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Guide to Getting Started Modding Ultimate - A Tutorial for Super Smash Bros. Ultimate


ARCropolis Update 7mo
  • Overhaul Rewritten to fully support ARCropolis
The big update is finally here. You no longer require a data.arc file to mod this game with the belated introduction of ARCropolis. ARCropolis allows for the loading of mod files directly off of the microSD card and eliminates all file size restrictions.

While ARCropolis has been released for a while, it hasn't been the most stable. I decided to wait out on making it the mod installer for the tutorial until most bugs were ironed out. However, ARCropolis has some of its own quirks you may need to cope with as it's not in its final state yet. If you are here looking to migrate from Ultimate Mod Manager to ARCropolis, please read the last FAQ entry at the bottom of the tutorial.

The old version of the tutorial has been archived here in the event you still want to use Ultimate Mod Manager or are having too many issues with ARCropolis.

1. Introduction

This tutorial will be guiding you through from scratch on how to begin modding Super Smash Bros. Ultimate.

1.1. Before Getting Started

This tutorial will contain the most up-to-date information on preparing to mod Ultimate.

NOTICE: As you read the guide directly below, please keep in mind creating an emuMMC is not required and generally not necessary for standard custom firmware usage. It is especially not recommended to use it online as opposed to sysMMC. If you would like to create one anyway, you may do so as you please.

If you are new to Switch hacking, please read up on this guide which will guide you from stock to custom firmware. Be sure to read the FAQ in that guide to answer some questions you may have regarding Switch hacking.

A microSD card formatted to FAT32 is recommended to avoid filesystem corruption. If you have the option to change the allocation unit size, set it to the maximum value of 64 KiB/65536 bytes. Windows OS by default is unable to format SDXC cards to FAT32, so please use the third-party software guiformat to do so.

To clear up the fear-mongering and misinformation spread throughout the Switch modding community regarding online activity with a modded Switch, please take a look through this GameBanana thread detailing what is safe and what isn't for both custom firmware usage and Super Smash Bros. Ultimate modding.

Having issues with the tutorial? Please consult me in the comments or join the Smash Ultimate Modding Hub Discord server and ask in the help channels there.


I assume no responsibility for any kind of ban or brick by following this tutorial. You take the risk by running the unsigned software. If you follow the instructions exactly as written, you will be fine.

2. Requirements

  • A microSD card of any capacity (a 32 GB microSD card is required if you plan to dump the game for mod creation)
  • ARCropolis

2.1. Preparing the microSD Card

  1. Download and extract the build of ARCropolis.
  2. Drag the atmosphere folder from the .zip file to the root of your microSD card. You will likely already have an atmosphere folder. If that is the case, merge its contents with the existing one.
  3. Create your mod directories. These are explained in the chapter directly below, so create the directory you take preference in. You may have both if you wish. Any folders you do not have already, create them (applies to the rest of the guide).

3. Adding Mods

3.1. Organized Layout (ARCropolis)

This layout allows for the sorting of multiple mods within an individually named mod folder. An example directory is given below, starting from the root of the microSD card:
sd:/ultimate/mods/Shiny Mewtwo (Default Eye)/
"Shiny Mewtwo (Default Eye)" is the named mod folder, and within it is the mod content's root arc folders (effect, fighter, stage, stream;, ui, etc.). You can have as many of these named mod folders as you want within the ultimate/mods/ directory and they can be named anything. 

Please make sure the named mod folder doesn't have one or more named mod folders within it. This will prevent ARCropolis from loading any mod content in that folder.
Do also note that if two mod folders replace the same file(s), the last iterated mod folder will take priority.

3.2. All-in-One Layout (ARCropolis)

You can also add all of your mod content into one single directory if that is your preference. The directory for that is as follows, starting from the root of the microSD card:
Inside of the arc folder is where you would place the mod's root arc folders (effect, fighter, stage, stream;, ui, etc.).

3.3. Skyline Plugins

NOTICE: Only necessary to follow if your mod comes with an .nro plugin designed for Skyline.

