Tools
The list below is curated to contain only tools which are most used and relevant to PAYDAY 3. For a more exhaustive list of Unreal Engine modding tools, see Buckminsterfullerene's repository.
Working with Pak files
Repak by truman
Repak is a popular community-made option, with support for both unpacking and packing assets. Repak is written in rust and crazy fast.
UnrealPak by Epic
UnrealPak is the official tool shipped with the Unreal Editor.
You can find it in your editor's install directory under Engine\Binaries\Win64\UnrealPak.exe.
While having the best compatibility due to it using the Engine's own logic to work with Pak files, it is by far the hardest to work with for mod developers. The command line interface is unintuitive, not well-documented, and AES encryption keys must be specially formatted into a configuration file.
Inspecting cooked assets
FModel
FModel is a very powerful interactive Pak viewer. It allows viewing most assets serialized as JSON, as well as playing back audio and viewing textures and models.
UAssetGUI
UAssetGUI is a tool to view and edit cooked assets. Compared to FModel, the GUI is usually easier to read than the raw JSON output of FModel. Unlike FModel, properties can be edited, and the asset can be re-exported to a cooked asset.
Stove
Stove is an editor for cooked Unreal Engine maps. It allows viewing where actors are positioned in the map and viewing their properties. It also has the ability to transform (move, rotate) actors and to transplant actors from one map to another.
Other Tools
DDS-Tools
DDS-Tools is a tool to used for texture modding. It's good for simple texture replacements and if you don't wish to install Unreal Engine.
Working with cooked Blueprint code (aka Kismet)
kismet-analyzer by truman
kismet-analyzer is a command line toolkit to help analyze and manipulate assets containing Kismet functions.
For your convenience, the control flow graphs generated using this tool are available on this website.
kismet-debugger by truman
kismet-debugger is an extension for UE4SS which allows you to interactively step through Blueprint functions.
Several people have reported issues getting this to work with PAYDAY 3. Should you get it to work, feel free to update this page with instructions.