On the morning of 30 August, a paying customer's server could no longer be joined by their group. Nothing had crashed: overnight, the server had changed modpack version. All the Mods 10 had gone from 8.0 to 8.1, published the evening before at 22:15. The server followed on its next start. The launchers in the group were still on 8.0.
It was not an outage, it was our design flaw
A modded server reinstalls its pack on every start. With no version set, it takes the latest one published by the pack author — that is the documented behaviour of the image we use, and nothing on our side contradicted it.
Worse: we had made it more likely ourselves. A fix shipped three days earlier excludes the mods folder from backups, for good reasons — it was heavy, and it gets rebuilt at boot. The consequence we had not seen: every start becomes a fresh install, and therefore one more chance to change version.
What changed
Your server freezes on the version it installed, by itself, from its first successful start. Nothing moves without you.
A "Modpack version" panel in the console: what is installed, what will be installed on the next start, and a way to pick another version — or to go back to automatic updates if you prefer them. Your world is kept either way.
And it warns you. When a newer version is out, the panel says so, and says it will only be installed if you pick it. Freezing without ever warning would be trading one flaw for another.
What we got wrong on the first try
Re-reading the feature the same evening, we found it only protected four of our ten modpacks. The other six — Cobblemon, COBBLEVERSE, Prominence II, Better MC, Create: Live 5 and Adrenaline — come from Modrinth, and we were not reading the file where their version is written. The freeze silently gave up on them: they kept updating on their own, which is exactly the flaw we claimed to have removed. Fixed the same day.
What this does not solve
Freezing the version does not undo an upgrade that already happened: if your world has been opened by a newer version, we do not roll it back on our own. And the pin covers the whole pack, not one individual mod.