UE Serialization & Save Games

You are an expert in Unreal Engine's serialization and save game systems. You implement save/load pipelines using USaveGame, FArchive, config files, and versioning so player progress persists correctly across sessions and game updates.

Step 1: Read Project Context

Read .agents/ue-project-context.md before giving any recommendations. You need:

• Engine version (UE 5.0+ has ULocalPlayerSaveGame; earlier versions differ)
• Module names (the save system lives in a specific module)
• Target platforms (console vs. PC save paths and user indices differ)
• Whether multiplayer is in scope (server-authoritative vs. client-local saves)

If the file does not exist, ask the user to run /ue-project-context first.

Step 2: Gather Requirements

Ask before writing code:

1. Save complexity: Simple key/value data, or complex world state with hundreds of objects?
2. Data types: Primitives, nested structs, asset references (soft vs. hard)?
3. Versioning needs: Live game with future patches? Old saves must keep working?
4. Multiple save slots: How many? Does each player/user get their own?
5. Async requirement: Can save/load stall the game thread, or must it be background?

Step 3: USaveGame Subclass

USaveGame is an abstract UObject from GameFramework/SaveGame.h. Subclass it and mark fields with UPROPERTY(SaveGame) for automatic tagged serialization by UGameplayStatics.

// MyGameSaveGame.h
#pragma once
#include "CoreMinimal.h"
#include "GameFramework/SaveGame.h"
#include "MyGameSaveGame.generated.h"

USTRUCT(BlueprintType)
struct FInventoryItemData
{
    GENERATED_BODY() // Required — missing GENERATED_BODY() breaks struct serialization silently

    UPROPERTY(SaveGame) FName  ItemID;
    UPROPERTY(SaveGame) int32  Quantity = 0;
    UPROPERTY(SaveGame) bool   bIsEquipped = false;
};

UCLASS(BlueprintType)
class MYGAME_API UMyGameSaveGame : public USaveGame
{
    GENERATED_BODY()
public:
    UPROPERTY(SaveGame) int32   SaveVersion = 0;      // Always include a version field
    UPROPERTY(SaveGame) float   PlayerHealth = 100.f;
    UPROPERTY(SaveGame) int32   PlayerLevel = 1;
    UPROPERTY(SaveGame) FVector LastCheckpointLocation = FVector::ZeroVector;
    UPROPERTY(SaveGame) FString PlayerDisplayName;
    UPROPERTY(SaveGame) float   TotalPlayTimeSeconds = 0.f;
    UPROPERTY(SaveGame) TArray<FInventoryItemData>   InventoryItems;
    UPROPERTY(SaveGame) TMap<FName, int32>            AbilityLevels;
    // TSet<FName> is also supported in UPROPERTY(SaveGame) fields and serializes/deserializes automatically.

    // Asset references: FSoftObjectPath stores a string path — safe across saves
    // Never use raw UObject* or hard TObjectPtr<> to content assets in save data
    UPROPERTY(SaveGame) FSoftObjectPath LastEquippedWeaponPath;
};

Saving and Loading

#include "Kismet/GameplayStatics.h"

static const FString SlotName  = TEXT("MainSave");
static constexpr int32 UserIdx = 0; // Always 0 on PC; use GetPlatformUserIndex() on console

// Create the object first, populate its fields, then save
UMySaveGame* SaveGame = Cast<UMySaveGame>(UGameplayStatics::CreateSaveGameObject(UMySaveGame::StaticClass()));
SaveGame->PlayerHealth = 75.f;
// Then pass SaveGame to SaveGameToSlot / AsyncSaveGameToSlot below

// Sync save (blocks game thread — avoid in gameplay)
bool bSaved = UGameplayStatics::SaveGameToSlot(SaveData, SlotName, UserIdx);

// Async save (preferred — does not block)
FAsyncSaveGameToSlotDelegate OnSaved;
OnSaved.BindUObject(this, &USaveManager::OnAsyncSaveComplete);
UGameplayStatics::AsyncSaveGameToSlot(SaveData, SlotName, UserIdx, OnSaved);

// Load
if (UGameplayStatics::DoesSaveGameExist(SlotName, UserIdx))
{
    UMyGameSaveGame* Save = Cast<UMyGameSaveGame>(
        UGameplayStatics::LoadGameFromSlot(SlotName, UserIdx));
}

// Async load
FAsyncLoadGameFromSlotDelegate OnLoaded;
OnLoaded.BindUObject(this, &USaveManager::OnAsyncLoadComplete);
UGameplayStatics::AsyncLoadGameFromSlot(SlotName, UserIdx, OnLoaded);

// Delete
UGameplayStatics::DeleteGameInSlot(SlotName, UserIdx);

Step 4: ULocalPlayerSaveGame (UE 5.0+)

ULocalPlayerSaveGame ties a save to a specific local player, tracks versioning via GetLatestDataVersion(), and provides HandlePostLoad() for migrations.

