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Document Control

Field Value
Document ID admins-foundation
Version 1.1.0
Status approved
Confidence Medium
Category (track) Admins
Build both
Owner PZ Knowledge-Base Pipeline
Created 2026-07-30
Updated 2026-10-08
Review due 2027-01-07
Game versions verified 41.78.16, 41.78.21, 42.20, 42.21

Executive Summary

A Project Zomboid dedicated server is a headless Java game-server process, distributed free of charge as Steam App ID 380870 and installable with an anonymous SteamCMD login on Windows or Linux [7]. This document is the foundation of the Admins track: it maps what the server is, which build branches exist and why build matching matters, where the configuration lives (servertest.ini vs servertest_SandboxVars.lua), which ports must be open, how memory is sized, how mods are wired in, and what the open-source admin tooling landscape looks like. Deeper Admins-track runbooks (installation step-by-step, settings reference, RCON operations, mod management) will hang off this overview.

The operational headline of mid-2026 is the branch split. Build 42.20 became the stable public build on 2026-07-29, the first stable release of Build 42 and the first stable build with B42 multiplayer [1] [2]. Build 42.21 has been the stable build since 2026-09-28 [22]. Communities that want to stay on Build 41.78 — whose saves cannot migrate — move server and players together to the legacy41 Steam beta branch [2] [3]; that branch is still maintained, with 41.78.21 a security hotfix on 2026-08-26 [21]. Build 42's multiplayer, restored in unstable 42.13 (2025-12-11), arrives at stable with a reworked, re-enabled anti-cheat and item-handling logic moved server-side, which changes both the security posture and the resource profile of a B42 server [1] [5] [6].

Document-level confidence is Medium: the branch, port, file-layout and anti-cheat spine rests on official announcements and current pzwiki revisions (High), but the hardware-sizing numbers that admins most want — RAM per player tier and CPU single-thread behaviour — exist only in hosting-company and community sources with conflicting values, and are quarantined below rather than stated as fact.

Key Takeaways

  • The dedicated server is a free Steam tool: SteamCMD App ID 380870, installed with login anonymous — no game purchase is needed on the host account (cited) (both)
  • Build 42.20.0 became stable on 2026-07-29 and 42.21 has been the stable build since 2026-09-28; B41 communities run app_update 380870 -beta legacy41 validate and keep their players on the legacy41 client beta — server and clients must be on the same build (cited)
  • Two config surfaces per server name: servertest.ini (server/network/mod settings) and servertest_SandboxVars.lua (gameplay rules), plus spawnpoint and spawnregion Lua files, all under Zomboid/Server in the server user's home directory (cited) (both)
  • Open UDP 16261 and 16262 to the internet; RCON listens separately on RCONPort (default 27015) guarded by RCONPassword (cited) (both)
  • Memory is set with JVM -Xms/-Xmx flags; the B42 StartServer64.bat ships with a 16 GB default you must edit down or the server can fail to start (cited) (B42)
  • The claim that B42 needs roughly +2 GB RAM over B41 at each player tier is hosting-company sourced and inconsistent between vendors — treat it as planning folklore, not fact (community, unverified)
  • 42.20 reworked and re-enabled anti-cheat, moved item anti-cheat server-side, and fixed item-spawning/XP/foraging exploits — B42 servers police more logic server-side than B41 did (cited) (B42)
  • Prefer open-source RCON tooling (pz-admin, zomboid-rcon, rcon-cli) over hosting-panel lock-in; all three are maintained, permissively or copyleft licensed, and work against any PZ server (cited) (both)
  • Mods are wired with the dual list WorkshopItems= (Workshop IDs) plus Mods= (mod loading IDs); the widely repeated B42 backslash-prefix convention for Mods= entries has no primary source and conflicting secondary ones (community, unverified)

Purpose

This is the first document a server administrator should read in this knowledge base. It answers: what process am I actually running, which build branch should my community be on, where do I change things, what must be open on the network, how big a machine do I need (and how much of the popular sizing advice is actually verifiable), and which tools should I administer with? It is a map, not a runbook — the deeper Admins-track documents carry the copy-pasteable procedures.

Scope

Covered: the dedicated server as a distribution and a process; SteamCMD installation at overview level; the stable/legacy41/42.19 branch picture and build matching; the configuration file surfaces and how changes are applied; required ports and RCON; memory sizing mechanics and the state of the evidence on RAM/CPU requirements; Build 42's server-side/anti-cheat shift; self-hosting (Ubuntu-first) versus managed hosting; the open-source admin tooling landscape; and mod wiring basics.

Not covered: a full step-by-step install runbook, the per-setting server.ini and SandboxVars reference, admin command and RCON operation catalogues, backup/migration procedures, and performance tuning — each is a deeper Admins-track document. Player-facing Build 42 changes are in players-foundation; mod development is in modders-foundation. Unstable-branch behaviour after 42.20 is out of scope.

