Quick Answer
Build 42.21 went Stable on September 28, 2026. What the official post records: the worst multiplayer zombie duplication issues and post-chunk-re-entry disappearances were addressed (some remained for a later update); XXL tree cutaway changed for players in vehicles (still in progress); loadstring/loadstream Lua methods, disabled in 42.20.4 during a security investigation, were re-enabled. Migration order: confirm 42.21 Stable on game, server, and every client; back up saves, profiles, and mod lists; launch a clean test before your main save; then recheck each mod's live Workshop page.
Table of Contents
Update decision
Treat the first launch after a patch as a controlled migration. 42.21 Stable landed 2026-09-28 — run each row in order and keep the old save and mod list available until the last one passes.
| Before launch | Do exactly this | If it fails |
|---|---|---|
| Branch | Read the version text on the game, the server, and every client — the unstable branch still exists alongside 42.21 Stable | Stop and align branches before touching mods — a version error looks exactly like a mod error at join time |
| Backup | Date-copy the save, the server profile, and the mod list into one folder before the first launch | Don't overwrite the only working copy — restore from the dated folder and retry |
| Clean test | Launch a clean, zero-mod session on the new branch before re-enabling anything | A clean failure means the game or the branch, not the mods — fix that layer first |
| Mod review | Re-enable mods in halves per launch — the captured 127-mod case needed about 7 restarts to bisect to the broken item | Keep bisecting; the half that still fails contains the culprit |
| Join test | Have one client join the clean session and copy any error message word for word | The exact missing-file or version text names the broken layer — match it against the mod list before changing anything |
What the release post actually fixed
Build 42.21 Stable, announced 2026-09-28. Each row is the official wording, with the caveat the wording carries — read the 'still open' column before closing your own bug report.
| Area | The official record | Still open |
|---|---|---|
| Zombie duplication and disappearance | The worst zombie duplication issues in multiplayer, and zombie disappearance after chunk re-entry in single-player and multiplayer, were addressed | Some instances remained for a later update — a duplication report on 42.21 is not automatically a 'you did something wrong' report |
| XXL trees | Cutaway behavior changed for players in vehicles, overhanging houses and furniture, and overall transparency | The work was still described as in progress — expect further changes, don't build guides on exact cutaway behavior |
| Lua: loadstring and loadstream | Both methods, disabled in 42.20.4 during a security investigation, were re-enabled after further investigation | Mods that avoided these methods for 42.20.4 compatibility may now have redundant workarounds — and any mod relying on them works again |
| Existing saves | 42.20.4 saves should not be affected | The official recommendation still stands: manually back up an existing save before testing it on the new branch |
The patch-day reports, and what they teach
The 42.21 transition produced a small pile of captured user reports. They are symptoms, not verdicts — but together they sketch the migration's real failure surface.
The launch failure: a game using the PZ Optimisation mod went from working to not-launching across the 42.20 to 42.21 boundary. One mod, one patch, total launch failure — the reason the migration order puts the clean test session before your main save. Note also what the thread proposed as a fix: a PowerShell command downloading and executing a remote script. Verify any script before running it; a patch-day panic is exactly when bad fixes look attractive.
The scale failure: a user with 127 installed mods reported menu, inventory, and trap-state problems with no single verified cause. That's the bisect situation — disable half, test, halve again — and also the bookkeeping argument: a written list of IDs and versions turns a 127-mod haystack into a searchable document. The invisible-subscription report (one subscribed mod present in the list, another missing) rounds out the set: patch days amplify every configuration ambiguity you were carrying.
The save boundary is the reassuring row: 42.20.4 saves should not be affected, per the official patch-notes topic. 'Should not' is the official phrasing, and the same topic recommends the manual backup before testing — which is the correct reading of every patch note ever written: the guarantee covers intent, your backup covers reality.
Reference data
These are release records from the official 42.21 post and patch-notes discussion.
| 42.21 change | What was reported | What to do |
|---|---|---|
| Stable release | Build 42.21 Stable announced on 2026-09-28 | Confirm the branch before loading a main save |
| Zombie behavior | Worst multiplayer duplication and chunk re-entry disappearance issues addressed | Recheck multiplayer after updating |
| Lua methods | loadstring and loadstream were re-enabled after investigation | Review scripts and server mods that depend on them |
| Save boundary | Patch notes recommend a manual backup before unstable-branch testing | Keep a dated save copy |
FAQ
When did Build 42.21 come out?
September 28, 2026 — the official blog announced Build 42.21 Stable on that date. That's your version-anchor for alignment: anything still reporting 42.20.x afterwards is on the wrong branch, holding an unfinished update, or deliberately pinned.
Does Build 42.21 break existing saves?
The official patch-notes topic says existing Build 42.20.4 saves should not be affected — and still recommends manually backing up a save before testing it on the new branch. 'Should not' covers intent; your dated backup covers reality. Keep the pre-update copy until you've verified your main save on 42.21.
Is the zombie duplication bug fixed in 42.21?
Mostly: the release post says the worst zombie duplication issues in multiplayer, and zombie disappearance after chunk re-entry in single-player and multiplayer, were addressed — while explicitly noting some instances remained for a later update. A duplication report on 42.21 isn't automatically user error; note it, capture the circumstances, and don't destroy a save over it.
Why won't my game launch after updating to 42.21?
The documented patch-day pattern is a mod: a captured report shows a game using the PZ Optimisation mod failing to launch right after the 42.20 to 42.21 update. Test a clean launch with mods disabled before blaming the patch — and read any community-proposed fix script (one thread proposed downloading and executing a remote install.ps1) before running it.
What happened to loadstring and loadstream in 42.21?
They're back: both Lua methods were disabled in 42.20.4 during a security investigation and re-enabled in 42.21 after further investigation. Mods that depend on them work again, and mods that carried 42.20.4 workarounds may now have redundant compatibility code — one more reason to recheck each item's live Workshop page after the update.