GC3 in 2026

Hi folks,

As promised in another thread, I'm posting a round-up of "GC3"
resources. But first, I'll summarize what this is about for people who
missed the summits/sprints where it was discussed.

The idea grew, from discussions with a few people, that it would be
good to level up group chats in XMPP, without breaking significantly
with the extremely entrenched MUC protocol.

In fact, we have already extended MUC (XEP-0045) in various ways, both
by updating XEP-0045 itself and through various add-on XEPs in recent
years, some with relatively rapid adoption (XEP-0421), and others not
so rapid (yet?).

There has been vague occasional talk of "MUC 2" for a very long time
(since before MIX), mostly various isolated ideas for improvements. I
wanted to try and finally glue some of these ideas together, without
necessarily "stealing" the title as an official MUC successor, so
"GC3" became a working title.

Shortly after I'd started gathering this together, we held a sprint in
the UK. A lot of that sprint ended up being reviewing the GC3
wishlist, priorities, eliminating some bad ideas, and clarifying some
possible approaches.

The slides of the GC3 presentation I gave at Summit 27, a few months
later, are here:
https://matthewwild.co.uk/uploads/xmpp-gc3-jan-2025.pdf

The main GC3 notes, including feedback from the sprint and summit, are
available here: https://pad.nixnet.services/s/7xKNOKxxE

In addition to the notes, there have been a couple of partial attempts
at turning it into something more like a specification.

This one by Jonas: https://pad.nixnet.services/s/yu7drWpml

This one by Kev: https://github.com/swift/protoxeps/blob/master/gc3.md

Both these are missing some things, they probably overlap in some
areas and conflict in others. But definitely together they form the
basis of anything that would become a GC3 XEP.

Apart from this, Kev also produced some additional related documents:

- https://github.com/swift/protoxeps/blob/master/acl.md
- https://github.com/swift/protoxeps/blob/master/related-entities.md

These were spawned from GC3 discussions, but could certainly stand as
XEPs in their own right. I don't want to speak for Kev, but I assume
if there is sufficient interest then he would be willing to work
further on these and get them submitted, or have someone else help to
do so.

Okay, that's the status of documentation.

Partly because of the numerous ideas floating around, I also wanted to
work on at least a rough prototype implementation of at least some
core parts of the code. Just to try out the approach and prove it can
work.

I know that implementation isn't required by the XSF in order to have
an experimental XEP. However, faced with the decision between spending
time on writing a GC3 XEP, and figuring out if it can even be
implemented, I personally chose to prioritize the latter. This isn't
any kind of statement about whether or not there should be a GC3 XEP
submitted at this stage, I *personally* just don't want to spend time
writing such a broad XEP for a protocol that may need drastic changes
as soon as anyone tries to implement it. I won't (and, anyway, can't)
stop someone else with time and motivation from doing what they want
(implementation or specification) with any of the "GC3" notes we've
gathered. I presented them at the summit precisely with the hopes that
everyone interested in the topic may do exactly that, so we can move
the thing forward.

After the summit presentation, I started making some necessary changes
to the Prosody MUC code to allow us to accommodate some of the
discussed GC3 features. Mainly internal refactoring of some APIs and
data structures to make it more extensible and support chats where we
have mixed MUC and GC3 clients.

The only externally visible change in Prosody so far, is that I began
implementing a participant list fetch protocol (behind an experimental
feature flag). Ironically, it's one of the parts of the protocol we
deemed during the sprint as high priority and yet we don't actually
have a protocol for this written anywhere in the several "GC3"-related
documents, yet.

One of the features of such a protocol would be paging, e.g. using
RSM. Prosody's implementation doesn't currently support this. I
immediately ran into the problem that to support paging, the
participant list needs to be ordered and stable. Currently Prosody
stores affiliations and participants in unordered hash tables. Another
thing that needs refactoring.

Okay, that's the implementation update from me.

Now, the status and future of GC3...

It appears from discussions that multiple people have come to view me
as having some kind of "ownership" over GC3. This was entirely
unintended and very unfortunate. It's true (I think?!) that I picked
the name, and I wrote the first draft of notes. However I think by now
that between various discussions online and offline across and venues
(regrettably not this list?), my personal contributions to the whole
thing are quite outnumbered by others.

Obviously if nobody else beat me to it, at some point writing a GC3
XEP was inevitably on my todo list. So I accept that I'm somehow the
"default" person in a way - going to push this forward, eventually, if
nobody else can/will. But it seems somehow this appeared as some kind
of exclusivity, and this was likely due to poor communication on my
part.

In that regard, I am glad that Dave ignored this and broke the ice
with the MUC2 proposal regardless, surfacing this whole issue.

I like collaborating with smart folk, it's why I am here and why I do
open-source. But I (and I'm not alone) have far more ideas, wishes and
responsibilities than I have time to work on all at once. Sometimes
this also means I get rather behind on emails, or that I might miss
MUC discussions.

If anyone else feels that I am somehow blocking them from doing what
they want - I'm always willing to talk it over. And if that fails,
please, just go ahead and do it.

Now, finally, GC3 future...

Realistically, I'm unlikely to change trajectory. I have a long list
of stuff to do, and honestly, GC3 is outranked by various other
priorities for the coming month or two at least.

This means, I see the following possible outcomes:

- MUC2 halts and nothing further happens until I finally get around to
a GC3 XEP (nobody wants this)
- MUC2 continues as a from-scratch parallel effort (not ideal in my
opinion, but I can't and won't block such an effort, especially not if
others think it's a good path)
- MUC2 is at least informed and improved by the prior work on GC3,
adopting as much or as little as deemed appropriate. Even if it
ultimately adopts none of it, I think this would be a win.

What I care about is that we arrive at a good MUC successor XEP with a
good path to adoption. I will say it again just to be clear here: I
don't care what it's called, or whose name(s) are on the document.

Regards,
Matthew
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