RETS vs RESO Web API: What Real Estate Professionals Should Know

Introduction

If you have ever been on a call about your website's listing feed, you have probably heard both terms used as though they were interchangeable. They are not, and the difference occasionally shows up in things you care about: what a vendor can build, how quickly, and whether your listing pages carry the fields your competitors' do.

You do not need to be able to write either integration. You do need enough to ask a straight question and recognise a vague answer.

⚡ Quick answer — What's the difference between RETS and the RESO Web API? Both are ways of moving MLS listing data from a board to another system. RETS is the older one, introduced in the late 1990s and specific to real estate. The RESO Web API is the current standard, built on general web technology — OData and JSON — rather than a real-estate-only protocol. RESO has deprecated RETS and no longer supports it, though existing RETS servers can still be running.

What RETS was, and why it existed

RETS — the Real Estate Transaction Standard — solved a real problem. Before it, every MLS exposed data its own way, and any vendor wanting to work across markets built a bespoke integration per board. RETS gave the industry one agreed way to request listings.

It was designed in an era before JSON, before OAuth, and before REST APIs were the default way software talked to software. So it invented its own conventions: its own query language, DMQL; its own compact data formats alongside XML; and HTTP digest authentication. Those choices were reasonable in 1999. They aged.

The practical consequence was that working with RETS required specialist knowledge and specialist libraries. A competent web developer could not simply pick it up, which kept the pool of people able to build real estate integrations small.

What RESO is, and what the Web API is

RESO — the Real Estate Standards Organization — is the body that maintains the industry's data standards. It is worth separating two of its outputs, because they are frequently conflated:

The Data Dictionary standardises what the data is called. RESO describes it as "the real estate industry's universal language for data," defining resources, fields and permitted values so that the same concept carries the same name across systems.

The Web API standardises how the data moves. RESO calls it "the modern way to transport data in the real estate industry."

You can have one without the other, which is why a board can be on the Web API and still expose local fields that no dictionary describes.

What changed technically

The Web API is built on OData v4, a general-purpose standard for building and querying APIs, and returns JSON. Authentication uses OAuth.

None of those are real estate technology. That is the entire point. A developer who has consumed any modern API already knows how to consume this one, and the ordinary tooling — HTTP clients, JSON parsers, API testing tools — works without adaptation.

Querying changed in the same direction. Where RETS required DMQL, the Web API uses OData query parameters, so filtering, sorting and pagination follow conventions used across the wider software industry.

Is RETS dead?

This deserves care, because the confident answer you will hear in either direction is wrong.

RESO's position is unambiguous about support: RETS "has been deprecated and is no longer supported by RESO." The RETS Workgroup published RETS 1.9 as its final release and was retired in 2017 after eighteen years, on the basis that there was "no more business to transact."

But deprecated is not the same as switched off, and RESO has been explicit about that too. Cary Sylvester put it directly at the time: "RETS is not dead, because it's not dead until you quit using it. But we are not going to extend it further. We are going to focus on the RESO Web API." A colleague offered the analogy that has stuck: think of it like Flash — "it's no longer developed, but it's still running, and a lot of people still keep running it."

So the accurate statement is narrow: RETS receives no further development and no RESO support, while some legacy RETS implementations remain in operation. Anyone telling you RETS no longer exists anywhere is overstating it; anyone treating it as a current standard is understating the direction of travel.

Where the transition stands

RESO publishes adoption figures. As of its June 2026 transition report, at least 90% of MLSs have RESO-certified Web API services, MLS feeds converted to the Web API cover 62.4% of US subscribers, and feeds committed to conversion cover 76.4% — up from under 5% of the industry in 2021.

Two readings of that. The direction is settled and not in dispute. But a meaningful share of subscribers is still served by feeds that have not completed conversion, which is why a vendor's answer about your board still matters more than the industry aggregate.

What this means if you're a broker or agent

Four things, and only four.

Ask which your board serves. If your MLS offers the Web API, a platform building against it will generally move faster and break less often. If your board still runs RETS, that is not a crisis, but it does mean your vendor is maintaining older machinery.

Data Dictionary conformance affects your pages. A board and platform both aligned to the Dictionary means fields land where they should. Where they diverge, someone has to map local fields by hand — and unmapped fields render as blanks on your listing pages.

"We support RESO" is not a complete answer. Supporting the Web API in general and holding an active, current feed with your specific board are different claims. Ask for the second.

Migration is not free. A board moving from RETS to the Web API is a project on the vendor's side too. If your platform is mid-migration, ask what changes for your site and when.

For how any of this data actually reaches a listing page, see MLS Integration: How Listing Data Actually Reaches Your Website.

FAQ

Do I need to know which standard my MLS uses? Not day to day. It becomes relevant when you are choosing a platform, or when a vendor explains a delay by pointing at your board's feed.

Is the RESO Web API faster? It is generally easier to build against and to query efficiently, which tends to show up as fewer integration problems rather than a faster website. Page speed is determined by your site, not your feed.

Will my listings look different after a board migrates? They can. Migrations sometimes change which fields are available or how values are expressed. It is a reasonable thing to ask your platform about ahead of a switch.

Is RESO the same as the MLS? No. RESO sets standards. Your MLS is the organisation holding the data and setting the rules for its use.

What is OData, in one sentence? A widely used standard for building web APIs — the same kind of technology behind ordinary web services, rather than anything specific to real estate.

In short

RETS solved a real problem with the tools of its era and is now deprecated and unsupported by RESO, though not universally switched off. The RESO Web API replaces it with general web standards — OData, JSON, OAuth — plus the Data Dictionary to keep field names consistent.

For most agents this is background. The one question worth carrying into a vendor conversation is whether they hold a current, active feed with your board — and what standard it runs on.

Check coverage on the MLS boards page.

Related reading

• MLS Integration: How Listing Data Actually Reaches Your Website
• Real Estate IDX Websites: The Complete Guide
• MLS & IDX Compliance: What Your Real Estate Website Must Handle

Research Integrity

Written 16 August 2026 against RESO's own documentation. The deprecation wording, the OData v4, JSON and OAuth details and the Data Dictionary description come from RESO's Web API and Data Dictionary pages. The RETS 1.9 final release, the 2017 workgroup retirement and both quotations come from RESO's announcement. Adoption figures are from RESO's Web API Transition Leaderboard, June 2026 report, and are cited with that date because they move.