the OpenPath connection as reinforcing the company's commitment to what he called durable income for developers of web content.

Neither statement, notably, included specific performance projections or revenue targets for the Dutch market - a contrast with how The Trade Desk has sometimes framed OpenPath announcements involving larger American publishers, where fill-rate and revenue figures have accompanied the rollout from the outset.

Why this matters for the marketing community

For advertisers and agencies operating in the Netherlands, the practical significance of a single mid-sized publisher joining OpenPath is modest on its own. Massarius is not a top-tier Dutch media property with the scale of, say, a national newspaper group or broadcaster. What the deal signals more clearly is directional: The Trade Desk continues to build out its OpenPath footprint market by market, publisher by publisher, following a pattern already visible in Germany, where three SSPs adopted a related deal-management API in February 2026, and in the United States, where OpenPath expansion has been central to nearly every Trade Desk earnings narrative since mid-2025.

Should the deal deliver results resembling those Hearst and the New York Post reported, that would offer a data point for other mid-sized European publishers weighing whether a flat 4.5 percent OpenPath fee beats the layered costs of their existing supply chains. Should it not, the absence of published performance figures in this announcement - unlike the US cases where fill-rate and revenue statistics accompanied the rollout - may prove telling in itself. For now, the marketing community gets a market-entry signal rather than a performance case study, set against a backdrop in which OpenPath's core transparency premise has drawn sustained scrutiny from three of the world's largest advertising holding companies over the preceding several months. Advertisers evaluating Dutch or Benelux inventory strategies now have one additional direct path to consider, and mid-sized publishers across the region have one more concrete example of what an OpenPath integration looks like in practice, separate from the largest examples the platform has publicized to date.

Timeline

  • 2012 - Massarius is founded in Amsterdam as a publisher-focused advertising technology company.
  • June 9, 2025 - The Trade Desk launches Deal Desk, the platform pillar whose training Massarius later completes ahead of its OpenPath integration.
  • February 6, 2026 - Three German supply-side platforms, Ströer, Virtual Minds, and YOC, integrate a related Trade Desk deal-management API, offering an earlier parallel to the Dutch integration.
  • February 20, 2026 - Dentsu and WPP exit OpenPath over transparency and fee concerns, according to Adweek reporting.
  • June 29, 2026 - Marketing Report publishes the announcement of The Trade Desk's first Dutch OpenPath partnership with Massarius, at 17:46 local time.

Summary

Who: The Trade Desk, a US-based demand-side advertising platform, and Massarius, an Amsterdam-based publisher house founded in 2012 with 11 to 50 employees.

What: The two companies formed a strategic partnership integrating Massarius into OpenPath, The Trade Desk's direct publisher-inventory access program, marking the first such OpenPath integration with a Dutch publisher.

When: The partnership was reported by Marketing Report on June 29, 2026. Massarius completed Deal Desk training ahead of the integration; Deal Desk itself launched on June 9, 2025.

Where: The deal applies to the Dutch advertising market, with Massarius operating from Amsterdam and additional offices in the United Kingdom and Spain.

Why: The integration reflects The Trade Desk's continued market-by-market expansion of OpenPath following earlier moves in Germany and the United States, and reflects growing demand among mid-sized European publishers for more direct, less intermediated access to advertiser demand at a moment when OpenPath's fee structure and transparency claims have drawn scrutiny from some of the industry's largest holding companies.