Internet Data Exchange is the National Association of REALTORS policy framework, implemented through each local MLS's own rules, that permits participating brokers to display other brokers' listings on their public websites. Access requires an executed IDX agreement with the MLS, broker participation, and compliance with display, attribution and refresh rules. It is the mechanism through which most public US listing data reaches the open web, and it is a contract rather than a statute.
IDX
Industry policy — not law United States (trade association / local MLS)
A binding rule made and enforced by a private trade association through membership and licensing, not by a legislature or regulator.
IDX is a licence, not a law — and in US residential real estate it is the instrument that actually determines whether anyone can reach listing data.
Under NAR policy, implemented through each local MLS’s own rules, a participating broker may display other participants’ listings on their public site. To consume that feed a developer executes an IDX agreement with the MLS, works under broker participation, and accepts rules on how listings are displayed, attributed and refreshed.
- Per-MLS, not national - There are roughly five hundred MLSs, each with its own agreement and its own terms. There is no single contract that unlocks American listing data.
- Broker-anchored - Access derives from broker or agent participation. A developer without that relationship generally routes through a reseller who has it.
- Display-constrained - IDX governs what may be shown and how, not merely what may be retrieved.
The State of US Real Estate APIs recorded an access gate for every organization researched, and IDX sits at the centre of the American ladder. The finding that gives this entry its point: sorted by access model, average API score runs backwards — licence-agreement 47.7, self-serve 47.6, application-approval 44.7, partner-only 37.2, membership-required 32.5, nothing-published 23.0. The most contractually locked tier has the best contracts.
This is the counterpart to NAR Policy Statement 7.90 and the reason that mandate returned so little. 7.90 compels the schema; IDX governs the access. A RESO-certified endpoint is genuinely conformant and still answers 401 to anyone without an executed agreement.
For the agentic turn this is the hard edge. An autonomous agent can parse an OData contract and follow a discovery document. It cannot execute an IDX agreement or obtain broker sponsorship. Every access tier above self-serve in this sector is a human legal process, and in the sixty-seven organizations newly researched for the study, only four were self-serve — none of them American.
Referenced in API Evangelist papers
This regulation shows up in my published research. These reports read the machine-readable evidence provider by provider — and put this regulation in the context of a real sector.
The State of US Real Estate APIs
The agreement a developer must execute to display MLS listings — the access layer that a RESO certification says nothing about, and the reason certified endpoints return 401.