UCLASS()
class MYGAME_API UMyLocalPlayerSave : public ULocalPlayerSaveGame
{
    GENERATED_BODY()
public:
    virtual int32 GetLatestDataVersion() const override { return 3; }
    virtual void  HandlePostLoad() override;

    UPROPERTY(SaveGame) TMap<FName, int32> UnlockedAbilities;
};

void UMyLocalPlayerSave::HandlePostLoad()
{
    Super::HandlePostLoad();
    const int32 Ver = GetSavedDataVersion(); // version when last saved

    if (Ver < 2) { UnlockedAbilities.Add(TEXT("Dash"), 1); }
    // Ver < 3 migrations go here
}

// Load or create (sync)
UMyLocalPlayerSave* Save = ULocalPlayerSaveGame::LoadOrCreateSaveGameForLocalPlayer(
    UMyLocalPlayerSave::StaticClass(), PlayerController, TEXT("PlayerSlot0"));

// Load or create (async)
ULocalPlayerSaveGame::AsyncLoadOrCreateSaveGameForLocalPlayer(
    UMyLocalPlayerSave::StaticClass(), PlayerController, TEXT("PlayerSlot0"),
    FOnLocalPlayerSaveGameLoadedNative::CreateUObject(this, &AMyPC::OnSaveLoaded));

// Save back
Save->AsyncSaveGameToSlotForLocalPlayer(); // async (preferred)
Save->SaveGameToSlotForLocalPlayer();      // sync

Step 5: FArchive and Custom Serialization

FArchive (from Serialization/Archive.h) is the base for all UE serialization. Key API:

Ar.IsLoading()    // true when deserializing — same operator<< handles both directions
Ar.IsSaving()     // true when serializing to output
Ar.IsError()      // true after any read/write failure — always check before continuing
Ar.Tell()         // current position (int64); -1 if not seekable
Ar.CustomVer(Key) // returns the registered version number for a FGuid key

FMemoryWriter and FMemoryReader

FMemoryWriter/FMemoryReader (from Serialization/MemoryWriter.h / MemoryReader.h) serialize to/from TArray<uint8>:

// Serialize to bytes
TArray<uint8> OutBytes;
FMemoryWriter Writer(OutBytes, /*bIsPersistent=*/true);
int32 Version = 2;
Writer << Version;          // Serialize version header first — always
Writer << SomeData;
checkf(!Writer.IsError(), TEXT("Serialization failed"));

// Deserialize from bytes
FMemoryReader Reader(OutBytes, /*bIsPersistent=*/true);
int32 LoadedVersion = 0;
Reader << LoadedVersion;
if (LoadedVersion < 1 || Reader.IsError()) { /* corrupt data */ return; }
Reader << SomeData;

FBufferArchive

FBufferArchive (from Serialization/BufferArchive.h) combines FMemoryWriter + TArray<uint8> — the object is the output buffer:

FBufferArchive Buffer(/*bIsPersistent=*/true);
int32 Magic = 0x53415645; // 'SAVE'
Buffer << Magic;
Buffer << MyStruct;        // requires operator<< overload
TArray<uint8> Bytes = MoveTemp(Buffer); // FBufferArchive IS a TArray<uint8>

Custom operator<< for Structs

Define operator<< to make a struct serializable via any FArchive (required when passing it to FBufferArchive, FMemoryWriter, etc.):

FArchive& operator<<(FArchive& Ar, FMyCustomData& Data)
{
    Ar << Data.Name << Data.Value << Data.Timestamp;
    return Ar;
}

Compressed Archives

For large saves, use FArchiveSaveCompressedProxy / FArchiveLoadCompressedProxy (from Serialization/ArchiveSaveCompressedProxy.h):

// Compress
TArray<uint8> Compressed;
FArchiveSaveCompressedProxy Comp(Compressed, NAME_Zlib);
Comp.Serialize(RawData.GetData(), RawData.Num());
Comp.Flush();

// Decompress
FArchiveLoadCompressedProxy Decomp(Compressed, NAME_Zlib);
TArray<uint8> Raw;
Raw.SetNum(KnownUncompressedSize);
Decomp.Serialize(Raw.GetData(), Raw.Num());

Custom Serialize() on UObject

Override Serialize(FArchive& Ar) for precise binary layout control:

void UMyObject::Serialize(FArchive& Ar)
{
    Super::Serialize(Ar); // always call Super first
    Ar << BinaryField;
    Ar << UniqueRunID;
    if (Ar.IsLoading() && Ar.IsError()) { /* handle corruption */ }
}

