Quick answer: The Indie Stone released Build 42.20 to the stable branch on 29 July 2026, multiplayer included. Build 42 is now the default on Steam, which means a fresh server install gets Build 42 without you choosing it. The part that matters most: Build 41 savegames are not compatible with Build 42. If you have a long running community world on 41, read this before you update anything.
What actually happened
Build 42 spent a long time in unstable. Multiplayer first appeared there in the 42.13 unstable update in December 2025, and the full stable release landed on 29 July 2026 as 42.20.
Two consequences for anyone running a server:
Build 42 is the default now. No -beta flag, no opt in. A new server install pulls Build 42. That is convenient if you wanted to move, and a trap if you did not.
Build 41 saves do not transfer. The developers were direct about this: Build 41 savegames are not compatible with Build 42. This is not a "might have issues" warning, it is a clean break.
If you have a world you care about
Do this in order, and do the backup first.
1. Back up before you touch anything. Copy your entire Zomboid/Saves folder somewhere off the server. This is the only step that cannot be undone later, so it is the one to do first rather than last.
2. Decide: move or stay. There is no partial answer here. Either your group starts a new world on Build 42, or you stay on Build 41 for now. There is no path that carries a 41 world into 42.
3. If you are staying on 41, switch the server to the legacy41 beta branch. That pins you to Build 41 and keeps your world working. Do this before your next server restart, not after, because a restart on the default branch will pull Build 42.
4. If you are moving, treat it as a fresh start. New world, and check every mod in your list for a Build 42 version, because a major build change breaks mods more thoroughly than a point release does.
The mod problem, honestly
This is where most communities lose a week.
A build change of this size means your mod list is not going to survive intact. Some mods will have Build 42 versions already, some will be abandoned, and some will exist but behave differently. The Workshop being full of mods that say they support 42 does not mean the specific combination you run works together.
The sensible order, and it is worth the patience:
Start the server on vanilla Build 42 first. Confirm everyone can join. Then add mods back one collection at a time, restarting between them. Adding twenty mods before your first successful join makes it very hard to work out which one broke it, and that is exactly the situation people end up in.
What Build 42 changes for the server itself
Beyond multiplayer reaching stable, 42.20 brought substantial crafting, map, animal and lighting work. From a hosting perspective the practical effect is that Build 42 worlds are heavier than Build 41 worlds were, because there is simply more being simulated.
The sizing logic for Project Zomboid has not changed shape though, and it is worth restating because people size these servers wrong:
Memory follows explored map area, not player count. Four players who have each explored a different town load four times as much of the map as four players in one place. That is why a server that ran fine for a month suddenly struggles with no config change: the group spread out.
| Setup | Realistic RAM |
|---|---|
| Small group, fresh map | 4GB |
| Established world, several towns explored | 8GB |
| Large group, long running world, mods | 12GB+ |
Our Project Zomboid hosting starts at EUR 4 a month for that first case and scales up as your world does.
The error you will see if mods are wrong
If a player gets "File doesn't exist on the client", the server is running content they do not have, or your WorkshopItems and Mods lines do not match each other. Both have to list the same content and every player has to have subscribed.
That error becomes much more common right after a build change, because half your players updated their mods and half did not.
Where you host it
Zomboid is not a twitchy game, but it is a long session one. Groups play for hours, and a server on the other side of the world makes that noticeably worse over an evening than it would in a game you dip into for twenty minutes.
We run Project Zomboid in the Netherlands, North America, Singapore and India. Pick the one closest to most of your group.
FAQ
When did Build 42 go stable? 29 July 2026, as version 42.20, with multiplayer included on the public stable branch.
Will my Build 41 world work on Build 42? No. The developers stated clearly that Build 41 savegames are not compatible with Build 42. Back up your save and either start fresh on 42 or stay on 41.
How do I keep my server on Build 41?
Switch to the legacy41 beta branch. Do it before your next restart, because Build 42 is the default now and a restart will pull it.
Do I still need the -beta flag for Build 42? No. Build 42 is the default branch since 42.20 went stable. The beta flag is now what you use to stay on the old build, not to get the new one.
Why do players get "File doesn't exist on the client"? The server has content they do not, or your WorkshopItems and Mods entries do not match. This spikes right after a build change because players update mods at different times.
How much RAM does a Build 42 server need? Start at 4GB for a small group on a fresh map. Memory grows with how much of the map has been explored, so established worlds are more comfortable at 8GB or more.
Related reading: Project Zomboid hosting plans from EUR 4 a month across four regions.