Definitions

  • Dedicated server — the standalone headless server distribution of Project Zomboid (Steam App ID 380870), as opposed to the in-client "Host" option that runs a server inside a player's game session [7].
  • SteamCMD — Valve's command-line Steam client, used to install and update server files with an anonymous login [7].
  • servertest — the default server name; config files and the save folder are named after the server name, so servertest.ini, servertest_SandboxVars.lua, etc. [7] [8].
  • SandboxVars — the Lua file of gameplay-rule settings (loot, zombies, world clock) for a server, distinct from the .ini server settings [7] [8].
  • RCON — remote console: a password-protected network interface for issuing admin commands to the running server from external tools [8] [13].
  • legacy41 — the Steam beta branch (client and server) that keeps Build 41.78 installed after Build 42 became the default stable build [2] [7].
  • GSP — game server provider; a company renting managed game servers. The Indie Stone runs a feedback channel for medium-to-large Zomboid GSPs [4].

Build Applicability

Build Applies Verified against Notes
B41 (legacy41) Yes 41.78.16; latest primary-attested legacy release 41.78.21 Server reachable via app_update 380870 -beta legacy41 validate [7]; B41-only values tagged (B41); 41.78.21 (2026-08-26) is a security hotfix [21]
B42 (stable) Yes 42.20; re-checked against 42.21 42.20.0 stable since 2026-07-29 [1] [2], 42.21 stable since 2026-09-28 [22]; B42-only values tagged (B42)

Revision note (2026-10-07): this document was re-checked against the Steam posts for 42.20.1, 42.20.3, 42.20.4/41.78.21 and 42.21 stable and the TIS forum 42.21 patch notes (selected items only) [19] [20] [21] [22] [23]. Only statements those sources affect were changed; everything else is carried forward unchanged from the 2026-07-30 review (41.78.16, 42.20) with no contradicting change found in them. It was not re-tested on a live server, and the cited pzwiki revisions were not refreshed against 42.21.

The pzwiki pages cited for the file layout, ports and settings are versioned against 42.20.0 [7] [8]. The same surfaces (App ID, ports, file names, dual mod lists) existed on B41, but this document's wiki verification is against the B42-era page revisions; where a value is only confirmed for one build it is tagged inline.

Reference

What the dedicated server is

The dedicated server hosts Project Zomboid multiplayer on either Windows or Linux [7]. It is a Java process — the launch scripts invoke java against the zombie.network.GameServer class with JVM arguments for memory and native libraries [7]. On Windows the install ships three launch scripts: StartServer32.bat, StartServer64.bat and StartServer64_nosteam.bat; on Linux the equivalent is start-server.sh, with a -nosteam argument for hosting non-Steam (e.g. GOG) players [7]. On first launch the server prompts for an admin-account password, then generates default config and world data [7]. Servers running in non-Steam mode require clients to also run non-Steam, via the -nosteam launch option [7].

Getting the server: Steam and SteamCMD

There are two supported install paths: the "Project Zomboid Dedicated Server" tool in the Steam library's tools section, or SteamCMD [7]. The SteamCMD sequence is force_install_dir <path>, then login anonymous, then app_update 380870 validate — the server is App ID 380870 and needs no owned account [7]. The pzwiki Linux instructions (versioned 42.20.0) are Debian/Ubuntu-first: install steamcmd from apt, create a dedicated non-root pzuser, install to /opt/pzserver, and drive updates from a reusable update_zomboid.txt SteamCMD script [7]. The community-maintained Bobagi guide follows the same shape for Ubuntu 22.04/24.04 LTS — UFW firewall rules, a dedicated Steam user, SteamCMD install of App 380870, and screen for a persistent console session [14]. The wiki warns against launching the server through the Steam client UI; if that happens, file integrity should be re-verified [7].

The branch picture: 42.21 stable, legacy41 and 42.19

Build 42.20.0 was released to the stable branch on 2026-07-29 [1]; four hotfixes followed (42.20.1 on 2026-08-05, 42.20.3 on 2026-08-17, 42.20.4 on 2026-08-26) [19] [20] [21], and 42.21 reached stable on 2026-09-28 after an unstable test cycle that began 2026-09-23 [22] [23]. The studio says its standard procedure going forward is unstable first, then stable [22]. Build 41 savegames are not compatible with Build 42, and The Indie Stone's guidance to B41 server owners and their players is to opt into the legacy41 beta branch — client side via Steam Properties → "Game Versions & Betas" → legacy41, and server side by adding the beta flag to the SteamCMD update command: app_update 380870 -beta legacy41 validate [2] [3] [7]. A separate 42.19 beta branch exists for finishing unstable-era 42.19 saves, which are likewise incompatible with 42.20 [2]. The legacy line is still patched: 41.78.21 shipped on 2026-08-26 in a single hotfix note shared with 42.20.4 and 42.19.2 [21]. For B42 saves, the 42.21 notes state that saves from 42.20.4 should not be affected by 42.21 and advise backing up first [23]. A 42.20.1 fix also stopped broken B41 worlds from being hosted on B42 servers [19].

