Binding of Isaac Multiplayer: How to Play Co-Op Guide

Twelve years after its release, The Binding of Isaac: Rebirth is enjoying a second wind: SteamDB recorded a peak of 156,156 concurrent players on July 7, 2026, and Edmund McMillen himself mentioned, in his Steam post on July 10, a massive influx of newcomers combined with a wave of returning players. The reason can be summed up in one word: since Repentance+, Isaac can be played online with up to four players.

The problem is that this mode remains officially in beta, that it's explained nowhere in the game, and that its latest addition — crossplay between Steam and the Epic Games Store, delivered with build 1.9.7.16 on April 11, 2026 — is enabled by manually editing a text file. None of this appears in a menu option.

Here's the procedure as of August 9, 2026: what you need to own, how to enable crossplay, what multiplayer changes about the rules, and the pitfall that can cost you an entire save file.

What you need before clicking Online

The online mode arrived with Repentance+, a free DLC released on November 18, 2024 for Repentance owners. It only exists on PC: as of August 9, 2026, no console has yet received Repentance+, despite the announcement of a Switch 2 edition in late 2025 and Edmund McMillen's confirmation, in July, that the complete collection will eventually reach next-gen consoles — with no date given.

Second requirement, the most commonly misunderstood: every player must own all the DLC, including Repentance. A friend who stopped at Rebirth or Afterbirth+ won't be able to join your lobby, even though Repentance+ is free on their end. Once the DLC is installed and checked in Steam, an Online entry is added to the main menu, alongside the single-player options.

Third point: mods are off-limits. The API changed with Repentance+ and Workshop creations don't carry over online; community workarounds cause desyncs as soon as two clients aren't loading the same thing. If you usually play with External Item Descriptions, get used to going without it.

A welcome consolation: since build 1.9.7.13 on September 10, 2025, the game includes its own native item descriptions. You need to have beaten Mom to unlock the feature, after which every collectible, consumable, or trinket picked up at least once reveals its text. Exactly what you need when guiding beginners.

Enabling Steam and Epic Games Store crossplay

This is the April 2026 addition, and it has no in-game switch. Nicalis shipped PC crossplay with build 1.9.7.16 on April 11, explicitly asking the community to serve as a testing ground. Everything happens in the OPTIONS.INI file, located by default in Documents\My Games\Binding of Isaac Repentance+. Its last line reads EosCrossplay=0: simply change the value, with the game closed, then save.

Two habits worth adopting. First, value 3 only matters if you actually have Epic-side contacts; otherwise value 2 is enough and spares you the linking screen on every launch. Second, the feature is presented as a test: Nicalis is asking players to report bugs by email. The same update also added two useful commands in the online chat, /mute username and /unmute username, which only require a fragment of the username.

The four matchmaking entry points

Repentance+ isn't limited to a single host or join button. The online menu comes in four flavors: Quick Match (automatic search), Public Match (an open lobby that you create and that strangers can join), Friend Match (a private invite-only lobby), and online Daily Run, which applies the daily seed to an entire team.

Each lobby holds up to four players, spread across slots P1 through P4, with a ready state to confirm and a launch countdown. The screen also shows the selected mode and two win-streak counters, which give multiplayer a competitive flavor absent from solo play. Worth noting: the reference wiki points out that several settings available in solo, including changing the mode and manually entering a seed, are not accessible online.

If you join a game already underway, you don't appear in the middle of the current floor: the game brings you in at the start of the next floor. It's cleaner, but it means arriving during the Caves can leave you as a spectator for several minutes.

What co-op changes in the rules

Isaac's multiplayer isn't a solo game with four cursors. Resources split into two very distinct categories:

• Shared: coins, bombs, and keys form a single stockpile visible to everyone. A single player can therefore empty the group's reserve.
• Individual: hearts, passive and active items, trinkets, cards, and pills belong to whoever picks them up. Nothing is shared.

Two adjustments compensate for the player count. On one hand, damage dealt to bosses is reduced by 20% in multiplayer. On the other, each Boss Room drops one extra item per additional player: the replacement items only appear once the first one has been picked up, and they are always drawn from the boss room item pool.

