99 Day Survival Guide has online co-op for up to four players. You host a lobby and invite friends through Steam, and everyone needs their own PC with the game: there is no split screen or local co-op, and you cannot join a run that has already started. Bigger squads get stronger team buffs and more loot, and downed teammates can be revived.
The game is a top-down zombie survival roguelite from a solo developer who goes by Sam, and it launched on Steam on November 11, 2026, for Windows. Sam has answered most of the co-op questions directly on the Steam forums, so everything below comes from the developer or the 99 Day Survival Guide Steam page.
Co-op at a Glance
| Question | Answer |
|---|---|
| Max players | 4, online |
| How to invite | Through Steam, to the host’s lobby |
| Split screen or local co-op | No |
| Join a run in progress | No, only while the host is still in the lobby |
| Solo play | Yes, the game was first designed for single-player |
| Platforms | Windows PC on Steam only |
How to Play With Friends
Online play runs through Steam. Sam’s instructions are short: “You can invite your friends to your lobby directly through Steam.” In practice:
- One player hosts and opens a lobby.
- The host invites friends from their Steam friends list.
- Everyone joins before the host starts the run.
That last step matters. Sam confirmed on launch day: “You can’t join a game that’s already in progress. You can only join while the host is still in the lobby.” If a friend is late, finish the run or back out to the lobby and start again together.
Is There Split Screen?
No. The store page listed local co-op for a while, and Sam said it was a mistake: “Looks like I accidentally listed local co-op as a feature.” The rule is clear: “The game requires two computers for online multiplayer and does not support split-screen play.” Each player needs a PC with the game installed.
There is no crossplay to worry about either, because the game is only on Steam for Windows. There is no Mac, Linux or console version.
What Changes With a Bigger Team
The store page says co-op is not just the solo game with more people in it:
- Team buffs scale with team size. “The larger your team, the stronger your team buffs and the more gear you can loot.”
- Revives. A downed teammate can be brought back: “No one gets left behind!”
- Shared runs. You explore together, share resources, take down bosses and split the loot.
- No getting stranded. Since an update in May, a player who dies too far from the group is teleported closer to their teammates.
The roster includes eight hidden characters you unlock by finding them, so a squad can mix roles: the store page highlights the Bombproof Officer’s forward shield, the Drone Engineer’s aerial drones, the Pyro Drunk’s Molotovs and the Demolition Expert’s grenades.
Multiplayer Fixes Since Launch
Co-op had some rough edges at launch, and Sam has been patching it daily:
- November 11: fixed an issue where players in certain regions could not auto-attack after joining another player’s lobby, caused by network instability. Sam said it was not yet certain the fix covered every region and asked anyone still affected to report it on the Steam forums.
- November 12: players are now invulnerable while picking upgrade cards in multiplayer, and the card selection countdown resets properly after several level-ups in a row. The same update lowered enemy health across all five difficulty levels and raised equipment drop rates.
If your auto-attack stops working as a guest, check that you are on the latest version and that the host is too.
Is It Fun Solo?
Sam’s answer to that question on Steam: “The game was originally designed for single-player, but I later added online co-op so friends could play together.” Solo is fully supported, with five difficulty levels and leaderboards per level and difficulty, including a friends leaderboard. There is a free demo on Steam, and Sam has confirmed that demo saves carry over to the full game, so you can try it alone before you round up a squad.
If you are after more games to play with friends, our list of the best PC co-op games of 2026 has plenty, and if you like horde survival in general, our guide to Warhammer Survivors’ multiplayer covers what another game in the genre offers groups.
Frequently Asked Questions
Does 99 Day Survival Guide have co-op? Yes. It has online co-op for up to four players through Steam.
Does 99 Day Survival Guide have split screen? No. The developer says it needs a separate computer for each player and does not support split screen or local co-op.
Can you join a friend’s game in progress? No. You can only join while the host is still in the lobby.
Is 99 Day Survival Guide crossplay? There is nothing to cross with. The game is only on Steam for Windows.
Can you play 99 Day Survival Guide solo? Yes. The developer says it was first designed as a single-player game, with co-op added later.
Does demo progress carry over? Yes. The developer has confirmed demo save data carries over to the full game.