Build 42 Multiplayer Guide — Playing With Friends on 42.21 & 42.22

Yes — multiplayer is fully in the stable Build 42 release (current stable: 42.21 since September 28, 2026; pre-release testing build: 42.22 on the new Pre-Release branch, which replaced Unstable). You no longer need any beta branch to play stable co-op, and 42.21 already includes the big networking and zombie synchronization fixes that used to be beta-only. This guide covers the two ways to play with friends (host-your-own vs dedicated server), how to join, and the settings that make a co-op server feel right. If you want a 24/7 world that survives with you offline, that's the dedicated server guide — this page is the player-facing version.

How to play with friends — two options

Option 1 — Host a game and invite friends

1. From the main menu pick Solo Player → Host (or Co-op in the multiplayer tab).
2. Set the world rules (Sandbox settings are recommended for first co-op runs — start gentle).
3. Launch, then in the pause menu note your options: friends can join via the Friends list (Steam invite) or via direct IP.
4. Every friend needs the same game version (42.20 stable) and ideally the same mod list.

Direct-IP joins need port forwarding: UDP 16261 on the host's router/firewall. Without it, only Steam friend invites work (which use Steam's relay and are flakier). Port issues and "can't connect" are diagnosed on the error fixes page.

Option 2 — Join a server

• Steam server browser (main menu → Multiplayer) lists public B42 servers, filterable by version, population and mods.
• Direct IP — most private/friend servers: Multiplayer → Join → enter the IP:port (e.g. 1.2.3.4:16261).
• You will be prompted to accept the server's Workshop mod list on first join if it uses mods — accept to auto-subscribe.

Settings that matter for a friendly co-op server

Whether hosting or renting, these decide whether your group has fun or fights:

Roles that make a co-op team work

Build 42's systems reward specialization. A balanced group of 3–4:

• Scavenger/fighter — loot runs, clears areas, kites zombies.
• Builder/farmer — base security, farming, rain collectors.
• Cook/medic — cooking, first aid, keeps everyone fed and stitched.
• Mechanic — vehicles, fuel, repairs, and the group's wheels.

Spread occupations so every skill has an owner — the skill leveling guide shows who levels what fastest.

Build 42 multiplayer gotchas

• Version match is mandatory. A client on 41, 42.19 or a different branch than the server fails to connect. In Build 42.21, the game now displays a helpful notification if you attempt to connect to a server with a mismatched game version.
• Horde persistence & duplication (42.21): The notorious bugs where zombies duplicated on multiplayer reconnects or completely vanished when leaving/re-entering chunks were officially resolved in 42.21.
• High-ping driving and Z-level desync (42.21): Driving across chunks with 150+ ms latency is significantly smoother with reduced road pop-in voids, and vertical Z-level coordinates now synchronize cleanly.
• Split-screen co-op fixes (42.21): Read book statuses and XP multipliers are no longer mistakenly mirrored between players, fishing works properly, and crashes when sharing a moving car have been resolved.
• "Loading Map" infinite hang: If someone gets stuck loading into the world, make sure the server and client are running 42.20.3 or higher, where this connection handshake bug was officially fixed.
• B41 mods don't work — apply the compatibility rules in the mods guide before the group joins. (loadstring was re-enabled in 42.21).
• "Kicked for moving too fast" — a server option (KickFastPlayers) that can bug out on 42.20; it's a known irritant, not a cheat accusation.
• 42.22 pre-release servers: if the group wants the new ladders and crowbar door-forcing, every client and the server must be on the Pre-Release branch; a mismatch is refused at connect. See the Pre-Release branch guide.

42.22 multiplayer fixes (Pre-Release)

The October 27, 2026 build targets the bugs that ended multiplayer sessions rather than adding features:

• Damage is calculated server-side. Hits that visually connected but applied no damage are the symptom this addresses — PvP damage authority moved off the client.
• The doppelganger bug is fixed. A second copy of your character could appear at a chunk edge and get the real player killed; the studio credits community member Six for helping isolate it.
• Trailers and towing. Multiple trailer-physics and towing-desync fixes, and a moving vehicle no longer disappears permanently after a reconnect or server restart.
• Map knowledge. "All Known On Start" works in MP again, map notes survive being stored in a container and retrieved, and the whisper-chat server-wide close on disconnect is fixed.
• Server stability and backups. Fixes for a freeze with "Entity is already registered", a StackOverflowError in ItemContainer.getCharacter(), and a hang from IsoGridSquare.BurnWalls on multi-tile objects — plus faster backup creation.

Remember: a 42.22 server needs every client on 42.22, and it is still a Pre-Release build (branch guide).

Verified on 42.21 stable & 42.22 Pre-Release · Last checked 2026-10-30

Multiplayer-in-stable status and dates from The Indie Stone announcements; post-launch multiplayer hotfix changes (42.20.1 through 42.20.4) and the Build 42.21 release ("Re-population of the Dead", Unstable September 23, stable September 28, 2026) verified against official release notes; settings and joining flow from hosting-provider B42 multiplayer guides. 42.22 multiplayer fixes from the official "42.22 Patch Notes" (October 27, 2026).