Death isn't final. A killed player turns into a ghost baby: it no longer attracts enemies, can no longer pick up items, its shots deal almost no damage, but it keeps collecting coins, bombs, and keys — and it revives with one or two hearts once the next boss room has been cleared. The reference wiki also notes several online-specific quirks: a resurrection machine usable without going through a boss, doors that stay locked for a second when changing rooms, enemy attack indicators in large rooms, and a ghost that's a bit less powerless than in local play.

Sharing the loot without breaking the run

This is where public games go off the rails. Since nothing is shared, whoever reaches a pedestal first takes everything, and a single attack item can shape an entire run. Three habits avoid most of the friction:

• Announce before entering a Treasure Room that takes the item. The emote wheel is made precisely for this: pointing at a tinted rock, suggesting a direction, or rolling a die to settle a dispute between two claimants.
• Leave Devil Deals to the dead player. A ghost who's about to respawn at the next boss can spend their heart containers without any real consequence: they're the group's best buyer.
• Avoid resurrection items during the run. They reset the boss's health bar to full when they trigger. The one exception: the fight against The Beast, where the effect becomes a safety net again.

Two other details matter for team composition: familiars disappear along with the player carrying them, and global passive effects — the kind of item that makes Devil Deals cheaper for everyone — stop working as long as the player holding them is dead. Handing these items to the most cautious member of the group genuinely changes a run's efficiency.

The shared save trap

Here's the most important point in this guide, and the least well known: an online session runs on the least-advanced player's save file. If a beginner joins your lobby, the whole group plays with their unlocks, including locked characters, items, and endings. This isn't a bug but the intended behavior, and it explains most of the "where did my items go?" posts on the forums.

Achievements remain active, with one exception: joining a game already in progress disables them for you. Finally, the most serious warning is spelled out plainly by the wiki: online co-op is still in beta, it can be unstable, and it sometimes corrupts save data. Copy the Documents\My Games\Binding of Isaac Repentance+ folder elsewhere before a big session; that's two seconds of effort against hundreds of hours of unlocks.

The Deathmatch and end-of-run titles

Finishing an online run doesn't end the session: everyone gets thrown into a Deathmatch, a 90-second player-versus-player minigame. All participants play as Isaac, in a normal room for two players, or a 2x2 room for three or four. Health is no longer counted in hearts but in bar segments: soul hearts, black hearts, and items that increase health add a segment, healing items only fill one, and invincibility frames after a hit are cut to the bare minimum. When the timer runs out, Fly Bombs appear and retractable spikes rise from the floor to force a decision.

Each Deathmatch randomly rolls its loadout: you might get an Ipecac variant, Mom's Knife, The Ludovico Technique, Spirit Sword, Mysterious Liquid, or even a Bomber Boy version with 99 bombs and a blindfold. Eliminated players come back as a ghost baby able to drop small bombs: no one is fully out of the game until the timer ends.

The end of a run also hands out titles, the sins, awarded based on each player's behavior: Wrathful for the most damage dealt, Big Boss for the player who landed the killing blow on the most bosses (five minimum), Expressive for the one who used the emote wheel the most (ten minimum), along with others such as Eternal, Heartless, and Savior.

What about the French version in all this?

Let's be clear about it, since the question comes up with every new wave of players: Repentance+ has no French translation. French did exist as an official language in Repentance — Nicalis had integrated biobak's community translation, and the Language option in the menu would switch the interface, items, and secrets to French — but this option disappeared with Repentance+, which has remained in beta since its launch. All the labels cited in this guide are therefore those from the English client, the only one displayed online.

Falling back is possible, but it comes at a cost: unchecking Repentance+ in the game's DLC list on Steam brings you back to classic Repentance, with French and Workshop mods, but with no access whatsoever to online mode. It's up to each player to decide based on what they're looking for.

What's left to expect

The last public build dates back to April 20, 2026 (1.9.7.17.J460), and Nicalis hasn't accompanied it with detailed patch notes, true to a habit the wiki has documented for years. In his July 10 update, Edmund McMillen mentioned a small update in the works, potentially releasing before the end of the year, and reiterated that the console port remained on the roadmap, still without a timeline.

In the meantime, the best way to approach online play is also the least spectacular: a two-player Friend Match, a save file set aside as backup, and a clear agreement about pedestals before even reaching the first floor. Public Quick Match remains fun, but it comes with two risks — the less experienced player who chips away at your unlocks, and desync issues. And if your friends play on the Epic Games Store, the EosCrossplay=2 line is well worth the thirty seconds of file editing it requires.