ARCropolis already comes bundled with the latest build of Skyline at the time of its latest release, but if you are not using ARCropolis and do not have Skyline or you just want to update it, here is a direct link to the latest beta. You can skip to the second step in this chapter if you already have Skyline and/or ARCropolis.

1. Extract the .zip file. Add the exefs folder into the following directory, starting from the root of the microSD card:
2. Add your plugin .nro file(s) into the following directory, starting from the root of the microSD card:
3. To load these plugins in-game, simply launch the game.

3.4. SaltyNX Plugins

NOTICE: Only necessary to follow if your mod comes with an .elf plugin designed for SaltyNX.

  1. Download the latest release of SaltyNX here. Extract the .zip file, merge the atmosphere folder with the one on the root of your microSD card, and add the SaltySD folder to the root of your microSD card.
  2. Inside of the SaltySD folder, create a new folder named plugins, and inside of that folder, a new folder named 01006A800016E000. Inside of that folder is where you're going to place your plugin .elf file(s).
  3. To load these plugins in-game, simply launch the game. Holding L to prevent mods from loading will not prevent SaltyNX from executing these plugins as SaltyNX is not a part of LayeredFS whatsoever.

4. Updating Compressed Mods

Compressed mods are mod files compressed with a lossless data compression algorithm known as Zstandard (or Zstd) which makes files smaller and allows the original data to be perfectly reconstructed from the compressed data when decompressed. Compressed mods are more often than not named by an offset, which an offset is either an absolute or relative address to a piece of data, and in this case are written in hexadecimal (e.g., 0x1360D7E68). Offsets for most files are version specific, meaning they are not supported on different game versions. ARCropolis is unable to parse files named by offset and without a valid file path.

  1. To update compressed mods, you will need a Python script called OffsetModUpdater. This Python script decompresses every compressed mod file and reorganizes everything into a structure parsable to ARCropolis.
  2. Download and install the latest Python 3 release if you haven't already.
  3. Move your target mod folder to be in the same directory as the Python script. Double-click the script. If it asks to install Zstandard, install it.
  4. When asked for the mod folder path, drag the folder you want to update into the Command Prompt/terminal window.
  5. When asked for the mod version, input the version that the mod's offsets were made for (e.g., 1.0.0). Usually the submission page for the mod says what version the offsets were for, so use that information.
  6. The new mod folder will output to wherever you have the script. The folder inside of output will contain the named mod folder you can use for mod installation.

5. Running Mods

To run mods, simply launch Super Smash Bros. Ultimate while in custom firmware. If you are using ARCropolis and this is your first time installing it, you should receive a fake error popup thanking you for installing ARCropolis. Close this fake error popup and allow the game to launch. If all went well, you should see ARCropolis' version on the title screen and your mods loaded at whatever point they should take effect. If no mods have loaded for one system or another, please see the first FAQ entry titled "Why are my mods not working?".

6. Conclusion

By the end of this guide, you should be comfortable with the basic process for modding Super Smash Bros. Ultimate. If you have any questions, please let me know and I'll be sure to answer you as soon as I can! Alternatively, please look through the FAQ to see if something there answers your question.

6.1. FAQ

Q: Why are my mods not working?
A: There could be a multitude of reasons for this, especially between which system and what "not working" means. The below will contain some select few solutions per each system.

  • Skyline not loading: Check to see if you have this file: sd:/atmosphere/config/system_settings.ini. If you do have it, open it and look to see if most lines are commented out with semicolons. If that is the case, delete the file. Reboot your console if done over FTP. If you do not have it and are on Atmosphère 0.12.0 or lower, you need to update Atmosphère, preferably to the latest release. If you do not have it and are on Atmosphère 0.13.0+, then this solution does not apply to you.
  • Skyline crashing: If you have both subsdk1 and subsdk9 in sd:/atmosphere/contents/01006A800016E000/exefs/, remove preferably subsdk1. Skyline is compiled into one of these subsdk files, so having two of them causes Skyline to load twice, resulting in a crash.
  • Skyline plugins crashing: This could be the cause of two plugins hooking the same function or if a plugin hooks a function via an offset not for your current game version. It's not exactly feasible to know how or if this is the case for a crash, but it is one worth noting. There's no real way to fix this without contacting the author about it or removing it.

