Project Zomboid Build 42: What Changed and Mod Compatibility
Build 42 is a large enough overhaul that "will my server still work" is a fair question. Here's what actually changed, and the specific, testable reason your old mod list probably won't load without checking first.
What changed in Build 42
Build 42 is a major multiplayer overhaul, not an incremental patch. The netcode was rebuilt, which is the difference server owners feel most directly — noticeably higher stable player counts than late-B41 servers could hold without desync. The map expanded with new underground areas. It's the full B42 feature set running server-side, not a subset.
The part that catches people off guard isn't the headline features, it's the two things that don't carry over from a B41 setup: your world save, and your mod list.
Saves don't carry over
The world save format changed along with the engine work. A Build 41 save will not open on a Build 42 server — there's no import or conversion path. If you're moving an existing group from B41 to B42, plan on a fresh world, not a migration of the old one.
Why Build 41 mods don't load on Build 42 — tested, not assumed
This is the part most write-ups gloss over, because it's easy to assume "the game updated, obviously old mods need updating too" without actually knowing why. We tested it directly on a real server rather than repeating the assumption: a mod built for Build 41 simply does not load on a Build 42 server. A mod built and tagged for B42, installed on the exact same box in the same test, loaded and ran correctly.
The reason is structural, not a compatibility flag somewhere. Build 41 mods use a flat folder layout — the mod's Lua, media, and metadata sit directly under the mod's own top-level folder. Build 42 introduced a versioned layout: a mod that supports B42 places its content under mods/<ModName>/42/mod.info, a subfolder the game specifically looks for on B42. A B41-only mod has no 42/ folder at all — there's nothing there for a B42 server to find, so it doesn't load, silently, rather than erroring.
That's the concrete thing to check before you assume a favourite mod is broken or abandoned: has the author actually restructured it with a 42/mod.info folder, or does it only have the old flat layout. If the mod's own Steam Workshop page doesn't explicitly say it supports B42, the safe assumption is that it doesn't yet.
What this means for your mod list
Go through your existing mod list one at a time and check each one's Workshop page for an explicit B42 statement, rather than assuming the whole list transfers or the whole list breaks. Large, actively maintained mods (map expansions, major overhaul mods) tend to move first; small quality-of-life mods from less active authors are the ones most likely to be stuck on B41 for a while.
If your server genuinely can't run without a specific mod that hasn't moved to B42 yet, staying on the mod's B41 requirement rather than upgrading the server is a legitimate call — it just means holding off on B42's netcode and player-count improvements until that mod catches up.
Check mod compatibility from the panel before you commit
Add a mod ID or a Workshop Collection URL and restart — if it's not built for B42, it just won't appear, no crash to debug. Try it free for 24-hour, no credit card required.
Frequently asked questions
Will my Build 41 mods work on a Build 42 server?
Not automatically, and in most cases not at all yet. We tested this directly: a mod built for B41 does not load on a B42 server, because the mod folder structure itself changed, not just the game underneath it. A B42-native mod loaded fine on the exact same box in the same test. If a mod's own page doesn't explicitly say it supports B42, assume it doesn't until you've confirmed otherwise.
What actually changed in the mod folder structure?
Build 41 mods use a flat layout: the mod's code and media sit directly under the mod's own folder. Build 42 introduced a versioned layout — a mod that supports B42 places its content under mods/<ModName>/42/mod.info, alongside (or instead of) the old flat structure, so the game can tell which version of the mod matches which build. A B41 mod simply doesn't have that folder, so B42 has nothing to load.
Are Build 41 world saves compatible with Build 42?
No. The world save format changed along with everything else in the B42 overhaul. An existing B41 save will not open on a B42 server — you would be starting a fresh world.
Is Build 42 worth it if most of my mods are not updated yet?
That depends entirely on which mods your group can't live without. B42 is a genuinely large multiplayer overhaul — improved netcode, higher stable player counts, and new content the base game didn't have — but if your server's identity is built around two or three specific mods that haven't moved to B42 yet, waiting or checking with those mod authors directly is a reasonable call before committing a group to a fresh world.
How do I check if a specific mod supports Build 42?
Check the mod's Steam Workshop page directly — most actively maintained mods state their supported build in the description or a recent update note. If it doesn't say, the safest assumption is that it doesn't support B42 yet. Adding an unsupported mod to a B42 server via the panel's Workshop Mods section will simply not load anything for that mod on restart, rather than error loudly.