Build matching between server and clients is enforced in two ways: operationally, the checklist post tells server owners to move themselves "(and your players)" to the branch together before the switch date, because a community split across builds cannot play together [2] [3]; and mechanically, the DoLuaChecksum=true server setting kicks clients whose game files do not match the server's [8]. 42.20.1 further improved Lua checksum validation for multiplayer anti-cheat [19], and 42.21 added a notification for players who try to connect to a server running a different game version [23].

Configuration surfaces: server.ini vs SandboxVars

Four files define a server's configuration, all named after the server name (default servertest) and stored in the server user's home directory — %USERPROFILE%\Zomboid\Server on Windows, ~/Zomboid/Server on Linux [7] [8]:

File Governs Cited at
servertest.ini Server, network, security and mod settings (ports, RCON, Mods=, WorkshopItems=, anti-cheat toggles, PVP, whitelist behaviour) [7] [8]
servertest_SandboxVars.lua Gameplay sandbox rules (the option space players know from sandbox mode) [7] [8]
servertest_spawnpoints.lua Custom spawn points [7]
servertest_spawnregions.lua Which named regions players may spawn in [7]

If these files do not exist on startup, the server generates them with defaults [7]. Settings can be edited either through the game client's Host → Manage Settings editor (saving under the name servertest) or directly in a text editor [7]. Two admin commands close the loop: showoptions prints the live option values, and — for servertest.ini specifically — the wiki documents that edits can be saved while the server runs and applied with the reloadoptions admin command [7]. No equivalent live-reload is documented for SandboxVars; the widely circulated blanket rule "always stop the server before editing config" is examined in the quarantine section (Claim 4). Renaming a server (-servername) switches which .ini/Lua set and which save folder is used [7] [9].

Ports and RCON

Two UDP ports must be reachable by clients: 16261 (the default port, DefaultPort=16261) and 16262 (the direct-connection port, UDPPort=16262) [7] [8]. On a Linux host with UFW, that is sudo ufw allow 16261/udp and 16262/udp followed by a reload [7]. Each additional server instance on the same machine needs its own pair of free UDP ports [7]. A UPnP=true option can attempt automatic port mapping on home gateways, falling back to defaults on failure [8]. Both ports can also be overridden at launch with the -port and -udpport startup parameters [9].

RCON is separate from the game ports: RCONPort (default 27015) with RCONPassword, which the settings reference tells you to pick strong [8]. Standard Source-RCON-protocol tools interoperate with it — the gorcon rcon-cli project lists Project Zomboid among its supported games [13]. Admin commands can equally be issued at the server console, or in-game with a leading / by users holding admin access [7].

Memory: how it is set, and what is actually documented

Server heap size is controlled by the JVM -Xms (initial) and -Xmx (maximum) flags in the launch script — the same mechanism as the client's launch options [7] [9]. Two documented facts anchor sizing (B42, wiki page versioned 42.20.0): the shipped StartServer64.bat specifies 16 GB by default, and the wiki warns that -Xms/-Xmx must be edited down to fit the machine's real RAM — oversized values stop the server from launching at all, exiting with memory errors [7]. The wiki's worked example runs a server at 6 GB [7]. This launch-script value is equivalent to the "Server Memory" option in the in-client Host screen [7]. The Ubuntu community guide configures the same limits via ProjectZomboid64.json [14]. Beyond these mechanics, The Indie Stone publishes no RAM-per-player table for either build; every such table in circulation is hosting-company or community material, and the popular "+2 GB for B42" figure is quarantined below (Claim 1).

Multiplayer scale and Build 42's server-side shift

When B42 multiplayer shipped in unstable 42.13 (2025-12-11), it was explicitly work-in-progress for stress testing: the release guidance was to prefer Steam co-op or whitelisted servers, keep dedicated servers to at most 20 player slots "for now", disable mods (even client-side ones), and avoid debug mode during MP sessions [5] [6]. Those are unstable-era operating limits, published for 42.13 and not re-stated (either as still-current or as lifted) in the 42.20 release notes [1] [5].

