Do you have a source for that agreement? Sounds like an EU fine of billions just waiting to happen.

Various carriers have had their own RCS apps for years, I doubt Google could’ve done anything about that. Of course you’ll need a carrier that supports RCS if you want to use non-Google RCS clients (unless you want to reverse engineer the protocol Google uses with their own servers) but RCS is a federated network between carriers (and, I guess, Google). Apps like Verizon Messaging use RCS and although Verizon is shutting down their app somewhere this month, I can’t find any indication that the has something to do with Google. I’ve certainly never heard of it and I can’t say I imagine this app being very successful.

An early protocol implementation for RCS has been around for years but it seems to be abandoned. A newer stack has appeared on Github last year but it only implements USIM authentication, so must have system privileges. This is only one of the four authentication methods built into RCS(page 183) and carriers that use either digests or username+password combinations should be usable with such stacks with some small changes. One of those methods involves the carrier adding a header to your HTTP call and authenticating you completely transparently, even.

The biggest problem with RCS isn’t Google gatekeeping it, but the general disinterested from the public combined with a very complex protocol that involves carriers and carrier support. Your carrier needs to set up and maintain a server and either they or someone else needs to make an app to use that server. As a result, Google is the only one that has actually bothered to set up both an app and maintained a service of their own, as carriers generally don’t like running extra servers almost nobody uses. I don’t know if Apple will use their own servers or if they’ll work through your carriers’ servers instead (like the protocol was designed to work) so perhaps there will be two non-carrier RCS networks out there soon.

There’s actually a second player: Samsung

But aside from the two of them, yeah, closed. Basically the protocol the servers use to talk to each other is open, but whether they’ll want to talk to your server is undocumented and unlikely, and the protocols Google and Samsung use to talk from their servers to their respective apps are closed.

There are no other implementations because no other app implemented RCS. Developers want Google to expose RCS the same way SMS and MMS are exposed but I’m not sure if they even can without sacrificing the ability to install Google Messages on a phone that didn’t come with it.

Even then, I’m not sure if Messages even works with carrier services (like RCS should). I’ve only seen it work with Google’s servers. Even then, I don’t think AOSP should contain a messaging API tied directly to Google’s servers.

There have been dozens of RCS clients in the past, all named things like “message+” or “Joyn”, but none of them ever got a serious user base. It took Google setting up servers and handing out free accounts for RCS to become relevant, but every RCS app seems to have already shut down or is in the process of shutting down.

I’m not surprised there’s no choice of clients with how little consumers actually care about RCS.

Luckily, RCS being just internet messaging allows for alternative clients in all authentication mechanisms except for SIM key authentication (that requires system privileges). I don’t know which carriers implement which authentication mechanism, but 3 out of 4 authentication mechanisms should Just Work with an RCS stack within an app.