> trying to build your own is explicitly blocked

Google doesn't offer an RCS API, but making your own API is not blocked, especially not on phones running MicroG.

For standard ROMs, RCS apps are not feasible because carriers expect interaction with the SIM module, and only privileged apps can do that. Google Messages blocks root access (probably because the RCS spec says they have to if they ever implement the money exchange feature for RCS) but that just locks out Google Messages.

LineageOS, GrapheneOS, /e/, and all the others could build their own RCS client. They may not be able to register with Google, but they can make an RCS app nonetheless. Rooted devices can also promote third party RCS apps to gain system permissions, so they would be able to use the same RCS apps. Thing is, like all telco protocols, RCS is a long spec with a billion features and acronyms made up of acronyms of even more acronyms, plus you can't easily set up your own server (though in theory a sister project for an open RCS server might be useful for the open 5G project).

I get why people want Google to just stuff their RCS library into open source Android the same way they do SMS/MMS, but to say it's impossible to write a client for, especially when running at the permission level MicroG runs at, is not the whole truth.

As for the whole "Google is the gatekeeper" thing: there are more RCS-for-business providers out there. Google's is probably the easiest to use by far, but Twilio has RCS too, as well as smaller companies such as LINK Mobility and Esendex. Sure, the people whose carriers don't support RCS might receive these business messages through Google's servers, but there's no need to pay Google a dime to make use of the RCS for Business specs.

> LineageOS, GrapheneOS, /e/, and all the others could build their own RCS client. They may not be able to register with Google, but they can make an RCS app nonetheless.

Building a spec compliant RCS client for the love of the game, I guess.

> but there's no need to pay Google a dime to make use of the RCS for Business specs.

Assuming we're talking of global RCS and not domestic deployments (China and Korea mostly) you pay Google indirectly in all cases as Jibe is monetized via A2P revenue share.

> They may not be able to register with Google, but they can make an RCS app nonetheless.

They can make a "standards compliant" RCS which will be able to connect to literally zero carriers or servers on the planet.

In fact Google Messages "RCS" doesn't even implement the RCS standard. They use a proprietary protobuf api exclusive to Google. Google messages can't connect to any "RCS" servers except Google's.

> Twilio has RCS too

Twilio, Bandwidth etc. are gatekept by Google and are basically just a middleman reselling Google's product.

> LineageOS, GrapheneOS, /e/, and all the others could build their own RCS client

And it will be useless. They likely won't be able to connect to their mobile network's RCS or to Google's RCS. A user with an official Android phone will be able to reach you only over regular SMS.

The way RCS works, the mobile _operator_ uses your corresponder's phone number to look up their RCS server. So that's also why RCS connectivity is so patchy, not all cell phone operators peer with each other.

> I get why people want Google to just stuff their RCS library into open source Android the same way they do SMS/MMS, but to say it's impossible to write a client for, especially when running at the permission level MicroG runs at, is not the whole truth.

It is. The Google's RCS endpoint requires attestation that is available through Play API only.

My personal advice is to avoid RCS at all costs, and use something that is not infested by mobile phone operators or Apple.