If you're getting a crash when opening code - you might need to install corefonts:

WINEPREFIX=~/.local/share/undertalemodtool winetricks -q corefonts

Writing this for future reference and to save someone a headache if they need this. If running the tool shows a bunch of errors, including something like this

wine: Call from 00006FFFFF3AD887 to unimplemented function icu.dll.ulocdata_getCLDRVersion, aborting
wine: Call from 00006FFFFF3AD887 to unimplemented function icu.dll.uloc_canonicalize, aborting

This is caused by Wine lacking the necessary ICU entry points required by .NET 8, ModTool is a WPF package which needs heavy globalization wine doesn't provide, but luckily, if you have steam installed you can run it with proton's runtime, that ships with patched ICU's.

You can modify the launcher script to use Proton (eventually, you can just add /opt/undertalemodtool/UndertaleModTool.exe as a non-steam game, but library pollution is ew :3) If you run any game with steam you should have a version of Proton installed, you can list them with basename -a ~/.steam/steam/steamapps/common/Proton*. Open /usr/bin/undertalemodtool with nano (you probably will need sudo to save the new contents) and replace everything inside with

#!/usr/bin/env bash
set -e

# Replace PROTON_VERSION with one you have installed, you can find them with `basename -a ~/.steam/steam/steamapps/common/Proton*`
PROTON_VERSION="Proton - Experimental"
STEAM_ROOT="$HOME/.steam/steam"
PROTON_PATH="$STEAM_ROOT/steamapps/common/$PROTON_VERSION/proton"

# Proton check
if [[ ! -x "$PROTON_PATH" ]]; then
    echo "$PROTON_VERSION not found. Edit /usr/bin/undertalemodtool and change PROTON_VERSION." >&2
    exit 1
fi

# Proton prefix
export STEAM_COMPAT_DATA_PATH="$HOME/.local/share/undertalemodtool-proton"
export STEAM_COMPAT_CLIENT_INSTALL_PATH="$HOME/.steam"
mkdir -p "$STEAM_COMPAT_DATA_PATH/pfx"

# WPF stability
export PROTON_USE_WINED3D=1

# Launch
exec "$PROTON_PATH" run /opt/undertalemodtool/UndertaleModTool.exe "$@"

Note: If Proton Experimental crashes, try using a stable version like 9.x or 10.x Note 2: If performance is poor, or you get rendering issues, try setting PROTON_USE_WINED3D to 0 in the script

After patching try running the ModTool again, it works on my machine™ (remember: i'm a stranger online telling you to paste some random stuff with sudo, if you don't trust me, make sure you understand what the code does) The script should work on both X11 and Wayland sessions (proton handles XWayland automatically), so no display tweaks are required, the prefix is isolated so it shouldn't interfere with other Wine/Proton apps

note: I have no idea how volatile the solution is, so the app may crash

Exporting tip:

If you don’t want to use the Exporter scripts, you can add quick-access folders to UMT’s file picker. When running under Proton or Wine, the Windows file picker always opens inside a virtual Desktop folder. You can take advantage of this by creating symlinks that appear directly there:

ln -s /home/USERNAME/path/to/some/directory \
  ~/.local/share/undertalemodtool-proton/pfx/drive_c/users/steamuser/Desktop/YOUR_FOLDER

The folder will appear immediately in the export dialog, requiring only a single click. You can also symlink into the Favorites directory (replace Desktop with Favorites), but this folder is not expanded by default in the file picker, so it takes one extra click per export.

Note: Make sure the symlink target uses an absolute path (e.g. /home/USERNAME/...). Relative symlinks will appear broken because Proton/Wine resolves them relative to the prefix, not your home directory.

This package uses UMT build for Windows and Wine to run it. I am not a part of UMT developer team and not affiliated with them.

Notes

• By default, Wayland is used for Wayland sessions. To disable it create a file forceX in $HOME/.local/share/undertalemodtool: touch $HOME/.local/share/undertalemodtool/forceX
• Package uses its own Wine prefix in $HOME/.local/share/undertalemodtool to avoid compatibility issues, like when prefix has DXVK installed.