How to send & receive RCS (Rich Communication Services) Messages from Android app?

Question

Google is promoting and using RCS in its messaging app.
As a consequence, it would be good (mandatory ?) for third party SMS apps to support RCS.

I have developed an SMS app in which I sometimes 'encounter' RCS messages.
My app cannot receive RCS messages, but sometimes, a previous app did receive some of these (eg: Google Message App), so my app 'find' these 'old RCS messages in Android SMS DB.

My app is able to read these (at least basic) old messages, but **I have no clue on how to respond to these messages**.

My app is based on android-smsmms library and I found some information (2020) about this RCS issue:

To summarize:

• Third party apps cannot support RCS at this time.
  
• Until Google provides APIs or builds RCS support into Android, third party apps will not be able to use it.
  
• You can simply disable RCS (or "Advanced Messaging")

src: https://web.archive.org/web/20201111221326/https://github.com/maplemedia/pulse-sms-issues/issues/143

Searching for an RCS library, I found nothing but an old unmaintained library rcsjta

---

Questions:

• Is RCS open to third party app as of now? Or will soon be? (It was not in 2021: see https://www.xda-developers.com/google-messages-rcs-api-third-party-apps/ )
• Where can I find accurate information about RCS API current status and roadmap?

Notes:
Here are some SO questions I've already looked at regarding this:

• How to read RCS messages from android device programmatically
• How does SMS work (with Android devices)?
• Android: How to incorporate Rich communication Services in Application

Floatable

Rcs is not a private API, but it is super complex. I am unaware of any open source implementation. Google doesn't offer an API to make it easy. – Ventriloquism

@MooingDuck you are right, my app can not receive RCS messages. It can read previously received RCS messages form SMS DB, but not receive them. I updated my question accordingly. Thanks for this precision :-) – Floatable

android.googlesource.com/platform/packages/services/Telephony/+/… Apparently Android released an open source app to use as a baseline. – Ventriloquism

Any luck finding a way to listen or send RCS messages in Android? – Attorneyatlaw

Still nothing on my side :/ – Floatable