Casualties: Unknown is single-player only at launch. Here is what the community multiplayer mod supports, what breaks, and how to set up a co-op run safely.
Casualties: Unknown Multiplayer: Official Status and Co-op Mod
Casualties: Unknown has no official multiplayer. Steam lists the game as Single-player only, and the store page has no co-op, PvP or online tag. If you are looking for a lobby browser, dedicated servers or matchmaking, it does not exist at launch.
There is a community multiplayer mod. It is ambitious, experimental and built on BepInEx. This page explains what it can do, what it cannot, and how to avoid losing a save to it.
Official source: Casualties: Unknown on Steam ↗
Official status
| Feature | Status |
|---|---|
| Online co-op | Not supported |
| Local co-op | Not supported |
| PvP | Not supported |
| Steam Workshop | Not supported |
| Community multiplayer mod | Available on Nexus |
The Steam category list is the authoritative source: Single-player and Family Sharing only. The store page does not mention multiplayer, and the developers have not announced it.
The community mod
The multiplayer mod lives on Nexus Mods and requires BepInEx 5 LTS. It works by synchronizing player entities, physics and some world state between a host and clients.
It is not a official feature, and it is not a drop-in "click co-op" button. Expect:
- Host-based sessions. One player hosts; others join.
- Version lock. Everyone must run the same game version and the same mod version.
- Manual setup. The mod page is the only reliable install guide; steps change between releases.
- No matchmaking. You need the host's address or a virtual LAN.
Read the mod page before installing. This guide will not replace it, because the mod's setup changes faster than any article can track.
What syncs and what breaks
The hard part of modding this game is that it simulates bodies, not just positions. The mod has to synchronize:
- Player position, velocity and ragdoll state.
- Limb damage, bleeding and fractures.
- Inventory and item pickup.
- Enemy AI and physics.
- Layer state, traps and doors.
In practice, the more of that list a mod version covers, the more stable it needs to be. Common symptoms:
- Desync: one player sees a dead enemy, the other sees it alive.
- Ragdoll mismatch: a player is standing on your screen and unconscious on theirs.
- Item duplication: both players pick up the same item.
- Save drift: the host's save and the client's state diverge.
- Crash on layer transition: the most common failure point.
None of these are fixed by "just reinstalling". They are the cost of a community networking layer on a single-player simulation.
Setting up a co-op run
A safe procedure:
- Back up saves. Both players. Before every session.
- Install BepInEx 5 LTS on both machines.
- Install the same multiplayer mod version on both.
- Launch once to generate configs, then close.
- Host starts a fresh run. Do not use an existing save for the first test.
- Client joins using the method on the mod page.
- Play 10 minutes. Test a fight, a layer transition and an item pickup.
- If it holds, continue. If it desyncs, stop and restart both games.
Use a fresh save for the first session. A mod that corrupts a new save costs ten minutes; a mod that corrupts a 20-hour save costs the run.
Performance
Multiplayer adds network and physics load. The host does most of the work.
- Host on a wired connection.
- Close background apps. The game is CPU-bound already.
- Lower the danger and loot settings if you are playing with more than two people; more entities mean more sync.
- Avoid heavy structure mods in multiplayer. They are the most common cause of layer-load crashes.
The game's minimum spec is modest, but the minimum spec is for single-player. Treat multiplayer as one tier higher.
Is co-op worth it?
For a duo that has finished the demo and wants to share the game: yes, with caveats. The tension of the medical system is better with someone who can bandage you.
For a first playthrough: no. Play solo first. The game's pacing assumes one person managing their own body, and a desynced co-op run will teach you the wrong lessons about the combat.
For a public server: no. There is no server browser, no moderation and no anti-cheat. Play with people you know.
Alternatives
- Steam Remote Play Together requires a local co-op mode. This game does not have one, so it does not apply.
- Screenshare and voice chat is not co-op, but it is a stable way to play "together" while one person controls the character.
- Parallel solo runs with a friend on voice chat is the most stable co-op experience the game currently offers.
Will official multiplayer come?
There is no announcement. The Steam page lists Single-player only, and the developers have not promised co-op. Treat the mod as the only option and do not buy the game for multiplayer.
If an official mode is announced, it will likely be a separate update rather than a launch feature. The release date guide tracks what is confirmed.
The multiplayer mod is experimental. A fresh save is the only safe way to test a new version.
FAQ
Does Casualties: Unknown have multiplayer? Not officially. It is single-player only on Steam.
Is there a multiplayer mod? Yes, a community mod on Nexus that requires BepInEx.
Can I play co-op with a friend? Yes, with the mod, a shared version and a backed-up save. Expect desync.
Does the mod work in the demo? Some versions do. Check the mod page for the current demo support.
Will multiplayer mods get me banned? There is no anti-cheat or ban system. The risk is corrupted saves, not bans.
Is there split-screen? No.
Sources and further reading
Related guides
More Casualties: Unknown Guides
Every weapon in the Casualties: Unknown demo ranked: damage, swing speed, durability, ammo and which weapon fits which layer.
How blood volume, bleeding rate, tourniquets and transfusions work in Casualties: Unknown, and the order to treat a bleeding limb.
The three playable characters in Casualties: Unknown, what Experiment can do in the demo, and what is actually confirmed about Milky and Dune.
How crafting works in Casualties: Unknown: the five tabs, Intelligence requirements, failure tiers, blueprints and the recipes worth making first.
What the free Casualties: Unknown demo includes, how long it takes, where it ends, what carries over and how to install it on Steam.
All six Casualties: Unknown presets plus the 41 custom run settings, with defaults, ranges and the options that actually change survival.
Casualties: Unknown endings explained: the confirmed story goal, the unreleased final layers, what the demo does at layer 5, and what is still speculation.
Every enemy in Casualties: Unknown, its layer, attack pattern and drops, plus the Elder Thornback fight and the traps that kill more players than monsters.