Use this skill when working with World Partition, level streaming, level travel, OpenLevel, ServerTravel, data layer, world subsystem, level instance, sub-level, seamless travel, open world, or HLOD. See references/streaming-patterns.md for configuration patterns by game type.

UE World & Level Streaming You are an expert in Unreal Engine's world management and level streaming systems. Context Read .agents/ue project context.md before advising. Pay attention to: Engine version — World Partition is UE5 only; sub level streaming works in both UE4 and UE5. Build targets — Dedicated server has no rendering driven streaming; streaming must be server safe. World size — Determines whether World Partition or manual sub level streaming is appropriate. Multiplayer — Seamless travel requirements and per player streaming radius. Information to Gather Before recommending a streaming approach, confirm: 1. World size and type : Is this an open world (World Partition), a set of discrete levels, or a hub and spoke map? 2. Multiplayer : Are you running a dedicated server? Are per player streaming radii needed? 3. Streaming control : Does gameplay code need to control load/unload explicitly, or should proximity drive it? 4. Level travel : Non seamless (lobby flows), seamless (multiplayer round transitions), or no travel? 5. Persistent data : What must survive a level transition — player state, inventory, session state? World Partition (UE5) Enabling World Partition Enable via the Level menu: World World Partition Convert Level . Once enabled, all actors in the level are managed by World Partition's grid. The level can no longer have traditional sub levels. Use One File Per Actor (OFPA) for collaborative editing: each actor is saved as its own .uasset under ExternalActors . Runtime Data Layers Data layers replace the old sub level toggle pattern. A runtime data layer can be loaded/unloaded at runtime without traveling to a new map. Data layer states: Unloaded — not loaded, not visible. Loaded — loaded into memory, not visible (pre warming). Activated — loaded and visible (fully active). Streaming Sources Each player controller is a streaming source by default. For custom sources (cinematic cameras, AI directors), implement IWorldPartitionStreamingSourceProvider . HLOD HLOD provides distant merged mesh representations of World Partition cells. Configure HLOD layers in the World Partition editor; build before shipping via Build Build World Partition HLODs . Without HLOD, content beyond the streaming radius is simply absent. Converting Sub Levels to World Partition Use Tools World Partition Convert Level . Actors migrate into the persistent level under WP management. Audit cross level references beforehand — hard references to converted actors become invalid. World Partition and Multiplayer In a multiplayer session, each player controller acts as a streaming source with a configurable radius. The server streams based on server side sources; clients receive visibility updates via AServerStreamingLevelsVisibility . On dedicated servers, rendering based streaming does not apply — streaming is driven by server side sources only. Streaming radius is configured per partition in the World Partition editor UI ( LoadingRange on URuntimePartition ), not via ini. Level Streaming (Manual Sub Levels) ULevelStreaming State Machine From LevelStreaming.h , the full state sequence is: Query state with: UGameplayStatics: LoadStreamLevel / UnloadStreamLevel For Blueprint friendly async streaming with latent actions (from GameplayStatics.h ): For soft object pointers (preferred for packaging safety), use LoadStreamLevelBySoftObjectPtr with the same arguments. ULevelStreamingDynamic: Runtime Level Instances Use ULevelStreamingDynamic::LoadLevelInstance to load the same level package multiple times at different transforms — for procedural dungeons, modular buildings, or instanced rooms (from LevelStreamingDynamic.h ): For networking: use OptionalLevelNameOverride to give all clients and server the same package name for a given instance. Without this, names are auto generated uniquely per process and will not match across connections. OnLevelShown / OnLevelHidden Delegates From LevelStreaming.h — four BlueprintAssignable delegates: OnLevelLoaded , OnLevelUnloaded , OnLevelShown , OnLevelHidden . Bind with AddDynamic : Streaming Volumes ALevelStreamingVolume automatically controls sub level loading when the player camera is inside or outside the volume. From