Multiplayer

Official categories say single-player. Casualties Together, started by Krokosha666 and now owned by Creature, adds Steam lobbies on top of BepInEx. The readme says it supports the 1.0 game. This page is the whole multiplayer answer so a second page does not compete with it.

Official

I am not calling co-op an official mode. The store page I read has no multiplayer category. Anything below is the mod.

Install

Install BepInEx, then unpack the MPMod archive so its mod folder contents land in the game root. A multiplayer button shows on the main menu. To go back to single-player, the readme says Settings, General, Deactivate MP Mod. Host-only rules use rule name value. rule sv_cheats 1 opens the command set, and a client needs adminpriv. Those are mod commands, not vanilla ones.

What the readme lists

• Steam lobbies, voice with in-game effects, middle-click pings, right-click to treat.
• Shared body temperature when you stand close, spectator after death, carrying a downed player.
• Sleep is disabled. Hunger and thirst slow down. A dead player returns on the next layer.
• Unchipped is per player. PVP can be turned on. The MP3 player becomes a boombox.
• Half the players must reach an exit. Only the host sees the continue prompt. Everyone must board the drill pod.
• No client prediction, no anticheat. The authors call it a simple co-op experience.

Radiation in co-op

Russian players ask how to remove radiation in the multiplayer mod. The readme I read has no line for that. I am not inventing a console toggle. The mod Discord linked from the repository is the place that discussion actually happens: discord.gg/SA6H7mA6De, with discord.gg/FrBX3srzzG as the backup invite printed in the same readme.

Return to single-player / disable the mod

The project readme names Settings → General → Deactivate MP Mod as the in-menu switch back to single-player. Use that documented option before treating uninstalling the loader as necessary. For a reversible file-based uninstall, close the game, keep a backup of the save and move the multiplayer mod’s own files out of the plugin folder. Keep dependencies needed by other installed mods. The installation guide explains how to restore an earlier working set if an update caused the problem.