Project Zomboid 42.21 Update: Patch Notes for Servers (2026)

Q: Is loadstring back in Project Zomboid?

Yes. 42.21 re-enabled the loadstring and loadstream Lua methods that 42.20.4 had disabled for security reasons.

Project Zomboid Build 42.21 reached the stable branch on 28 September 2026, five days after it entered Unstable, per The Indie Stone's Steam announcement. It is the first incremental update after Build 42 went stable, and it fixes zombies vanishing and duplicating in multiplayer, improves XXL tree cutaways and re-enables the loadstring Lua methods that 42.20.4 removed.

What is in Project Zomboid 42.21?

The headline fixes, from the 42.21 Unstable notes and the stable post:

Disappearing zombies fixed. Zombies could vanish after a player left and re entered a chunk, most visibly on high population settings. Fixed in singleplayer and multiplayer.
Zombie duplication in MP. The worst cases are fixed. The Indie Stone says some instances remain and are planned for the next update.
XXL trees. The big trees added in 42.20 now cut away better for players in vehicles, no longer hide houses and furniture they overhang, and are more transparent overall.
loadstring and loadstream are back. They were disabled in 42.20.4 for security reasons. After further investigation, The Indie Stone re-enabled them.

What changed for multiplayer servers?

42.21 has a long multiplayer section. The parts that matter to admins:

Change	What it means
Expanded anti-cheat, safehouse exploits fixed	Fewer ways to grief or dupe on public servers
Steam authentication exploit fixed	Players could join without proper Steam auth, which bypassed SteamID bans
Version mismatch notification	Players on a different game version now get a clear message
Server browser shows the last wipe	"Server Update" now means the last wipe, not the last restart
High ping driving fixes (about 150 ms and up)	Less map pop in, passenger teleporting and juddering
Z level desync fix	Better sync of player coordinates between floors

There are also gameplay and balance changes your players will notice: characters re-equip items after exercise, 86 more fluid containers can purify water in ovens, washing machines clean dirty rags and bandages, fridges warm gradually on the day the power goes out, explosives work in basements, and Welders now start with welding recipes instead of blacksmithing.

Will 42.21 break my save?

The Indie Stone said existing saves on 42.20.4 "should not be affected," and the stable post reported no major issues found during Unstable testing. They still recommend backing up before playing an existing save on a new version, and so do we. Note that the zombie disappearance fix changes how chunks reload, so a long running world may suddenly feel busier in areas that had quietly lost zombies.

How did we get here? Build 42 stable timeline

Date	Release
29 July 2026	Build 42.20.0 stable: multiplayer on the stable branch, map overhaul, XL and XXL trees
5 August 2026	42.20.1 and 42.20.2: chunk unloading lag fix for high population servers, memory leak fixes, `%%` in mod text
17 August 2026	42.20.3: up to 254 players, admin access on full servers, "Loading Map" hang fix
26 August 2026	42.20.4 plus 41.78.21 legacy: security fixes, `loadstring` removed
23 September 2026	42.21 Unstable
28 September 2026	42.21 stable

The Indie Stone says this is now the standard process: every update goes to Unstable for community testing first, then to Stable. Build 41 remains on the legacy41 branch and received the 41.78.21 security hotfix.

Why did my mods break in August?

If a mod stopped working after 42.20.4 with Lua errors about loadstring, that is why. 42.20.4 removed loadstring and loadstream to close a security hole, and mods that used them, often to run code sent from the server, broke. 42.21 restores both, so updating the server and clients to 42.21 should bring those mods back without the author changing anything.

Separately, 42.20.2 changed how % is handled in mod text files: mods should now write %% to show a percent sign. A temporary workaround accepts both for now, but The Indie Stone said it will be removed in a future Unstable update, so mod text may look wrong later if authors do not update.

What this means for your server

Back up, then restart to update. A dedicated server needs to be on 42.21 for 42.21 clients, and vice versa. Steam updates players automatically.
Run checkModsNeedUpdate after the restart and restart again if any Workshop mod updated. A mismatch shows up as the workshop item version is different than server's error.
Expect more zombies in old areas if your world suffered from the disappearing zombie bug.
Check your ban list still works. SteamID bans now hold because the authentication exploit is closed.
Players on Unstable after 42.21 went stable should switch back to the default branch to match the server.

Updating a GamezServers Project Zomboid server

Our servers check Steam for a new version every time they start, so press Backup, then Restart, and the server comes back on 42.21. Use the Check mod updates button afterwards. If your group is finishing a Build 41 world, the Build 41 server type stays on legacy41 and is not affected. Plans are 6, 8 and 12 GB on Project Zomboid server hosting.

FAQ

When did Project Zomboid 42.21 come out?

42.21 entered Unstable on 23 September 2026 and was released to the stable branch on 28 September 2026.

Does 42.21 fix zombies disappearing?

Yes. The bug where zombies vanished after leaving and re entering a chunk is fixed in both singleplayer and multiplayer.

Is loadstring back in Project Zomboid?

Yes. 42.21 re-enabled the loadstring and loadstream Lua methods that 42.20.4 had disabled for security reasons.

Do I need a new world for 42.21?

No. The Indie Stone says 42.20.4 saves should not be affected. Back up first anyway.

What is the next Project Zomboid update?

The Indie Stone has said it is working on a Build 42 support update through the rest of 2026 focused on optimization, modding support and polish, plus release of its mapping and animation tools. No date has been given.