Project Zomboid Build 42.21 went from the Unstable branch to Stable on 28 September 2026. It is the first incremental update since Build 42 went stable, and The Indie Stone has made it the template for every release from now on: Unstable first, tested by players, then Stable.
What changes in multiplayer
Most of 42.21 is fixes, and several of them are about servers:
- Zombies no longer vanish when a player leaves a chunk of the map and comes back, in single player as in multiplayer. Zombie duplication in multiplayer has also been tackled. Both were most visible on high-population servers.
- Steam authentication: some players could enter a dedicated server without being properly authenticated by Steam, which made SteamID bans useless. That is fixed, so a ban sticks again.
- A broader anti-cheat, safehouse exploits closed, and fewer desyncs when driving with a high ping (map not loading ahead of the car, passengers teleporting).
- A clear message for a player who tries to join a server running a different game version.
loadstringandloadstreamare back. The 42.20.4 hotfix had turned these Lua functions off for security reasons; some mods depend on them.
Outside multiplayer: the XXL trees get out of the way when you drive and no longer hide the houses under them, washing machines clean rags and bandages, and explosives work in basements. The full list is in The Indie Stone's patch notes.
On TickServ: nothing to do
Your Project Zomboid server fetches the latest server image every time it starts, so a stopped server comes back on 42.21 directly. The ports (16261 and 16262, UDP) and the location of your world are unchanged, and The Indie Stone says existing saves are not affected.
When the version of your server changes, we tell you in your dashboard and on your Discord channel if you have linked one. And when there is a backup from before the update, we set it aside so the usual rotation doesn't delete it.
The day we missed
42.21 went stable on 28 September around 2 pm (Paris time). The server image we build on stayed on 42.20 for about a day: its automatic build failed on Steam's side. Players' games had already updated themselves, and Steam offers no way back to 42.20, so nobody could join a Project Zomboid server we started during that time.
On 29 September in the morning, we switched to an image already on 42.21 and messaged every Project Zomboid customer to restart their server. The image we normally use was fixed that same day, and we are back on it.
What this doesn't solve
- We still depend on that image. If its build breaks again on the day of an update, there will be a gap again. Now we know to watch for it, and how to work around it.
- Mods that haven't been updated for 42.21 can still prevent your server from starting, or break your game. Turn off the latest mod you added before anything else.
- There is no going back to 42.20. Steam doesn't offer a 42.20 branch, so neither your players nor your server can go back to the previous version.