Step 6: Versioning

Integer Versioning in USaveGame

namespace ESaveVersion
{
    enum Type : int32
    {
        Initial          = 0,
        AddedInventory   = 1,
        SoftRefForWeapon = 2,
        VersionPlusOne,
        Latest = VersionPlusOne - 1
    };
}

void USaveManager::RunMigrations(UMyGameSaveGame* Save)
{
    if (Save->SaveVersion == ESaveVersion::Latest) { return; }

    if (Save->SaveVersion < ESaveVersion::AddedInventory)
        Save->InventoryItems.Reset();

    if (Save->SaveVersion < ESaveVersion::SoftRefForWeapon)
    { /* convert old FName field to FSoftObjectPath */ }

    Save->SaveVersion = ESaveVersion::Latest; // stamp after migration
}

FCustomVersionRegistration (FArchive-based saves)

// Declare version enum + GUID (generate once with FGuid::NewGuid(), then hardcode)
struct FMySaveVersion
{
    enum Type { Initial = 0, AddedQuestData = 1, VersionPlusOne, Latest = VersionPlusOne - 1 };
    static const FGuid GUID;
};
const FGuid FMySaveVersion::GUID(0xA1B2C3D4, 0xE5F60718, 0x293A4B5C, 0x6D7E8F90);

// Register globally (module startup or static):
FCustomVersionRegistration GReg(FMySaveVersion::GUID, FMySaveVersion::Latest, TEXT("MySave"));

// In Serialize():
Ar.UsingCustomVersion(FMySaveVersion::GUID);
const int32 Ver = Ar.CustomVer(FMySaveVersion::GUID);
Ar << CoreData;
if (Ver >= FMySaveVersion::AddedQuestData)
    Ar << QuestData;
else if (Ar.IsLoading())
    QuestData.Reset(); // Initialize missing data on old saves

Struct Field Migration

When a struct field is renamed or its type changes, override Serialize() on the struct to migrate old data:

void FMyStruct::Serialize(FArchive& Ar)
{
    Ar.UsingCustomVersion(FMySaveVersion::GUID);
    if (Ar.CustomVer(FMySaveVersion::GUID) < FMySaveVersion::RenamedHealthToHP)
    {
        float OldHealth;
        Ar << OldHealth;
        HP = OldHealth; // Migrate old field name to new
    }
    else
    {
        Ar << HP;
    }
}

Step 7: Config Files

UGameUserSettings (user preferences)

UCLASS()
class MYGAME_API UMyGameUserSettings : public UGameUserSettings
{
    GENERATED_BODY()
public:
    UPROPERTY(Config, BlueprintReadWrite, Category="Game")
    float MasterVolume = 1.0f;

    UPROPERTY(Config, BlueprintReadWrite, Category="Game")
    bool bSubtitlesEnabled = true;

    void ApplyAndSave() { ApplySettings(false); SaveSettings(); }
};
// Register in DefaultEngine.ini:
// [/Script/Engine.Engine]
// GameUserSettingsClassName=/Script/MyGame.MyGameUserSettings

UDeveloperSettings (project settings)

UCLASS(Config=Game, DefaultConfig, meta=(DisplayName="My Game Settings"))
class MYGAME_API UMyProjectSettings : public UDeveloperSettings
{
    GENERATED_BODY()
public:
    UPROPERTY(Config, EditAnywhere, Category="Save") int32 MaxSaveSlots = 5;
    UPROPERTY(Config, EditAnywhere, Category="Save") bool  bEnableAutoSave = true;
    UPROPERTY(Config, EditAnywhere, Category="Save") float AutoSaveIntervalSeconds = 300.f;
    static const UMyProjectSettings* Get() { return GetDefault<UMyProjectSettings>(); }
};

GConfig Direct Access

#include "Misc/ConfigCacheIni.h"

FString Value;
GConfig->GetString(TEXT("/Script/MyGame.MyConfig"), TEXT("Key"), Value, GGameIni);
GConfig->SetString(TEXT("/Script/MyGame.MyConfig"), TEXT("Key"), TEXT("Val"), GGameIni);
GConfig->Flush(/*bRemoveFromCache=*/false, GGameIni);

MyObject->SaveConfig();  // writes UPROPERTY(Config) fields to .ini
MyObject->LoadConfig();  // reloads from .ini

INI section naming: Section [/Script/ModuleName.ClassName] maps to the CDO. SaveConfig() writes from the object to INI; LoadConfig() reads INI into the object and is called automatically for the CDO at startup. Custom section names require overriding OverrideConfigSection(FString& SectionName).

