Project Zomboid: switch to Build 42Stable branch, multiplayer, mods & moving on from Build 41

Published on August 4, 2026 · last updated on October 21, 2026

Build 41 vs. Build 42: where things stand

For more than seven years, Build 41 (B41) was the stable branch of Project Zomboid. Build 42 ran as an optional beta for a long time, initially without any multiplayer at all; from December 2025, multiplayer could be tested in the unstable branch. With version 42.20 on 29 July 2026, The Indie Stone finally made Build 42 the stable default - including multiplayer and dedicated servers.

For server owners, that reverses the starting position: anyone who starts Project Zomboid via Steam now automatically ends up on Build 42. Build 41 has not disappeared, but has become a special case that you choose deliberately. For new rounds, Build 42 is the obvious way; Build 41 remains for existing saves and mod collections that do not (yet) exist for Build 42.

Staying on Build 41: the legacy41 branch

If you do not want to give up your running round, you stay on Build 41. For that, Steam has the legacy41 branch: right-click Project Zomboid → Properties → Game Versions & Betas → select legacy41.

Very important: server and client have to run on the same build. A Build 41 server only accepts Build 41 clients and vice versa. If your server stays on Build 41, every player has to activate the legacy41 branch - otherwise Steam automatically gets them Build 42, and joining fails. If you switch the server to Build 42, then conversely everyone who had chosen legacy41 or a beta branch in the meantime has to go back to the default.

Saves: no transfer from Build 41

The hardest point first: a Build 41 save cannot be opened in Build 42. The developers have said so clearly - world format, map and data structures differ too much. Worlds from the last unstable state before 42.20 are not compatible with the stable release either, because of the new map content.

You therefore have two clean options: the old world keeps running on legacy41, or you start a new world for Build 42. There is no middle way in which the existing base lives on in Build 42. That is a decision for the whole group and should be settled before the switch, not after it.

Checking mods for Build 42

The second stumbling block when switching: mods for Build 41 do not run reliably on a Build 42 server. Every mod has to be explicitly updated for Build 42. Before the switch:

• Check on the Workshop page of every mod whether it supports Build 42.
• Temporarily remove incompatible mods from WorkshopItems and Mods, see Install mods.
• Check maps and frameworks particularly carefully - they reach deep into the game and are often the last to catch up.

Since the stable release, many authors have been catching up, but not all. If your round depends on a mod that only exists for Build 41, that is a good reason to stay on legacy41 for the time being.

Switching cleanly

The switch itself is done quickly; the preparation is the important part:

1. Back up the Build 41 save. Backups are self-service with us throughout - you fetch the save folder via WebFTP or SFTP, see Create backups.
2. Clean up the mod list and only take over mods that work with Build 42.
3. Choose the version in the panel and let a new world be created.
4. Check the settings before everyone joins - especially the SandboxVars and servertest.ini.

Keep the Build 41 backup even if the switch goes well: that keeps the way back open at any time, and you can start the old world on legacy41 again if needed. With automatic updates, the server then stays on the current state of Build 42.

What Build 42 brings

The scope of the changes explains why the switch is not just a version issue. Build 42 rebuilds central systems instead of merely adding content:

• Crafting and production chains. Considerably more depth in manufacturing, processing and workshops - probably the most visible difference in everyday play.
• New lighting system. Nights are darker and light sources more important, which noticeably shifts how the game feels.
• Revised map. Knox County has been rebuilt and expanded, plus basements under many buildings.
• Animal husbandry. Livestock as a separate source of food and resources - a building block for long-term bases.

For a server, that means above all: the sandbox settings partly have a different effect than you are used to. If you take over a balanced configuration from Build 41, check it again after the switch - especially the loot and XP values, see SandboxVars & difficulty. In addition, the server does more work itself in Build 42 that used to be done by the client. If you have calculated tightly so far, keep an eye on the load after switching - guideline values are in How much RAM do I need?.

Which build for which server?

The decision depends less on the features than on your starting position:

• Build 42 is right for every new round. It is the default that Steam delivers to all players, it gets further development, and nobody has to change a branch in the properties.
• Build 41 via legacy41 is right for a grown world that you do not want to give up, or for a mod collection that does not exist for Build 42 yet.

The most important organizational point: every player has to use the same build. Since Build 42 became the default, this mainly affects Build 41 servers - Steam automatically updates new and inattentive players to Build 42, and the person affected sees no helpful message, only a failed connection attempt. How joining otherwise works is described under Join a server, further causes are collected in Connection problems.

Frequently asked questions