  • ARCropolis not loading: Check the first solution to Skyline problems as ARCropolis is a plugin to be executed by Skyline.
  • ARCropolis crashing: An ARCropolis crash can likely be due to bad or conflicting mods. Find which mods are causing the crash by isolating the problem mod (folder), then ideally the problem mod file(s). Report the issue to the mod author, preferably. If you believe this is an ARCropolis issue, please report the issue on its GitHub repository and provide as much detail as possible.
  • ARCropolis loading but not loading mods: If you are certain your mods are laid out properly in either of the mod directories, then this may be due to the archive bit being set on a folder. To keep it brief, when the archive bit is set on a folder it is read as a file on the Switch's OS. To unset it for Hekate users, under "Tools > Arch bit • RCM • Touch • Partitions" is an option titled, "Fix archive bit." Select this option to automatically "fix" the bit, which unsets it in areas where it should not be set and sets it where it should be set.
  • Game takes a long time to boot: Often times this is a result of the ARCropolis update server being down and you being online. It takes a pretty long time for it to time out. To resolve this, you can either go offline or you can edit sd:/atmosphere/contents/01006A800016E000/romfs/arcropolis.toml and set the IP address to This will disable contact to the ARCropolis server and prevent you from receiving beta updates but will solve the problem.

  • SaltyNX plugins crashing: This could be the cause of two plugins hooking the same function or if a plugin hooks a function via an offset not for your current game version. It's not exactly feasible to know how or if this is the case for a crash, but it is one worth noting. There's no real way to fix this without contacting the author about it or removing the plugin.

Q: I want to start making my own mods. How can I get started?
A: Granted, with ARCropolis there are more steps to write out into one place, so this question will be split up into three different questions below to avoid clutter and easier searching.

Q: What is the data.arc file, why do I need it, and how do I dump it?
A: The data.arc file is specific to Super Smash Bros. Ultimate for containing all of the game's assets into an efficiently packaged file archive within the ROM filesystem of the cartridge/title. You will need it to gain access to the game's assets for mod creation. To dump it, follow the steps below:
For simplicity's sake, you should use Ultimate Mod Manager for nothing more than dumping the game.
1. Put the .nro file from the embedded link above inside of the /switch/ directory on the root of your microSD card.
2. Launch Ultimate Mod Manager from the Homebrew Menu by holding R and launching Smash. You should immediately be presented with the application's dumper screen.
3. To perform a verified dump, hold R and press A or Y (depending on the format of your microSD card). Doing this should help ensure that the dump is a "good" file. A list of good MD5 hashes for the data.arc file can be found here.
4. After the dump has completed, shut down your Switch and insert your microSD card into your computer. The dump will be stored in the following directory:

The result will either be a file or a folder named data.arc. On exFAT formatted microSD cards, it could be either because there is no practical file size limit. With FAT32 formatted microSD cards, there is a 4 GiB file size limit, so any files greater than that size will need to be split and inserted into a folder with a special bit known as the archive bit applied onto the folder for it to be read as a single file on the Switch's OS. If data.arc is a file, simply move it to your computer or preferred disk and read the Q&A after the following one. If it is a folder, follow the Q&A directly below.

Q: How do I combine the data.arc folder into a file?
A: For Windows, run the following command in Command Prompt inside of the dump's romfs folder (see video here on opening Command Prompt in a specific directory) (do not replace %UserProfile%):
copy /b data.arc\* "%UserProfile%\Desktop\data.arc"
This may take some time to complete. Once you are able to type in the console again, it has completed. Confirm the data.arc is one file within your output directory and that it opens in CrossArc (as discussed in the next entry). If all is well, delete the data.arc folder on your microSD card.

For either Linux or macOS, open a terminal window with the current working directory being the dump's romfs folder and run the following command:
cat 00 01 02 03 > ~/Desktop/data.arc
This may take some time to complete. Once you are able to type in the console again, it has completed. Confirm the data.arc is one file within your output directory and that it opens in CrossArc (as discussed in the next entry). If all is well, delete the data.arc folder on your microSD card.

