CoStar has filed an amicus brief—a brief filed by a non-party, or a “friend of the court”— urging a federal judge to deny Zillow’s effort to block Midwest Real Estate Data (MRED) and Compass from cutting off its listing feed, arguing the portal giant is seeking court-ordered access to MLS listings while hoarding its own exclusive pre-market inventory.
The brief, which CoStar requested to file on June 10 in the U.S. District Court for the Northern District of Illinois before Judge John Tharp, marks the first formal intervention by Zillow’s chief portal rival in the closely watched case. CoStar, which operates Homes.com, says it competes directly with Zillow for the “valuable pre-market listings” at the center of the dispute.
“Zillow wants a court order forcing MLSs to hand over their listings while Zillow hoards its own exclusive pre-market inventory—a breathtaking ‘heads I win, tails you lose’ proposition,” said Gene Boxer, CoStar’s general counsel, in a statement.
Zillow, in a statement sent to RISMedia, noted that CoStar and Compass are making the same flawed argument—that premarketing and private marketing are the same thing. Zillow Preview is pre-marketing—publicly visible for any buyer to see it, save it and connect with the listing agent directly for free; no buyer is required to work with any specific brokerage to access it, their spokesperson explained.
“Compass Private Exclusives are pay-to-play private marketing. Those listings are hidden from buyers unless they work with a Compass agent,” they continued. “The explicit purpose is to route listings through Compass’s own network before—or instead of—making them available to the public. Calling those the same thing is a word game designed to muddy a clear distinction, and it is exactly the kind of conflation that harms buyers and sellers when it goes unchallenged.”
Zillow sued MRED and Compass in May, alleging the MLS and brokerage conspired to cut off its listing feed in retaliation for Zillow’s ban on privately marketed listings. On May 20, the majority of listings in the Chicago area disappeared from Zillow’s feed. Just two days later, a judge ruled for the MLS to restore the feed to Zillow.
Zillow’s motion asks the court to force MRED to restore its listing feed while preserving Zillow’s right to block what it describes as “anti-consumer pre-market listings.” CoStar argues that framing collapses under the weight of Zillow Preview, the pre-market product Zillow launched with exclusive agreements covering more than 60 brokerages.
“Zillow has grown so large that it believes a listing on Zillow’s network alone is, by definition, pro-consumer,” the brief reads. “And a listing not available to Zillow’s network is, by definition, anti-consumer. Zillow needs a reality check.”
CoStar is not a party to the lawsuit or directly involved in the dispute. The company, though, has long set its sights on supplanting Zillow in the residential portal space, and hasn’t shied away from public attacks against its rival.
Zillow previously accused CoStar of “weaponizing” courts to attack business rivals, in a separate lawsuit where CoStar has accused the portal of using listing photos copyrighted by CoStar.
CoStar’s brief contends Zillow Preview replicates the dynamics Zillow attacks in its complaint—pre-market inventory kept exclusive to one platform and unavailable to rivals—and goes further, alleging the product funnels consumers into an ecosystem built on “lead diversion, mortgage steering, and other practices that benefit Zillow at consumers’ expense.”
CoStar further argues Zillow “should come to court with clean hands,” arguing the portal cannot claim irreparable harm from conduct it could avoid, and is itself committing at a larger scale. The brief points to the FTC’s September 2025 suit against Zillow and Redfin over their rental-listing agreement as evidence of a pattern.
The portal did not immediately respond to RISMedia’s request for comment.