Later hotfixes extended the multiplayer picture. 42.20.1 fixed a chunk-unloading performance problem on multiplayer servers that the notes link to server lag on high-population servers [19]. 42.20.3 improved server player-limit handling, including support for up to 254 players and administrator access when a server is full [20]; the post does not say what population is performant, so it does not by itself rescind the 42.13-era slot advisory. 42.21 added a server-browser change (the update column shows the last wipe rather than the last restart), fixed players entering dedicated servers without correct Steam authentication (which had blocked SteamID bans), expanded the anti-cheat system, and fixed a memory leak caused by eating from a vehicle trunk (selected items from the forum notes) [23]. Memory-leak fixes also appear in 42.20.1 and 42.20.3 [19] [20].

At stable, 42.20's release notes describe a security-hardened server: anti-cheats were "Re-Worked and Re-Enabled"; the antiCheatItem mechanism was "removed as now server-side"; anti-cheat logging was improved and false positives fixed; and a series of exploits was closed — arbitrary item spawning, illegal XP gains, foraging manipulation, and a hole that let clients duplicate a dedicated server's map data (B42) [1]. The current server-settings reference exposes a family of AntiCheat* toggles — among them checksum, hit, no-clip, packet-exception, permission, player, safehouse, safety, speed and XP checks — alongside a SteamVAC switch (B42, page versioned 42.20.0) [8]. 42.20 also added server-operator conveniences: a "Show coordinates" server option, ZNet/packet-logging improvements, object-pool statistics for monitoring, and fixes for server hangs during chunk generation and for map-visited data not persisting across restarts [1]. The community claim that this server-side shift makes B42 meaningfully heavier on CPU than B41 is quarantined below (Claim 3).

Mod wiring basics

Mods are declared in servertest.ini on two paired lists: WorkshopItems= carries the Steam Workshop item IDs the server should download, semicolon-separated (the reference example is WorkshopItems=514427485;513111049), and Mods= carries the mod loading IDs — the internal IDs found in each mod's info.txt under the Workshop content folder [7] [8]. The wiki's workflow is: collect the mods in a Workshop collection, extract the paired IDs (it points to a community ID-grabber tool), then paste them into the two lists [7]. Workshop mods added to a server are downloaded automatically by connecting clients [7]. Mod and server authors should also know the loadstring and loadstream Lua methods were removed in 42.20.4 as part of a security fix [21] and re-enabled in 42.21 [22] [23]; the same 2026-08-26 hotfix note also covers the 41.78.21 legacy build, and no source reviewed says whether legacy41 got them back. On B42, mods themselves use the new versioned folder layout (a mandatory common/ folder plus 42.x version folders) rather than B41's flat media/ layout, which is why B41-era mods needed restructuring for B42 servers (B42) [10]. The claimed B42-only backslash prefix on Mods= entries could not be verified against any primary source and is quarantined below (Claim 2).

The admin tooling landscape

Three open-source tools cover most day-two administration without tying you to a hosting panel:

Tool What it is License Notes
pz-admin (beyenilmez) Desktop app (Windows/Linux) managing servers over RCON: terminal, server options, player management, bans, XP/items/vehicles, world save MIT Actively developed [11]
zomboid-rcon (jmwhitworth) Python library (pip install zomboid-rcon) wrapping PZ RCON with 30+ command methods for scripting and bots GPL-3.0 On PyPI [12]
rcon-cli (gorcon) Generic Source-RCON CLI with explicit Project Zomboid support; single binary, Docker image MIT Fits cron jobs and CI [13]

For process supervision on Linux, the wiki documents a systemd unit + FIFO socket pattern but notes that the developers explicitly discourage systemd management — the in-place shutdown safeties should save the world, but there are no guarantees — and its own baseline instructions use tmux instead [7]. The Ubuntu community guide uses screen for the same purpose [14].

Self-hosting vs managed hosting

Self-hosting is fully supported by the free server distribution and first-class Linux instructions [7] [14]. Managed hosting outsources the machine but not the knowledge: every surface in this document (branch selection, .ini/SandboxVars, mod lists, RCON) still has to be operated, typically through the vendor's panel. The Indie Stone signalled at the stable release that it is engaging game server providers directly — medium-to-large GSPs are invited to a feedback channel via it@theindiestone.com — which is the closest thing to an official hosting-ecosystem programme [4]. Hosting-company knowledge bases are useful procedure mirrors but are marketing-adjacent; this knowledge base treats their numbers as corroborate-only (see Claims 1 and 3).

