Modding (Afterbirth †)

Modding is now officially supported in Afterbirth+, enabling users to mod the game using the Lua programming language and to upload their creations to the Steam Workshop.

Installing Mods

Mods can be installed in a few different ways, depending on how the author chose to share it. Mods can typically be found in the Steam Workshop or in various community sources.

Steam Workshop

Mods found in the Steam Workshop can be installed by navigating to the mod's page and clicking Subscribe. The game must be started to install the mod into the game.

Manual installation

Mods that do not use the Steam Workshop can be installed by downloading them and extracting them to the appropriate location.

OS	Path
Windows	`Documents/My Games/Binding of Isaac Afterbirth+ Mods`
Linux	`~/.local/share/binding of isaac afterbirth+ mods`
Mac	`~/Library/Application Support/Binding of Isaac Afterbirth+ Mods`

Enabling unlocks

In Afterbirth+, having any enabled mod will disable the ability to unlock any achievements to encourage new players to play the game without any. To enable unlocking achievements with mods enabled, you need to defeat Mom at least once with all of your mods disabled.

Ensuring stability

While introducing official modding support, mods for this game are infamous for not working properly due to how the API was constructed and the lack of suitable, comprehensible documentation.

Save data

Many mods do not or cannot retain save data in a continued game. This could be due to the mod author not knowing how to enable this, or the mod's data being too complicated to save. It is recommended to avoid continuing any saved games if any mods are enabled.

Some mods may have their save data cleared if one of the situations below occurs:

The mod was recently updated.
The mod was unsubscribed from and then resubscribed to, forcing the mod to be downloaded again.

If any installed mods contain save data that must be kept between runs (for example, custom unlockables) then this would be a big concern, as deleting the mod's save data would remove this progress. Some ways to work around these issues include:

Backing up the save.dat files included in the mods after most sessions.
Installing the Mod Compatibility Hack, which will back up the saves of other mods within itself.

Assets

Most mod assets do not load properly if one of the situations below occurs:

Playing the game after the first time the game has loaded after subscribing to or unsubscribing from a mod on the Steam Workshop.
Playing the game after a mod on the Steam Workshop updates.
Continuing to play the game after enabling or disabling a mod from within the in-game mods menu. (this can also lead to a crash when leaving the menu).

When this happens, many textures and animation files will not be applied in-game. For example, a custom monster added by a mod affected by this issue may appear invisible. Some ways to work around these issues include:

Restarting the game after loading it if a mod was recently subscribed to. If there is a long pause and a black screen when starting the game, the game may be installing the mods. Restart the game after it has finished loading past this screen.
When the process of enabling or disabling mods in-game is complete, do not leave the menu. Instead, press Alt+F4 to close the game without ever leaving the menu and risking a crash or improperly loaded assets. Then simply start up the game again.