CoStar filed an amicus brief today opposing Zillow’s motion for a preliminary injunction in its federal lawsuit against Midwest Real Estate Data (MRED) and Compass.
The filing argues that Zillow is demanding court-ordered access to MLS listings while running its own pre-market listing program that withholds inventory from the broader market.
CoStar is requesting the court to deny Zillow’s motion.
What Led to the Zillow vs. MRED Case
The backstory goes back to April 2025, when Zillow created its Listing Access Standards (LAS), prohibiting agents from publicly marketing listings outside of Zillow first. The policy was widely read as a direct response to Compass’s private listing strategy, which routes listings through a phased marketing process before they hit the broader market.
Compass filed to block Zillow from enforcing the policy in July 2025. After a four-day hearing, the court denied Compass’s motion in November 2025, allowing Zillow to keep enforcing LAS while the suit proceeded.
Compass dropped its case entirely in March 2026 after Zillow clarified that listings first marketed on Compass-owned sites or Redfin wouldn’t automatically be banned.
That same month, Zillow launched Zillow Preview, its own pre-market listing product.
In April 2026, MRED opened its private listing network to all licensed agents nationwide, with Compass as the first to join.
By May, Zillow had filed a federal antitrust lawsuit against MRED and Compass, alleging they conspired to withhold Chicagoland listing data unless Zillow agreed to display Compass private listings nationally.
Here’s how the case escalated from there:
- May 19, 2026: MRED sets a deadline for Zillow to restore all eligible listings or face suspension of its IDX and VOW feeds
- May 22, 2026: Judge John Tharp grants Zillow’s injunction in part, ordering MRED to restore thousands of Chicagoland listings, but rules Zillow cannot ban listings in ZIP codes where MRED had listings between April 2025 and April 2026
CoStar’s Filing & Claims
CoStar Group is the parent company of Homes.com, one of Zillow’s primary competitors in the residential listings portal space. A hearing on the motion is scheduled for early July.
The central argument in CoStar’s court filing is that Zillow is asking the court to force MLSs to hand over listing data. At the same time, Zillow is running Zillow Preview, its own program that withholds pre-market listings from the broader market.
The brief states that Zillow Preview “mirrors the exact conduct of which Zillow complains in this case, just on a larger scale.”
CoStar also claims that Zillow’s long-term play is to replace the nonprofit MLS system with a Zillow-controlled ecosystem, keeping MLSs feeding it data until Zillow no longer needs them.
To support that argument, the brief states:
- The FTC previously sued Zillow for paying a rival to stop competing
- Zillow faces class action suits over allegedly steering consumers toward overpriced mortgages
- 99.7% of users on the platform don’t know who they’re actually contacting when they click “Contact Agent”
CoStar’s General Counsel Gene Boxer stated in a release accompanying the filing:
“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.
“The Court should see this motion for what it is: an attempt to weaponize the judicial system to entrench Zillow’s dominance at the expense of competition, consumers, and the MLS system that has served the industry for decades.
“Zillow’s strategy is simple and troubling: use its dominance to replace the non-profit MLS system with a Zillow-controlled ecosystem, locking out competitors while forcing MLSs to keep feeding Zillow data until Zillow no longer needs them.
“This is not a company defending consumer choice; this is a company that the FTC already sued for paying a rival to stop competing, that faces class actions for steering consumers to overpriced mortgages, and that feasts on the 99.7% of its users who don’t know who they’re contacting when they click ‘Contact Agent’ on Zillow’s platform. Zillow’s motion is a message to every MLS in the country: don’t interfere with Zillow’s takeover, or face litigation.
“CoStar filed this amicus brief because the Court deserves the full picture, and the full picture reveals that Zillow’s claims of concern for consumers and competition are a smokescreen for its own anticompetitive ambitions.”
Read CoStar’s full court filing and the full Amicus Curiae brief for more details.
The hearing is set for early July. Stay tuned for more as this story develops.