B41 vs B42 Delta

Area Build 41.78 (B41) Build 42.20 (B42)
Branch Served from the legacy41 beta branch; installed with app_update 380870 -beta legacy41 validate [2] [7]; latest primary-attested release 41.78.21 [21] Default stable branch since 2026-07-29 (42.20.0), 42.21 since 2026-09-28 [1] [2] [22]
Saves & matching B41 worlds live only on legacy41; server and players must move branch together [2] [3] B41 and 42.19-unstable saves cannot migrate to 42.20 [2]
Multiplayer maturity Long-stable MP [2] MP restored in unstable 42.13 (2025-12-11) with a 20-player-slot advisory and mods-off guidance; first stable MP in 42.20; up to 254 players supported by the server limit handling from 42.20.3 [1] [5] [6] [20]
Anti-cheat B41-era anti-cheat model Anti-cheat reworked and re-enabled at 42.20; item anti-cheat moved server-side; item-spawn/XP/foraging exploits and a map-data-copy exploit fixed [1]
Server-side surface Fewer server-side checks AntiCheat* toggle family (checksum, speed, XP, safehouse, etc.) plus improved server logging/monitoring in the 42.20-era settings reference [1] [8]
Mod layout served Flat media/ mod structure [10] Versioned common/ + 42.x mod folder structure; B41 mods need restructuring [10]
Default launch memory Not verified against a B41-era page revision in this document StartServer64.bat defaults to 16 GB -Xms/-Xmx, must be edited to fit the host [7]
Config surfaces Same four-file surface (.ini, SandboxVars, spawnpoints, spawnregions) — continuity implied by the legacy41 install path sharing the workflow, verified only against the 42.20-versioned pages [7] [8] Same, verified at 42.20.0 [7] [8]
Hardware appetite Baseline reference point in community sizing tables [15] Community and hosting sources agree B42 is heavier, disagree on magnitude — see Claims 1 and 3 [15] [16]

The one-line version: the server you operate is recognisably the same artifact on both builds — same App ID, ports, files and mod lists — but B42 changes what runs inside it (server-side anti-cheat, heavier world simulation) and what runs beside it (versioned mod structure), and the branch split makes build matching an explicit operational duty [1] [2] [7] [10].

Practical Guidance

  • Pick the branch before you build the server. A community's build choice is a migration decision, not a config flag: B41 worlds stay on legacy41, and your players must switch their clients the same day you switch the server. Put the branch in your server's documentation and announce switch dates ahead of time.
  • Pin your update command. Encode the branch into the SteamCMD script (app_update 380870 validate vs app_update 380870 -beta legacy41 validate) so a routine update can never silently hop builds.
  • Treat servertest.ini and SandboxVars as two different change classes. .ini changes can be applied to a running server with reloadoptions; sandbox and mod-list changes should be treated as restart-required. When in doubt, schedule a restart — it is never wrong, while an edit the server ignores can be.
  • Size memory from evidence, not the shipped default. Start from the wiki's worked 6 GB example for a small server, set -Xms equal to -Xmx, and grow based on observed usage — do not leave the 16 GB B42 default on an 8 GB VPS (it can refuse to start), and do not buy RAM off a hosting table without reading Claim 1.
  • Favour clock speed when choosing hardware, per the corroborated-but-unofficial single-thread picture in Claim 3 — and verify with your own tick-lag observations rather than CPU marketing.
  • Standardise on RCON tooling early. rcon-cli for scripts and cron, zomboid-rcon for anything Python, pz-admin for a GUI. All three work identically against self-hosted and rented servers, which keeps your operations portable if you change hosts.
  • Wire mods as pairs and stage them. Every entry in WorkshopItems= needs its partner entries in Mods= (a Workshop item can contain several mod IDs). Test mod-list changes on a copy of the server first: B42's stricter server-side checks and the 42.13-era "mods off" guidance both signal that mods are the least-stable layer of a B42 server.
  • If you self-host on Linux, follow the Ubuntu-first path — apt SteamCMD, dedicated non-root user, /opt/pzserver, tmux or screen — and avoid systemd supervision until the developers' discouragement changes.

Common Pitfalls & Troubleshooting

  • Server updated to 42.20, players on legacy41 (or vice versa). Mismatched branches cannot play together; DoLuaChecksum kicks mismatched clients even on the same branch if files differ [2] [8]. Fix the branch on the SteamCMD command line, not by guesswork.
  • B42 server fails with "Assertion Failed: Illegal termination of worker thread". Documented when B42 server files land on a machine that previously hosted a B41 server: confirm steam_appid.txt contains only 108600, back up your .ini/Lua/saves, then remove the old Zomboid data folder and let the B42 server recreate it [7].
  • Server exits at startup with memory errors. The B42 StartServer64.bat default of 16 GB exceeds many hosts; edit -Xms/-Xmx first, not last [7].
  • Launched the server via the Steam client and things broke. The wiki explicitly warns against launching through Steam; verify file integrity and use the launch scripts [7].
  • Mods listed but not loading. Check the pairing (WorkshopItems= and Mods= both populated), the separator (semicolons in the documented WorkshopItems example [8]), and — on B42 — whether the mod actually ships the B42 common/ + 42.x structure [10]. Be aware of the conflicting backslash advice in Claim 2 before copying a hosting KB's format.
  • "It worked while I edited it live, then my change vanished." Only .ini + reloadoptions is a documented live path [7]; treat everything else as restart-required and see Claim 4 for the community rule and its status.
  • Ports open but nobody can join. Both UDP ports (16261/16262) must be forwarded and allowed by the OS firewall; a second instance on the same box needs its own additional UDP pair [7].

