Fix: Unreal SaveGame Not Loading After Class Rename

Here is how to fix Unreal SaveGame not loading after a class rename. You shipped your game with a UMySaveGame class. Players have save files. You renamed the class to UPlayerSaveData during a refactor, and now UGameplayStatics::LoadGameFromSlot returns nullptr for every existing save file. New saves work fine, but all existing player progress is gone. This is a solved problem in Unreal — CoreRedirects exist specifically for this scenario.

The Symptom

LoadGameFromSlot returns nullptr. The save file exists on disk (you can find it in the SaveGames directory). New save files created after the rename load correctly. Only saves created before the rename fail. The log may show a warning about a missing class or struct during deserialization.

This also happens when you rename UPROPERTY fields, move the class to a different module, or rename the module itself. Any change to the serialized name breaks backward compatibility with existing save files.

What Causes This

1. Class path mismatch. Unreal’s serialization stores the fully qualified class path: /Script/MyGame.MySaveGame. When you rename the class to PlayerSaveData, the path becomes /Script/MyGame.PlayerSaveData. The old save file still references MySaveGame, which no longer exists. Deserialization fails.

2. Property name mismatch. Serialized properties are stored by name. If you rename PlayerLevel to CharacterLevel, the old value under PlayerLevel has no target property in the new class. The data is silently discarded.

3. Module rename or move. Moving a class from module MyGame to module MyGameCore changes the package path. The old save references /Script/MyGame.MySaveGame but the class now lives at /Script/MyGameCore.MySaveGame.

The Fix

Step 1: Add CoreRedirects to DefaultEngine.ini.

; DefaultEngine.ini
[CoreRedirects]
; Class rename
+ClassRedirects=(OldName="MySaveGame",NewName="PlayerSaveData")

; Property rename within the class
+PropertyRedirects=(OldName="PlayerSaveData.PlayerLevel",NewName="CharacterLevel")

; Module/package rename
+PackageRedirects=(OldName="/Script/MyGame",NewName="/Script/MyGameCore")

; Struct rename
+StructRedirects=(OldName="InventorySlot",NewName="ItemSlotData")

; Enum rename
+EnumRedirects=(OldName="EWeaponType",NewName="EEquipmentType")

Step 2: Verify the redirect works. Load an old save file and check that all properties deserialize correctly:

void AMyGameMode::TestSaveLoad()
{
    USaveGame* LoadedSave = UGameplayStatics::LoadGameFromSlot(
        TEXT("TestSlot"), 0);

    if (!LoadedSave)
    {
        UE_LOG(LogTemp, Error, TEXT("Load failed - check CoreRedirects"));
        return;
    }

    UPlayerSaveData* SaveData = Cast<UPlayerSaveData>(LoadedSave);
    if (!SaveData)
    {
        UE_LOG(LogTemp, Error, TEXT("Cast failed - class redirect wrong"));
        return;
    }

    UE_LOG(LogTemp, Log, TEXT("Level: %d, Gold: %d"),
        SaveData->CharacterLevel, SaveData->Gold);
}

Step 3: Add a version number to your save data. Future-proof your saves by including a version field that lets you handle migrations in code:

UCLASS()
class UPlayerSaveData : public USaveGame
{
    GENERATED_BODY()

public:
    UPROPERTY()
    int32 SaveVersion = 2;

    UPROPERTY()
    int32 CharacterLevel = 1;

    UPROPERTY()
    int32 Gold = 0;

    virtual void Serialize(FArchive& Ar) override
    {
        Super::Serialize(Ar);

        // Handle migration from older save versions
        if (SaveVersion < 2)
        {
            // Migrate data from v1 format
            Gold = Gold * 100; // v1 used dollars, v2 uses cents
            SaveVersion = 2;
        }
    }
};

Understanding the issue

Save data is forever. Once a save format ships, players will have that format in their files; you can't take it back. Bugs in save serialization compound over releases.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unreal. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unreal-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Unreal, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“CoreRedirects are not just for saves. They work for any serialized data: Blueprints, DataTables, Level assets. If you renamed anything and something stopped loading, a redirect is the answer.”

Why This Works

Unreal’s serialization system processes CoreRedirects before attempting to resolve class and property names. When the deserializer encounters MySaveGame in a save file, it checks the redirect table, finds the mapping to PlayerSaveData, and uses the new class. The redirect is transparent — no special code needed, no data migration. The save file still contains the old name, but the runtime maps it correctly. Redirects must stay in your config permanently because you cannot control which version of the save a player might have.

Related Issues

If your save file loads but DataTable row references inside it resolve to nullptr, see Fix: Unreal DataTable Row Not Found at Runtime for FName resolution issues that affect saved row references.

For Blueprint class renames that break level actors, the same CoreRedirect system applies. Add a ClassRedirect for the Blueprint path: +ClassRedirects=(OldName="/Game/BP/BP_OldName",NewName="/Game/BP/BP_NewName").

Rename the class, add the redirect. Remove the redirect, lose the saves. Keep them forever.