UP TO FOUR PLAYERS.

UNDERTALE CO-OP

Keep one launcher. Get new mod releases automatically when you open it.

DOWNLOAD WINDOWS APP (.EXE) ↓
  1. Save Play Undertale Co-op.exe somewhere easy to find, like your desktop.
  2. Open Steam, close Undertale, then double-click the launcher.
  3. Choose CO-OP / PARTY → HOST or JOIN. The host can start Story Mode or choose a one-off fight in Battle Mode.

Every player must own Undertale through Steam and have the Windows version installed. This mod supports Undertale v1.08.

NOW AVAILABLE · r64

Active combat stays on its normal clock with monitor-sync pacing removed. Battle Mode ends at defeat or spare, returns to its browser, and clears temporary ending data so it cannot reload Flowey. Fresh launches open the main menu.

WHAT'S INCLUDED

Steam parties · 1–4 players

Host or join through your Steam friends list, with names, profile pictures, and party counts such as 2/4. Join requests ask the host to accept or decline. No IP address or lobby code to copy.

Stay together in a menu lobby with a party panel. The host can remove a member or close the party; guests can leave. When the host starts Story Mode, existing members join automatically after the host save loads.

Shared Story Mode

See your friends move with their chosen appearances. Either player can interact, solve shared puzzles, and take everyone through a door or room transition. Shared encounters, dialogue, NPC movement, and story flags follow the host's world. Joining a story session backs up your personal save first.

Battle Mode

Browse 112 native encounter groups across Bosses, Normal Encounters, and Special Battles, including unused and hard-mode variants. Play solo or let the party host pick a fight. Win or lose, return to the battle browser; these fights use temporary data and do not advance or overwrite your story save.

Individual turns & souls

The active player's Steam name turns yellow and they see YOUR TURN. Their action finishes before the next living player chooses; then the monster acts. Encounters with a monster-first opening keep it.

Each player has separate HP, items, and a soul to dodge with. Host attacks deal 100% damage; guests deal 30%. Downed players spectate until victory or a full-party defeat. Winning revives a downed player with 1 HP. An idle character sidebar and health bars show the party during battles.

Keybinds & character design

Rebind directional controls for walking, battle menus, and soul movement. Pick hats, outfits, and colors, or play as Frisk, Sans, Papyrus, Toriel, Undyne, Alphys, Mad Mew Mew, Sonic, or Foxy. Appearances are cosmetic.

Settings, updates & recovery

FPS settings support solo presentation up to 120 FPS; connected co-op uses Undertale's native timing for stability. The launcher checks for signed mod updates before play and keeps a verified installation available offline. Session reports and disconnect recovery help track problems.

Compatibility is still being tested. Full genocide/pacifist runs, every boss, ACT option, and cutscene are not certified crash-free. Unused/test fights may need scripted story setup. The separate Photoshop Flowey sequence is not part of the 112 encounter groups.

Updates install between game sessions. If an update check is offline, your last verified installation is still available. Keep this launcher for future published releases.