Why does my Unreal PAK mod fail to load?

PAK mods placed in Content/Paks/~mods/ usually auto-mount in shipping builds. If they do not, check Project Settings > Packaging > Use Pak File. Confirm the mod was packaged with the same engine version and project as the host build; mismatches cause silent skip.

How do I mount a PAK file at runtime in Unreal?

Use FPakPlatformFile::MountPakFile or call IPlatformFilePak::Mount. Pass the .pak path and a priority. Higher priority wins for asset lookups. Subscribe to FCoreDelegates::OnMountPak to react to successful mounts.

Why does my mod overwrite work in editor but not in shipping?

Editor uses loose files which override pak content. Shipping uses paks; mod paks must mount with higher priority than the base game pak. Set priority above the default.

Fix: Unreal PAK File Mod Loading Failed

Quick answer: Place mods in Content/Paks/~mods/ for auto-mount. For runtime mounting use FPakPlatformFile::MountPakFile with a priority. Mod must be cooked against the same engine version and project as the host.

Here is how to fix Unreal user mods packaged as .pak files that do not load in shipping builds. Auto-mount has specific path conventions; manual mount needs the right API and priority.

The Symptom

Mod .pak placed in user-discoverable folder. Game starts, mod content not present. No errors in player log.

What Causes This

Wrong path. Auto-mount looks at Content/Paks/~mods/. Other folders are not scanned by default.

Version mismatch. Mod cooked against UE 5.3 will not mount in 5.4 host.

Priority too low. Mod assets need higher priority than base assets to override.

Encryption mismatch. If host uses encrypted paks but mod does not, or vice versa.

The Fix

Step 1: Use the standard auto-mount path. Tell players to place mods in <GameDir>/Content/Paks/~mods/ (note the tilde). Auto-mount scans this folder at engine startup.

Step 2: Manual mount in C++ for custom layouts.

void UMyModSubsystem::LoadMod(const FString& PakPath)
{
    FPakPlatformFile* PakFile = (FPakPlatformFile*)FPlatformFileManager::Get().FindPlatformFile(TEXT("PakFile"));
    if (!PakFile)
    {
        UE_LOG(LogTemp, Warning, TEXT("PakFile platform missing"));
        return;
    }
    if (PakFile->Mount(*PakPath, /*PakOrder*/ 100))
    {
        FCoreDelegates::OnMountPak.Broadcast(*PakPath, 100, nullptr);
        UE_LOG(LogTemp, Log, TEXT("Mounted: %s"), *PakPath);
    }
}

Priority 100 typically beats default game paks (priority 0–10).

Step 3: Verify version compatibility. Provide a mod SDK matching your engine version. Document required UE version in mod readmes. Reject mods packaged for other versions.

Step 4: Handle encryption consistently. If host uses encryption, distribute the encryption key with the SDK so modders can cook compatible paks. Or build host without encryption for mod-friendly distribution.

Step 5: Refresh asset registry after mount.

IAssetRegistry& AR = FAssetRegistryModule::GetRegistry();
AR.SearchAllAssets(/*bSync*/ true);

Newly mounted paks add assets; force a registry rescan so they are visible.

“~mods folder for auto-mount. Manual mount with priority for custom paths. Same engine version always.”

Related Issues

For asset registry empty, see AssetRegistry Empty. For data asset cook, see Data Asset Soft Refs.

~mods path. Manual mount with priority. Asset registry refresh. Mods load.