Community Notes & Unverified Claims

Claim 1 — A B42 server needs roughly 2 GB more RAM than a B41 server at every player tier

  • Claim: Hosting knowledge bases advise sizing B42 servers by adding about 2 GB to each B41 tier (e.g. 4 GB → 6 GB for a small vanilla server); one vendor KB states it as "add roughly 2 GB to every B41 number" [15]. Other vendor guidance is materially higher, calling 10 GB "the practical entry point" for B42 [16].
  • Why unverified: No official Indie Stone sizing table exists for either build; all sources are hosting-company KBs (marketing-adjacent), and their absolute numbers conflict with each other by roughly a factor of two even while agreeing B42 is heavier.
  • Confidence: Low. The direction (B42 needs more RAM than B41) is consistently reported, but the specific "+2 GB per tier" figure is effectively single-sourced and contradicted by other vendors' tables.

Claim 2 — B42 requires a backslash before each Mod ID in the Mods= line

  • Claim: Several hosting guides state that Build 42 changed the Mods= format to require a leading backslash per entry (Mods=\ModOne;\ModTwo), calling it a top cause of "installed but not loaded" mods on B42 [17].
  • Why unverified: No primary source documents it; the pzwiki Dedicated server page (versioned 42.20.0) shows the Mods= workflow with no backslashes [7], and at least one other hosting KB's B42 example also omits them [18]. Secondary sources therefore conflict.
  • Confidence: Low. The claim circulates widely enough to matter operationally, but the best fact-only source contradicts it; it may describe a transient unstable-era behaviour or a particular panel's convention.

Claim 3 — Server performance is dominated by single-thread CPU speed, and B42's server-side logic makes it heavier on CPU than B41

  • Claim: Hosting and community sizing guides consistently state that the PZ server's main simulation loop is effectively single-threaded, so clock speed beats core count (one KB's example: a 3.8 GHz desktop CPU outperforming a 2.4 GHz many-core Xeon), and that B42 is hungrier for CPU as well as RAM [15] [16].
  • Why unverified: No Indie Stone statement or profiling publication confirms the threading model; the primary record confirms B42 moved anti-cheat/item logic server-side [1] but says nothing about CPU cost.
  • Confidence: Medium. Multiple independent secondary sources agree and the mechanism is plausible given the documented server-side shift, but it rests entirely on non-primary sources.

Claim 4 — Always stop the server before editing any config file, or your edits will be lost

  • Claim: Community and hosting guidance widely instructs admins to fully stop the server before touching servertest.ini or SandboxVars, on the basis that the running server holds settings in memory and rewrites the files; one vendor KB notes mod loading "happens at boot only" [18].
  • Why unverified: As a blanket rule it conflicts with the wiki's documented live path — .ini edits saved while running and applied via reloadoptions [7]; no primary source states that the server overwrites on-disk edits at shutdown.
  • Confidence: Medium. Restart-required is demonstrably the safe assumption for mod lists and anything undocumented, making the rule sound operational advice; its factual justification ("the server overwrites your edits") is the unverified part.

Risks & Caveats

  • Fast-moving stable. 42.20.0 was followed by hotfixes within days and by 42.21 two months later [19] [20] [21] [22]; the anti-cheat toggles and server options cited from 42.20-era pages were not re-read against 42.21, which fixed server options such as Safety Disconnect Delay and expanded anti-cheat [23].
  • Unstable-era operating limits. The ≤20-player and mods-off guidance was published for 42.13 unstable and has not been re-stated or rescinded for 42.20 [5] [6]; 42.20.3 added support for up to 254 players without commenting on performance [20]. Treating the old advisory as still-binding is conservative, not confirmed.
  • Wiki verification is B42-sided. The cited Dedicated server and Server settings revisions are versioned 42.20.0 [7] [8]; B41 continuity of those surfaces is asserted from the unchanged workflow and the legacy41 install path, not from a B41-era page revision.
  • All hardware-sizing numbers are non-primary. Claims 1 and 3 are the load-bearing caveat of this document: no RAM table or threading statement here comes from The Indie Stone.
  • Steam announcement mirrors. Primary citations use steamcommunity.com announcement URLs (verified via the ISteamNews API mirror per project source policy); these hosts bot-block automated link checkers.