Cloud Save Integration

// Platform save systems (Steam, EOS, console) provide ISaveGameSystem
// Access via IPlatformFeaturesModule:
ISaveGameSystem* SaveSystem = IPlatformFeaturesModule::Get().GetSaveGameSystem();
if (SaveSystem && SaveSystem->DoesSaveSystemSupportMultipleUsers())
{
    // Platform handles cloud sync — use UGameplayStatics normally
    // Steam: auto-syncs Saved/SaveGames/ via Steam Cloud if configured in Steamworks
    // EOS: use IOnlineSubsystem → IOnlineTitleFileInterface for explicit cloud read/write
}

// Cross-platform pattern: serialize to TArray<uint8>, then write via platform API
TArray<uint8> SaveData;
FMemoryWriter Ar(SaveData);
SaveObject->Serialize(Ar);
// Upload SaveData via platform SDK

// Steam Cloud — write save slot directly via Steamworks API
ISteamRemoteStorage* SteamStorage = SteamRemoteStorage();
if (SteamStorage && SteamStorage->IsCloudEnabledForApp())
{
    SteamStorage->FileWrite("SaveSlot1.sav", SaveData.GetData(), SaveData.Num());
}
// Read back: SteamStorage->FileRead("SaveSlot1.sav", Buffer, Size)

Save Data Encryption

// Use FAES for symmetric encryption of save data
#include "Misc/AES.h"
// Build a zero-padded 32-byte FAESKey from a string.
// Do NOT use Key.Left(32): if the string is shorter than 32 chars it silently
// produces a truncated key, corrupting every encrypt/decrypt call.
static FAESKey MakeAESKey(const FString& KeyString)
{
    FAESKey AESKey;
    FMemory::Memzero(AESKey.Key, FAESKey::KeySize);
    const FTCHARToUTF8 Utf8(*KeyString);
    FMemory::Memcpy(AESKey.Key, Utf8.Get(), FMath::Min(Utf8.Length(), FAESKey::KeySize));
    return AESKey;
}

void EncryptSaveData(TArray<uint8>& Data, const FString& KeyString)
{
    int32 PaddedSize = Align(Data.Num(), FAES::AESBlockSize);
    Data.SetNumZeroed(PaddedSize);
    FAES::EncryptData(Data.GetData(), PaddedSize, MakeAESKey(KeyString));
}

void DecryptSaveData(TArray<uint8>& Data, const FString& KeyString)
{
    FAES::DecryptData(Data.GetData(), Data.Num(), MakeAESKey(KeyString));
}

Why encrypt: Prevents casual save editing for competitive/economy-sensitive games. Not foolproof — determined players can still extract keys from the binary. Combine with server-side validation for authoritative saves.

Step 8: Common Mistakes

PIE vs. Packaged / platform paths: In PIE, saves go to <Project>/Saved/SaveGames/. Packaged Windows builds write to %LocalAppData%/<ProjectName>/Saved/SaveGames/. Console platforms use title storage APIs. UGameplayStatics::SaveGameToSlot abstracts all of this through the platform's ISaveGameSystem — never hardcode OS paths; use FPaths::ProjectSavedDir() only for debug logging.

Advanced Edge Cases

Corruption recovery: When Ar.IsError() returns true mid-read or magic/version checks fail, discard the corrupt data and fall back to a fresh save. Optionally maintain a backup slot (write to Slot_Backup before overwriting Slot_Primary) so players never lose all progress:

USaveGame* LoadedSave = UGameplayStatics::LoadGameFromSlot(PrimarySlot, 0);
if (!LoadedSave)
    LoadedSave = UGameplayStatics::LoadGameFromSlot(BackupSlot, 0);
if (!LoadedSave)
    LoadedSave = UGameplayStatics::CreateSaveGameObject(UMySaveGame::StaticClass());

Large saves — chunked approach: Split world state across multiple slots by subsystem (e.g., Save_World_00, Save_Inventory, Save_Quests). Load each with AsyncLoadGameFromSlot in parallel. This prevents single-file bottlenecks and lets you load only what's needed for the current level.

Multiplayer save ownership: Shared world state (quests, economy, enemy state) belongs to server-authoritative saves — the server's AGameMode writes these; clients send state changes via RPCs, never write shared saves directly. Per-player preferences (keybinds, UI layout) remain client-local via ULocalPlayerSaveGame. This split prevents desync and cheating.

Module Dependencies (Build.cs)

PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine" });
// For UDeveloperSettings:
PublicDependencyModuleNames.Add("DeveloperSettings");

Related Skills

• ue-cpp-foundations — UPROPERTY, USTRUCT, UObject lifetime
• ue-data-assets-tables — FSoftObjectPath patterns for asset references in saves
• ue-gameplay-framework — GameInstance as save manager host; GameMode auto-save integration

Reference Files

• references/save-system-architecture.md — Full slot manager subsystem, metadata bank, multi-user patterns, and migration pipeline