Q: How do I browse the data.arc file?
A: You will need an application called CrossArc to do so. Open the executable, but before opening your data.arc file, click the "Update Hashes" option to update yourself to the latest list of cracked hashes. You do not need to do this every time you launch CrossArc, only when you first download the program and some time after a new update for the game is available. If you would like to keep up on the hash cracking process, see this GitHub repository.
Once you have updated your hashes, simply click on File>Open ARC and select your data.arc file to open. The rest should be self-explanatory.

Q: Am I able to add mods without constantly taking out my microSD card?
A: Yes you can! In fact this is recommended as constantly ejecting and inserting the microSD card will quickly wear out the console's reader. By using an FTP client, you will be able to send files to your microSD card over the network. I personally recommend the sys-ftpd-light sysmodule to use anywhere, anytime. From there, you can setup an FTP client to access the server and send files and/or folders to your microSD card. Tutorials for setting up a client can be found elsewhere online.

Q: How do I fully migrate from Ultimate Mod Manager to ARCropolis?
A: All listed directories start from the root of the microSD card:
1. Delete sd:/atmosphere/contents/01006A800016E000/romfs/data.arc
2. Delete sd:/UltimateModManager/backups/ and sd:/UltimateModManager/compTable.backup
3. Rename the UltimateModManager folder to ultimate.
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  • Where do I find the data.arc file when it's finished merging?
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  • Whenver I install any kind of mod that changes a character's appearance, the game crashes once I launch it. The same thing applies to some mods that alter the gameplay. I tried all the methods mentioned in the FAQ to fix it but nothing is working.
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  • So I am storing most of my games on a micro sd card, I was wondering if I had to get a new sd card of if I could use the one i've been using to mod smash
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  • Hey i was wondering how you would add mods that affect the same character alt and make one affect a different alt, as far as i know i cant find any information on it and the things i have tried haven't worked.
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  • JDarraz avatar
    JDarraz Joined 1mo ago
    Hi, I’m using quite an old laptop to get my switch into custom firmware (I have a Mac but it’s a lot easier for windows by the look of things). It’s worked a few times but mostly it just loads the Hekate intro screen and then doesn’t load. I bought a rcm loader, do you think this would help resolve the issue with it not loading cfw consistently?  
    URL to post:
  • Gol D Roger avatar
    Gol D Roger Joined 1mo ago
    I am sorry but I read all the comments, checked all the answers that you gave but no one worked. Even tho I have an unpatched switch, followed exactly this guide, ARCropolis does not boot when I launch SSBU. Could you please help me ?
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  • SwootyR avatar
    SwootyR Joined 2mo ago
    Hey, I just had a quick question. Every time I add any sort of mod, it says corrupted data and forces me to update. I have the 90DNS to block Nintendo connections so I can't update it normally. I'm able to update it through local users with my other switch. I was just asking, why does it do this? I can still play the mods I added after I update ,but it is just annoying. Any way I can avoid it?
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  • NeoNairb avatar
    NeoNairb Joined 2mo ago
    Sorry, i have a question, what SX OS users can do?, putting the atmosphere folder in my SD card it just didn't work
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  • Scarlet Jile avatar
    Scarlet Jile Joined 5y ago
    847 points Ranked 43,555th
    What's it supposed to look like when you try to dump the game with Ultimate Mod Manager?

    It tells me to overwrite the game and then launches Ultimate, so I have no way to gauge progress or know what's happening with the dump.

    Is Ultimate just supposed to keep running? Does it matter if the Switch goes to sleep? It's sort of unclear, so I'm stuck here.
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  • Texthead avatar
    Texthead username pic Joined 12mo ago
    Outta the Game
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    Texthead avatar
    Texthead’s Studio Flag
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    2mo 2mo
    My boot times for smash bros are insanely long with and without mods. It was working fine before and then I downloaded a modpack and everything went downhill. I even completely remade my SD card so nothing from before could possibly exist and redownloaded some mods but this problem seems to persist. No obvious problems seem to exist nor do any of the FAQ answers seem to work. Help would be appreciated :)
    What have I done... avatar
    What have I done...
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