Verification Steps

  1. Confirm the release/branch facts: query https://api.steampowered.com/ISteamNews/GetNewsForApp/v2/?appid=108600&count=25&maxlength=0 and read "Build 42.20.0 Stable Released" (2026-07-29) and "B42 CHECKLIST" (legacy41/42.19 instructions).
  2. Confirm the install path: run SteamCMD login anonymous + app_update 380870 validate against a scratch directory and verify the server files (including the three StartServer scripts on Windows) appear.
  3. Confirm the config surface: start the server once, then list Zomboid/Server in the server user's home directory and check for the four servertest* files.
  4. Confirm ports and RCON: read DefaultPort, UDPPort, RCONPort in the generated servertest.ini; connect with rcon-cli against RCONPort using RCONPassword and issue showoptions.
  5. Test the live-reload boundary: change a benign .ini value while running, run reloadoptions, and confirm with showoptions; then verify a SandboxVars edit does not apply until restart.
  6. Resolve Claim 2 empirically: on a B42 test server, load the same Workshop mod with and without a backslash prefix in Mods= and record which variant loads (check the server console mod list at boot).
  7. Probe Claim 1's direction: run the same small scenario on a legacy41 server and a 42.20 server with identical -Xmx and compare steady-state memory in the server statistics/logs.

Open Questions

  • Do the 42.13-era MP limits (≤20 player slots, mods discouraged) still reflect The Indie Stone's guidance on current stable, or has scale headroom improved? 42.20.3 raised the supported player-limit ceiling to 254 [20] but states no recommended population; a dev statement or Thursdoid would resolve it [5] [6].
  • Were loadstring/loadstream re-enabled on legacy41 as they were on 42.21 stable [21] [22] [23]? No source reviewed says.
  • Is the backslash Mods= convention (Claim 2) real on 42.20, and if so where did it originate — game code, a specific unstable build, or hosting-panel tooling? The empirical test in Verification Steps resolves the behaviour; a changelog entry would resolve the provenance.
  • What does the B42 server actually thread across cores (Claim 3)? Profiling on 42.20, or developer comment via the GSP feedback channel, would resolve it [4].
  • Will The Indie Stone publish official server sizing guidance now that a GSP engagement channel exists [4]?
  • Does the announced post-release Build 42 patching and support work change server resource behaviour later in 2026 [3]?

References

Primary Sources

  • [1] The Indie Stone — Build 42.20.0 Stable Released (Steam announcement, 2026-07-29; retrieved via Steam news API, ISteamNews app 108600). https://steamcommunity.com/games/108600/announcements/detail/1839676055882259. Accessed 2026-07-30.
  • [2] The Indie Stone — B42 CHECKLIST (Steam announcement, 2026-07-28). https://steamcommunity.com/games/108600/announcements/detail/1839041357038237. Accessed 2026-07-30.
  • [3] The Indie Stone — BUILD 42 STABLE PLANS (Steam announcement, 2026-07-23). https://steamcommunity.com/games/108600/announcements/detail/1839041357029453. Accessed 2026-07-30.
  • [4] The Indie Stone — NEXT STEPS (Steam announcement, 2026-07-09; game server provider initiative). https://steamcommunity.com/games/108600/announcements/detail/1836506165584147. Accessed 2026-07-30.
  • [5] The Indie Stone — Unstable 42 MP Released (Steam announcement, 2025-12-11). https://steamcommunity.com/games/108600/announcements/detail/1818752592123010. Accessed 2026-07-30.
  • [6] The Indie Stone — Build 42.13.0 UNSTABLE Multiplayer Released (Steam announcement, 2025-12-11). https://steamcommunity.com/games/108600/announcements/detail/1818752592122972. Accessed 2026-07-30.

Fact-Only Sources (no prose reuse) — pzwiki (CC BY-NC-SA 3.0): cite URL + revision id; facts only, never prose.

  • [7] PZwiki — Dedicated server (revision 1443349; page versioned against 42.20.0). https://pzwiki.net/w/index.php?title=Dedicated_server&oldid=1443349. Accessed 2026-07-30. Fact-only source.
  • [8] PZwiki — Server settings (revision 1443167; page versioned against 42.20.0). https://pzwiki.net/w/index.php?title=Server_settings&oldid=1443167. Accessed 2026-07-30. Fact-only source.
  • [9] PZwiki — Startup parameters (revision 1393745). https://pzwiki.net/w/index.php?title=Startup_parameters&oldid=1393745. Accessed 2026-07-30. Fact-only source.

  • [10] PZwiki — Mod structure (revision 1443271; page versioned against 42.14.0). https://pzwiki.net/w/index.php?title=Mod_structure&oldid=1443271. Accessed 2026-07-30. Fact-only source.

  • [19] The Indie Stone — 42.20.1 STABLE Hotfix Released (Steam announcement, 2026-08-05). https://steamcommunity.com/games/108600/announcements/detail/1840310314338766. Accessed 2026-10-07.

