Use the community mod for co-op
No, the official Steam listings advertise Casualties: Unknown as single-player, with no native co-op mode. Playing with friends uses the third-party Casualties Together mod, whose compatibility requirements, multiplayer features and bugs belong to the community project.
Start by matching your game to the mod's supported build. Casualties Together 5.2.0 targets Casualties: Unknown Demo v7.0.1, so the installation below is for that Steam demo pairing. Do not apply it to an itch prototype or a full-release executable simply because the game name matches.
Match the game, loader and mod package
The supported pairing described here is Casualties Together 5.2.0 with Steam Demo v7.0.1 and BepInEx win-x64 5.4.23.5. That mod release does not establish full game 1.0 compatibility. Before copying files, have the host and joining player compare their game build, mod package and loader installation.
Open the mod author's installation instructions.
Check the mod author's release notes for the package you intend to use.
Use the author's instructions to obtain the Steam demo, the specified BepInEx archive and the multiplayer package from Nexus Mods. Keep these downloads together while installing so you can repeat the same setup on each player's machine. Matching the package matters more than merely having multiplayer files present.
Install the supported Steam demo setup
- Open the demo's game root through Steam: right-click the game in your Library, then choose Manage > Browse local files. This takes you to the installation folder used by the author's instructions and helps avoid copying the packages into the wrong game directory.
- Extract the specified BepInEx archive into that game root directory. Use the loader package linked by the author for this supported setup. Keep the extraction destination consistent with the game folder you just opened through Steam.
- Extract the multiplayer archive, open its mod folder, and move that folder's contents into the game root directory. Follow the author's distinction between the outer archive and the contents of the mod folder so the files reach the intended destination.
- Launch the demo from your Steam Library. Multiplayer-related buttons should appear on the main menu after installation. Use their appearance as the immediate check that the mod is active before you spend time troubleshooting a lobby.
- Have the joining player complete the matching installation and check their main menu too. Compare game and mod versions before attempting to connect. This separates a mod-loading problem on a player's machine from a failure that happens after both installations reach the multiplayer menu.
Separate missing buttons from lobby failures
If multiplayer buttons are missing, return to the installation checks before investigating the connection. Compare the targeted Steam demo build, mod version and BepInEx setup, then check that the packages were placed in the game root as instructed. Both players should reach the modded main menu.
If the buttons appear but Steam lobbies fail, launch through the Steam Library as the author recommends. Next, compare the host and joining clients' files and inspect the mod's release notes for known synchronization problems. Avoid changing several things together; check the result after each correction.
The author recommends a good internet connection and describes the project as a simple co-op experience. Players have also reported lag and disconnections. A failed connection is therefore not proof that the base game's official multiplayer is broken, since this multiplayer implementation comes from the mod.
Expect different survival rules in co-op
Casualties Together adds Steam lobbies, voice chat with in-game effects, player healing interactions and carrying incapacitated players. Sleeping is disabled and hunger and thirst are slowed. Treat these as mod behavior when planning a group run rather than assuming your single-player routine transfers unchanged.
Right-click another player to open the interaction menu, where you can inspect their health panel, access their inventory or use usable items on them. Staying close also shares body temperature. These interactions give you practical ways to help a struggling teammate instead of only watching their condition deteriorate.
Return to single-player through the mod setting
- Open Settings > General and press Deactivate MP Mod when you want to return to single-player. This is the author's documented deactivation method. Use that explicit option rather than guessing which installed files to delete.