How to Switch to Project Zomboid Build 42 Server

Build 42 is now the stable branch (updated 29 July 2026)

Build 42.20 was released to the public stable branch on 29 July 2026, after 19 months on the unstable branch. That changes what this guide is for. A Project Zomboid server ordered from DoomHosting today installs Build 42 automatically, with no branch variable to set. If you already run a Build 41 server, this guide walks you through moving it to Build 42.

Two things to know before you touch anything:

• The unstable branch name has been retired. If your Startup tab still has unstable in the Steam Beta Branch box, because you followed the earlier version of this guide, clear it. Leaving a branch name that no longer exists on Steam is the most common reason a reinstall fails right now.
• Your existing Build 41 server will not change on its own. Our Project Zomboid egg ships with Steam Auto Update set to 0, so the server keeps running the files already on disk. Build 42 only arrives when you reinstall or enable auto-update. Nothing breaks tonight while you decide.

If you would rather stay on Build 41 for now, do not follow this guide. Read How to Keep Your Project Zomboid Server on Build 41 instead, which covers pinning the legacy41 branch so a future reinstall cannot pull Build 42.

What is Build 42?

Build 42 is the largest update in Project Zomboid's history. The headline changes that matter to a server owner:

• A heavily expanded and reworked Knox Country map, extended westward with revamped existing towns.
• A rebuilt crafting system, including workstations and real material processing.
• Animal husbandry, so livestock and wildlife are part of the world simulation.
• Basements and a new lighting model.
• A rewritten modding framework.

The map and crafting changes are why saves cannot carry over, and the modding rewrite is why your Build 41 mod list will not work as-is.

Before You Start

Before switching to Build 42, it is essential to back up any worlds you still care about from Build 41. The process of switching to Build 42 requires wiping all server files, which is a destructive process.

Warning: The wipe is permanent and will delete all your server files, including save data. Make sure to back up any important worlds before proceeding.

Our backup and restore guide covers pulling a copy of your save down first. Do that even if you plan never to go back, because a Build 41 save is the only way to revisit that world later.

Step-by-Step Guide to Switching to Build 42

To move an existing Build 41 server to Build 42:

1. Open your server in the DoomHosting game panel.
2. Back up your world first. This step is not optional if the save matters to you.
3. Navigate to the Startup tab, where you can set install variables for your server.
4. Find the Steam Beta Branch [requires reinstall] variable and make sure it is empty. Empty means the public stable branch, which is now Build 42. If it still says unstable or b41multiplayer, delete the value.
5. Save the startup settings.
6. Go to the Settings tab and click Reinstall Server.
7. When reinstalling, choose the option to wipe / clear all server files. This is crucial because Build 42 save data is not compatible with Build 41, and mixing old files with the new install causes crashes, missing assets, and corrupt save states.
8. After the reinstall finishes, start your server.

The first Build 42 boot takes longer than usual, because the server generates the new map chunks. Give it a few minutes before assuming something is wrong.

Client-Side Steam Branch Setup

This has flipped around compared to the old advice. Steam now serves Build 42 to every player by default, so nobody needs to opt in to a beta to join your Build 42 server.

The mismatch happens the other way now: a player who deliberately selected legacy41 in Steam cannot join a Build 42 server. If one player reports a version mismatch while everyone else connects fine, have them right-click Project Zomboid in Steam, choose Properties, open Game Version & Betas, and set it back to None.

Mods and Build 42

Build 42 rewrote the modding framework, so Build 41 mods do not work on a Build 42 server. This fails in a way that is easy to misread: Workshop IDs are not version-aware, so SteamCMD happily downloads every Build 41 mod onto your Build 42 server. The server starts, the mod list looks correct, and the mods simply do not function or throw errors in the console.

Rebuild your mod list from scratch against Build 42 versions rather than carrying the old one over. Our mod installation guide covers the correct Mods= and WorkshopItems= pairing.

Troubleshooting

If your server will not start after switching to Build 42, the most common causes are:

• The Steam Beta Branch field still contains a branch name. It should be empty for Build 42. Old values like unstable and b41multiplayer no longer exist on Steam.
• The reinstall was done without wiping files, so Build 41 assets are conflicting with the new install.
• Build 41 mods were carried over and are erroring on load. Clear the mod list and start the server clean to confirm.
• The player who cannot connect is pinned to legacy41 on their own Steam client.

Build 42 also uses more memory than Build 41 for the same player count. If the server starts and then dies under load, check our RAM requirements guide and consider adding a GB or two in the panel.

How to Switch Back to Build 41

Build 41 is still available on Steam as a permanent legacy branch. To go back, set the Steam Beta Branch [requires reinstall] variable to legacy41, then reinstall and wipe files again. Note that the old b41multiplayer branch name no longer works, and that your Build 42 world will be lost in the wipe, exactly as your Build 41 world was on the way in.

Full instructions, including how to stop a future reinstall from silently pulling Build 42, are in How to Keep Your Project Zomboid Server on Build 41.

Conclusion

Now that Build 42 is the stable branch, switching is mostly a matter of clearing the old beta branch value and doing a clean reinstall. The two things that catch people out are carrying a Build 41 save or mod list into a Build 42 install, and leaving a retired branch name in the Startup tab. Back up first, wipe properly, and rebuild the mod list.

You can rent a Project Zomboid server from DoomHosting with Build 42 installed by default, and manage branches, mods and backups yourself from the game control panel. If you get stuck, our support team is available 24/7.