  • [20] The Indie Stone — 42.20.3 STABLE Hotfix Released (Steam announcement, 2026-08-17). https://steamcommunity.com/games/108600/announcements/detail/1840944183785895. Accessed 2026-10-07.
  • [21] The Indie Stone — 42.20.4 STABLE & 42.19.2 UNSTABLE & 41.78.21 LEGACY Hotfixes Released (Steam announcement, 2026-08-26). https://steamcommunity.com/games/108600/announcements/detail/1842212951296601. Accessed 2026-10-07.
  • [22] The Indie Stone — Build 42.21 Stable Released (Steam announcement, 2026-09-28). https://steamcommunity.com/games/108600/announcements/detail/1844751498231307. Accessed 2026-10-07.
  • [23] The Indie Stone — 42.21 Patch Notes (TIS forum topic 101693, selected changes only; 2026-09-23). https://theindiestone.com/forums/topic/101693-4221-patch-notes/. Accessed 2026-10-07.

Secondary & Corroborating

  • [11] beyenilmez — pz-admin (MIT; RCON desktop admin app for Project Zomboid servers). https://github.com/beyenilmez/pz-admin. Accessed 2026-07-30.
  • [12] jmwhitworth — zomboid_rcon (GPL-3.0; Python RCON library, PyPI package zomboid-rcon). https://github.com/jmwhitworth/zomboid_rcon. Accessed 2026-07-30.
  • [13] gorcon — rcon-cli (MIT; Source RCON CLI listing Project Zomboid support). https://github.com/gorcon/rcon-cli. Accessed 2026-07-30.
  • [14] Bobagi — Project-Zomboid-Ubuntu-Server (MIT; Ubuntu 22.04/24.04 dedicated-server guide, App ID 380870). https://github.com/Bobagi/Project-Zomboid-Ubuntu-Server. Accessed 2026-07-30.
  • [15] DoomHosting — Project Zomboid Server RAM Requirements (Build 41 vs Build 42) (hosting KB; corroborate-only). https://www.doomhosting.com/help/articles/project-zomboid-server-ram-requirements. Accessed 2026-07-30.
  • [16] Pinehosting — How Much RAM Do You Need For Build 42 Project Zomboid Server Hosting? (hosting KB; corroborate-only). https://pinehosting.com/blog/how-much-ram-do-you-need-for-build-42-project-zomboid-server-hosting/. Accessed 2026-07-30.

Community & Creator

  • [17] Pinehosting — Project Zomboid Build 42 Mods: Install And Fix Guide (hosting blog; source of the backslash claim). https://pinehosting.com/blog/modded-project-zomboid-server-hosting-build-42-install-steam-workshop-mods-fixes/. Accessed 2026-07-30.
  • [18] DoomHosting — How to Install Mods on a Project Zomboid Server (Build 42) (hosting KB; backslash-free counter-example). https://www.doomhosting.com/help/articles/how-to-install-mods-project-zomboid-server-build-42. Accessed 2026-07-30.

Further Reading

Further Reading

  • The Steam news API mirror used to verify all primary announcements: https://api.steampowered.com/ISteamNews/GetNewsForApp/v2/?appid=108600&count=25&maxlength=0
  • Valve's SteamCMD documentation, referenced by the wiki install instructions: https://developer.valvesoftware.com/wiki/SteamCMD
  • Tiiffi/mcrcon, a minimal C RCON client also usable with PZ: https://github.com/Tiiffi/mcrcon
  • The official blog / Thursdoid feed (bot-blocks automated checkers; read in-browser): https://projectzomboid.com/blog/

Related Documents

  • players-foundation — the Players-track foundation (the branch split and save compatibility from the player's side).
  • modders-foundation — the Modders-track foundation (the B42 mod structure your server's mods must follow).
  • creator-foundation — the Creator-track foundation (multiplayer servers as content settings).
  • lore-foundation — the Lore-track foundation (what the world you are hosting actually is).
  • meta-style-guide — the style rules this document conforms to.

Revision History

Version Date Author Change Approved By
0.1.0 2026-07-30 KB Pipeline (virtual agent) Initial draft. —
1.0.0 2026-07-30 Orchestrator (KB Pipeline) Approved and frozen — foundation cluster release kb-release-2026.07.30. Standing mandate (2026-07-30)
1.0.1 2026-07-30 Orchestrator (KB Pipeline) License-hygiene prose rewrites after arming the pzwiki n-gram gate (no factual changes). Standing mandate (2026-07-30)
1.1.0 2026-10-07 KB Pipeline (revision worker) Re-baselined to 42.21 stable and legacy 41.78.21 against the 42.20.1, 42.20.3, 42.20.4/41.78.21 and 42.21 stable Steam posts and the 42.21 forum notes [19] [20] [21] [22] [23]: branch/version facts, 254-player limit, server-browser/auth/anti-cheat changes, loadstring status, save-safety note. Project owner (user instruction 2026-10-08)