Hello. In July of 2024 Rusted Moss released the modding update. Since then, I've spent most of my free time writing mods, development tools, and generally letting Rusted Moss take over my life. If you're in the official Discord, you've seen most of this, but it's time to spread the word.

Want to customize Ameli and Maya's colors like you can with Fern? Want to free the Lab monster? Want to play custom levels as Maya or Ameli? Well, now you can, with the power of mods!

But HOW ???

Rusted Moss doesn't have workshop support for mods and Google can't tell me anything, so where are they ??? The official Rusted Moss Discord (linked on each Steam announcement or their website) has a "modding-discussion" and "mod-share" channel, which contains mods. I've moved my mods to a Github repository[github.com], but I still recommend joining the Discord to keep updated with the latest modding news or download mods from other users.

NOTE: The easiest way to uninstall mods is to delete the entire "mods" folder or simply verify the game's installation with Steam. I also recommend making a backup of your saves before installing mods. Saves are located in the "%LocalAppData%/Rusted_Moss" folder, named "f_save_X.sav".

Mods live inside the "mods" folder in the game's Steam directory, accessible using the "Browse local files" option. Because of boring technical reasons, you can't normally install more than one mod at a time, so most mods will come as a zip file you can extract into this folder, replacing its contents.

Thankfully, we live in the future, and we've got better options.


Rusted Moss Mod Loader (or RMML) is a mod developed by me to fix a number of issues with modding. It's "the only mod you need," because it comes with Rusted Moss Mod Manager (or RMMM), a mod that lets you download mods off the internet.

To install RMML, visit its Github repository[github.com] and grab the latest release on the right. Then, you can follow the instructions in the repository, extracting "rmml_X_X.zip" to the Steam mods folder from earlier. After installing, you can simply use the menu in-game to manage your other mods.

Unless there's a large community of modders that aren't in the official Discord, most mods already support RMML and can be placed into the "mods/rmml" folder in the Steam directory and managed with RMMM.

That's cool, but what if I want to make my own mods ??? Rusted Moss lets you write unrestricted* Game Maker Language code for twenty-one Events for six moddable Game Objects. In short, anything you could do with Gamemaker code, you can do with the modding support.

Before continuing, if you want to use RMML (because it's easier to make mods with), you can safely ignore this section and read this instead[harlem512.github.io].

Yeah, but where do I start? In the "mods" folder, you'll find "meta_info.ini". Setting "enabled=1" in this file will enable mods, with the "mod_0=..." entry referencing a mod file. These mod files are in the "mods" folder and (such as "sniper_bnuy.ini") look like this:


The headers (surrounded by [brackets] ) denote a Game Object. The keys underneath it are Event names, and the key (after the equals = sign) is Catspeak code. The code is assigned to the Game Object, and runs whenever that Event would fire. In the example mod, the "i = 0; while ..." code runs when the "room_start" Event fires for a Controller Game Object.

Rusted Moss creates a single Controller Game Object when the game starts (or restarts, such as after returning to the main menu). The code you write is in Catspeak[www.katsaii.com], which is very similar to GML or Javascript. I highly recommend reading over GameMaker's architecture before modding.

The easiest way to distribute your mods is to post your files somewhere and post a link in the official Rusted Moss Discord. Other users need to download and install it manually, but you don't need to do any extra work.

If you want your mod distributed with Rusted Moss Mod Manager, you can message me (Harlem512) in the official Discord and I'll look at it. There are distribution rules[harlem512.github.io] for RMMM, but most of them are minor and I'm willing to work with you on to get your mods hosted, within reason.

If this is still confusing, someone (me) has spent a long time writing documentation[harlem512.github.io] on how to mod Rusted Moss. Writing it has been slow, but I'm gradually filling it with everything I've figured out about Rusted Moss modding. There's also links to useful tools, including Undertale Mod Tool, a GML function reference sheet, and a VS Code highlighter.

If something is missing or still confusing, you ask for help in the official Rusted Moss Discord and someone will probably respond.

Rusted Moss is really cool. The devs were kind enough to add modding support for me specifically (sarcastic) and I've tried my best to make modding slightly easier for everyone else. Also, please don't post bugs if you have mods installed. Mods can produce weird side effects, so don't expect to receive any help from the Rusted Moss developers if your game crashes. Mod authors still want to hear about your bugs though, so go message them instead (be nice).

Every mod developer is doing this in their free time. I won't speak for anyone else, but my motivation for this is driven solely by my enjoyment of programming and the thrill of the hunt.

Join the Discord. Use Undertale Mod Tool. Install RMML. Abuse Catspeak. Break the rules. And, most importantly, have fun!