How to unpack/pack utoc+ucas in Unreal Engine 4 & 5 games

PromiX · Jan 11, 2026

I spent considerable time figuring out the correct way to make this work, so I hope this guide helps you avoid the same frustrations.
What This Does:

This process creates Zen archive patches (.utoc/.ucas) for Unreal Engine games that use the newer Zen loading system (UE4.26+, UE5+). It lets you modify existing game files without repacking entire archives.
Requirements:
Windows 10/11
UnrealReZen from GitHub
Unpacked game files (romfs/Content/Paks)
Modified files with same folder structure as original

Key Points Learned:
Structure Matters: Modified files must maintain the exact folder hierarchy as the original game files
Path Selection: The game directory should point to where original .utoc/.ucas files are located
Engine Version: Critical for compatibility - use ue4cfgdumper to determine
File Priority: Patch files must load after originals (use naming conventions like zzz_ prefix)

Quick Process:
Prepare ModifiedFiles folder with your changes
Drag it onto the script below
Follow prompts for game path and engine version
Optional: Configure compression, AES key, or mount point if needed
Copy generated files to game's Paks folder

DropHere.cmd

Code:

@echo off
setlocal EnableDelayedExpansion
title UnrealReZen Packer (Fixed)
color 0A

if not exist "%~dp0UnrealReZen.exe" (
    color 0C
    echo [ERROR] UnrealReZen.exe not found in current folder!
    echo Please put this script next to UnrealReZen.exe.
    pause
    exit /b
)

set "CONTENT_PATH=%~1"

if "%CONTENT_PATH%"=="" (
    color 0E
    echo ==========================================
    echo      UNREAL REZEN PACKING ASSISTANT
    echo ==========================================
    echo.
    echo Usage:
    echo Drag and drop your "ModifiedFiles" folder onto this icon.
    echo.
    pause
    exit /b
)

set "CONTENT_PATH=%CONTENT_PATH:"=%"

if not exist "%CONTENT_PATH%" (
    echo [ERROR] Folder does not exist: "%CONTENT_PATH%"
    pause
    exit /b
)

echo [SOURCE]: "%CONTENT_PATH%"
echo.

:AskGameDir
echo [INPUT] Enter the path to the game's Paks folder.
echo Example: C:\Games\Game\Content\Paks
echo (You can drag and drop the Paks folder here)
echo.
set /p "GAME_DIR=Path: "
set "GAME_DIR=%GAME_DIR:"=%"

if not exist "%GAME_DIR%" (
    echo [ERROR] Path not found. Try again.
    goto AskGameDir
)
echo [GAME DIR]: "%GAME_DIR%"
echo.

:AskEngine
echo [INPUT] Enter Unreal Engine Version (e.g. GAME_UE5_2)
echo Press Enter to use default: GAME_UE5_2
set "ENGINE_VER=GAME_UE5_2"
set /p "ENGINE_VER=Version: "
echo [VERSION]: %ENGINE_VER%
echo.

echo [INPUT] Enter AES Key (starting with 0x...)
echo Press Enter if not encrypted.
set "AES_KEY="
set /p "AES_KEY=Key: "

echo.
echo [INPUT] Enter output filename (without extension).
echo Example: zzz_Patch_P
echo Press Enter to use default: zzz_Patch_P
set "OUT_NAME=zzz_Patch_P"
set /p "OUT_NAME=Name: "

set "OUTPUT_FULL=%GAME_DIR%\%OUT_NAME%.utoc"

cls
echo ==========================================
echo           STARTING PACKING...
echo ==========================================
echo Source:  "%CONTENT_PATH%"
echo Target:  "%OUTPUT_FULL%"
echo Engine:  %ENGINE_VER%
echo.

set "CMD_LINE="%~dp0UnrealReZen.exe" --content-path "%CONTENT_PATH%" --game-dir "%GAME_DIR%" --output-path "%OUTPUT_FULL%" --engine-version %ENGINE_VER% --compression-format Oodle --mount-point "../../../""

if not "%AES_KEY%"=="" (
    set "CMD_LINE=!CMD_LINE! --aes-key %AES_KEY%"
)

echo Running command:
echo !CMD_LINE!
echo.

!CMD_LINE!

echo.
if exist "%OUTPUT_FULL%" (
    color 0A
    echo [SUCCESS] Patch created successfully!
    echo Check folder: "%GAME_DIR%"
) else (
    color 0C
    echo [ERROR] Failed to create patch. Check the error message above.
)

pause

masagrator · Jan 11, 2026

Why are you forcing zlib when it's inferior to Oodle which is a default compression algorithm for Zen packages?

etzquizz · Jan 15, 2026

Sorry I dont undertand "romfs" what you mean how I get that

lextra · Feb 4, 2026

How can I "uncook" the files exported by FModel so I can edit them?

In other words: How can I edit .uasset files?

Edit: I've asked Gemini

UAssetGUI: This is the most popular tool for low-level editing. It allows you to open a .uasset, view its internal data tree, and change values (like health stats, movement speed, or item references)

So I've downloaded UAssetGUI, which of course, threw me an error, because why should changing a simple singular texture be easy. So it told me to go to ZenTools-UE4 which I did, and I've followed the instructions.

Anyway, long story short, the example provided there is lacking and the output is basically empty.

Needless to say, I'm livid. F-ck that.

masagrator · Feb 4, 2026

lextra said:
How can I "uncook" the files exported by FModel so I can edit them?

You can't, the same way you can't uncook a dish. Some informations that make it human readable are lost forever.

lextra said:
In other words: How can I edit .uasset files?

Uasset is a format that adjusts to data defined by devs. There are not that many common stuff between them, you need to figure out it yourself on game by game basis. FModel supports parsing only some common informations and some unique informations for specific games.

lextra · Feb 4, 2026

masagrator said:
Uasset is a format that adjusts to data defined by devs. There are not that many common stuff between them, you need to figure out it yourself on game by game basis. FModel supports parsing only some common informations and some unique informations for specific games.

The only type of game modding I'll be doing from now on is simple config tweaks IF the game supports it (.pak only)
I won't be wasting any more time on this needlessly obscured garbage.

Thanks for the quick response though. I appreciate it.

Otakugamer1412 · Friday at 10:10 PM

this tool have issue with some games in unreal engnine 5 , some game crashes and same games meshes skeleton being corrupted and appear as t-pose

Post automatically merged: Friday at 10:12 PM

this tool have issue with some games in unreal engnine 5 , some game crashes and same games meshes skeleton being corrupted and appear as t-pose

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