Project Zomboid Build 42 Server Upgrade & Migration

This guide explains how to upgrade an existing Project Zomboid dedicated server to Build 42 and migrate your save files, mods, and settings without losing progress. You will learn the exact steps for both Steam and standalone servers, what breaks during the update, and how to fix the most common migration errors.

Why Build 42 Requires a Careful Upgrade

Build 42 is a major engine and content update for Project Zomboid, introducing new crafting systems, animals, and significant changes to the map and item IDs. Unlike a typical patch, the save format and mod structure changed enough that simply launching the new server executable on an old world often results in corrupted chunks, missing items, or outright crashes. The official upgrade path is not a one-click process; you must back up, convert, and verify your server files in the correct order.

Before You Start: Backup and Prerequisites

Before touching anything, make a full copy of your server directory. On a Linux standalone server, this is usually the folder you extracted the dedicated server files into, plus the Zomboid folder in your home directory where saves and configs live. On Windows, this is typically C:\Users\\Zomboid.

• Check your server OS: Build 42 server files are distributed as a separate download on Steam (for the SteamCMD method) or as a zip from the official site. The upgrade steps differ slightly between Linux and Windows.
• Check your mods: Many popular mods are not yet updated for Build 42. Visit each mod’s Steam Workshop page and confirm it lists Build 42 compatibility. Remove any mods that are not updated before starting the server.
• Check your save version: Build 41 saves are not directly loadable in Build 42. You will need to use the built-in save converter, which is included in the new server files.

Step-by-Step Upgrade for SteamCMD Servers

If you run your server via SteamCMD (the command-line tool for downloading dedicated server files), the upgrade process is straightforward but requires you to delete the old server files first.

1. Stop the server completely. Do not just close the console; use the shutdown command or kill the process.
2. Back up your entire Zomboid folder (saves, configs, mods, logs).
3. Update SteamCMD to the latest version by running steamcmd +login anonymous +app_update 380870 validate +quit (this is the Project Zomboid dedicated server app ID).
4. Delete the old server files in the directory where you installed the server (e.g., steamapps/common/Project Zomboid Dedicated Server). This is critical because leftover Build 41 binaries can conflict with the new ones.
5. Re-download the server using the same SteamCMD command. This will fetch the Build 42 files.
6. Run the save converter – the new server will automatically detect a Build 41 save and offer to convert it on first launch. If it does not, look for a convert command in the server console or a separate utility in the server folder.
7. Update your mods – after the server is running once, check the console for mod errors. Remove any mods that are not compatible.

Migrating Saves: The Converter and What It Does

The save converter in Build 42 is a one-way process. It reads your old map chunks and rebuilds them in the new format, but it does not preserve every single detail. Items that no longer exist in the game (e.g., old weapons or crafting materials) are dropped. Vehicles are preserved, but their condition may be reset. Base building structures are kept, but any wall or floor that used a removed tile type may appear as a plain placeholder.

To run the converter manually, navigate to your server folder and execute the server binary with the -convert flag (exact flag may vary; check the server README). The converter will ask for the save name and the target version. It is safer to convert a copy of your save, not the original, in case something goes wrong.

Common Upgrade Problems and Fixes

Even with a perfect backup, you will likely hit one of these issues:

• Server crashes on startup with a “mod not foundâ€� error. This means a mod in your mods folder or in the server’s servertest.ini is not compatible. Remove it and restart.
• Players cannot connect after the upgrade. Their game clients must also be on Build 42. If you are running a public server, tell players to update their game. Also, check that your server’s servertest.ini has the same public/private settings as before.
• World items are missing or floating. This is normal after conversion. The converter cannot place items that have no equivalent in the new version. You may need to use admin commands to clean up stray items.
• Performance is worse than in Build 41. Build 42 has higher system requirements. Check your server’s RAM and CPU usage. You may need to reduce the server’s MaxPlayers or ZombieCount settings.
• Save converter does not appear. If you are launching the server with a script that skips the interactive prompts, the converter may not run. Try launching the server manually in a terminal to see the prompt.

Migrating Mods and Workshop Collections

Mod migration is often the most tedious part. Build 42 changed the mod load order and the way mods access game data. Even mods that are “compatible� may need to be re-ordered.

1. Go to your Steam Workshop subscriptions and note which mods you have.
2. For each mod, check its Workshop page for a “Build 42� tag or a recent update post.
3. Remove any mod that does not explicitly state Build 42 support. Do not trust “should work� comments.
4. After the server is running, load the game client, join the server, and check the in-game mod list to see if any are red (error).
5. If a mod is essential to your server (e.g., a map mod), check if there is a separate Build 42 version. Sometimes the mod author releases a new workshop item instead of updating the old one.

Downgrading: Can You Go Back?

Yes, but only if you kept your Build 41 backup. The converter is one-way; once you run a Build 42 server on a converted save, the save is no longer readable by Build 41. To downgrade, you must restore the original Zomboid folder from your backup and reinstall the Build 41 server files (using the SteamCMD beta branch or an older zip). This is why the backup step is non-negotiable.

Frequently Asked Questions

Do I need to delete my old server files before upgrading?

Yes. The Build 42 server is a separate binary, and leaving the old files in place can cause conflicts. Always back up first, then delete the old server folder and re-download the new one.

Will my base and vehicles survive the save conversion?

Most bases survive, but not perfectly. Walls, floors, and containers are preserved. Vehicles are kept, but their condition may be reset to a random value. Items that no longer exist in Build 42 will be deleted.

Can I run a Build 42 server with Build 41 clients?

No. The client and server must be on the same version. Players must update their game to Build 42 to join your server.

What should I do if a mod breaks my server after the upgrade?

Remove the mod from the server’s servertest.ini and the mods folder, then restart. If the server still crashes, check the server log for the specific mod ID that is causing the error.

Is there a way to skip the save converter?

No. The converter is mandatory for Build 41 saves. Starting a new world is the only alternative, but that loses all progress.

How long does the conversion take?

It depends on the size of the map and the server hardware. A small 2-3 player server might take 10 minutes, while a large, long-running server could take over an hour. Do not interrupt the process.

Related

• Screen Mirror iPad to Firestick: Full Guide
• Best Disk Space Analyzers to Find Lost Gigabytes
• Fish a Slime Codes? What They Are & How They Work