Running Project Zomboid on the unstable branch
The Steam unstable branch for Project Zomboid is the live Build 42 development feed. The Indie Stone publishes patches here every few weeks and they are explicit that ongoing multiplayer games on this branch will be disrupted by future updates. If you have just picked unstable in the panel, this page is the short version of "what you have signed up for and how to switch back."
Why unstable exists
Build 42 is a multi-year overhaul of Project Zomboid: new map work, the animal husbandry system, the basements and crawlspace system, an internal renderer change, and a rewrite of the multiplayer code. The Indie Stone runs unstable as a public preview so adventurous players can test the next patch and report bugs back. Single-player on unstable is reasonably playable; multiplayer is rough.
The current upstream guidance is: do not run more than 20 players on a Build 42 multiplayer server, and prefer whitelist over public-listed.
Three things you should expect
1. Some sandbox settings revert on restart
Not all sandbox settings are wired through to dedicated servers yet. Zombie speed is the classic example: you can set it via the panel, the server accepts it, the file is written, and on the next restart it is back to default. This is not a Supercraft bug; it is an upstream limitation that ships with the build.
If a value you change keeps resetting, search the PZ wiki page for that setting before assuming the panel is broken. Settings that work in single-player do not always work on the dedicated server during Build 42.
2. Save format changes between patches
The unstable branch's save format has been changed multiple times. Saves created before the 42.13 patch are not compatible with later patches and the conversion process is known to break elements of the world. If a new unstable patch lands and your server fails to start with a save-format error, restoring from a pre-patch backup is the only safe path.
Take a Backup Now from the Installation tab before every unstable patch. Daily auto-backups also run, but a pre-patch manual backup gives you the cleanest rollback point.
3. Restart loops and clean exits that don't come back
The unstable branch's MP code path has rough edges. Two symptoms we see weekly:
- The server "exits cleanly" right after saving — the log shows
SaveAll took NNN ms,Saving finish, and then the process simply does not return. The panel's auto-restart picks it up after a few minutes, the server runs for a while, and the same thing happens again. - The world starts but the spawn point is in a wall, the floor, or the void. This is Build 42 spawn-point logic interacting with new map data.
For the first one: take a Backup Now and switch to the stable branch (see below). For the second one: edit the spawn region in the panel's Config tab, or use a sandbox spawn-point mod compatible with B42.
How to switch from unstable back to stable
- Click Stop in the panel.
- Take a backup. Save format diverges; do not skip this.
- Open the Installation tab, click Reinstall, choose the stable branch.
- Start the server.
Important: your unstable-branch world will probably not load on the stable branch. The unstable branch is a future version of the game and stable does not understand its file formats. Plan to start a fresh stable world, or wait until Build 42 promotes to stable before switching back.
If you absolutely need the world data, the right path is to wait for Build 42 to ship on stable, then come back to unstable's save and let the stable branch convert it. There is no supported way to downgrade a save.
When unstable makes sense
Unstable is the right branch for you if:
- You are testing a mod that targets Build 42 features (animals, basements, new map zones).
- You are previewing the next major patch so your community knows what is coming.
- You explicitly accept that the world may not survive a future patch.
It is the wrong branch if you want a stable long-running world with a regular play group. Pick the stable branch for that — Build 41 multiplayer is still actively patched and broadly compatible with every mod published in the last two years.
Where to check what is currently in unstable
The Indie Stone posts patch notes for each unstable bump in their dedicated forum thread. Steam News for the game also picks up the bumps. Glance at the most recent post before you click Reinstall — if there is a known-broken feature that matters to you, wait a week.
Ready to play it safe? Get a Project Zomboid server with one-click branch switching and daily backups across both branches.