LevelStreamingVolume.h : Volumes are assigned to a sub level via its EditorStreamingVolumes array. Disable volume driven streaming for a level with ULevelStreaming::bDisableDistanceStreaming = true when you want code only control. Manual Visibility Control Force flush all streaming (blocks until complete — use sparingly): Level Instances ALevelInstance places a level as a reusable chunk in the editor. Actors inside are editable as a unit. For runtime instancing, see ULevelStreamingDynamic above. Packed Level Actors merge instance meshes into a single static mesh for performance. Enable via right click on Level Instance → Pack Level Actor . Per instance property overrides (UE5.1+): Each placed ALevelInstance can override individual actor properties (materials, gameplay values) without modifying the source level. Configure overrides in the Details panel; overridden values bake into packed level data at cook time. Level Travel Non Seamless: UGameplayStatics::OpenLevel Destroys the current world and loads a new one; all clients disconnect. From GameplayStatics.h : Server Travel (Multiplayer, Non Seamless) Initiated on the server; all connected clients follow ( World.h ): Seamless Travel Seamless travel loads the destination map in the background via a transition (midpoint) map. Clients stay connected. From World.h : Setup requirements: 1. Set bUseSeamlessTravel = true on AGameModeBase : 2. Set a transition map in DefaultEngine.ini : 3. Override GetSeamlessTravelActorList to control which actors persist: 4. Trigger on server: Travel sequence: current world transition map destination world. Use SetSeamlessTravelMidpointPause(true) to pause at midpoint for pre loading. Client Travel For client initiated travel (join server, change options), call APlayerController::ClientTravel(URL, ETravelType::TRAVEL Absolute) from the player controller. World Subsystems UWorldSubsystem (from Subsystems/WorldSubsystem.h ) is auto instantiated once per UWorld . It is destroyed when the world is destroyed — including on level travel. It is the correct place for per world singleton logic: streaming managers, zone trackers, world state caches. Access from anywhere with a world context: UTickableWorldSubsystem For per frame updates (distance checks, zone detection). Inherit from UTickableWorldSubsystem . Must call Super::Initialize and Super::Deinitialize to enable/disable ticking. Implement GetStatId returning a RETURN QUICK DECLARE CYCLE STAT . Persistent Data Across Level Transitions Mechanism Lifetime Use Case UGameInstance Entire application session Cross level player state, session config UGameInstanceSubsystem Entire application session Services that outlive any world Seamless travel actor list Transition only Actors that physically cross (GameState, managers) USaveGame + SaveGameToSlot Disk persistent Long term saves, progression UWorldSubsystem Per world World scoped cache; push data to UGameInstance in Deinitialize() before travel clears it GameInstance Pattern Store cross level data in UGameInstance properties (survives all level travel). Access from anywhere with a world context: Common Mistakes and Anti Patterns Loading everything at once. Setting bShouldBlockOnLoad = true on many sub levels causes hitches. Use async loading and the latent action pattern. Only block on load when the game is behind a loading screen. Streaming volume gaps. Overlapping volumes cause spurious unload/reload cycles. Use MinTimeBetweenVolumeUnloadRequests on the streaming level to add a cooldown and prevent flickering. Broken seamless travel in multiplayer. If bUseSeamlessTravel is true but no transition map is set, seamless travel silently falls back to non seamless. Always set TransitionMap in DefaultEngine.ini . Cross level hard references. Hard object references ( UPROPERTY() UObject ) between actors in different streaming levels cause the entire referenced level to stay loaded. Always use TSoftObjectPtr or TSoftClassPtr across level boundaries. Dynamic streaming level names not matching server and client. When using ULevelStreamingDynamic::LoadLevelInstance , each process generates a unique name. In multiplayer, supply OptionalLevelNameOverride with the same name on server and all clients. World Partition on dedicated server. The server does not use rendering driven streaming. Streaming sources must be explicitly added server side (e.g., player positions) or World Partition will not stream in actors correctly on the server. Modifying StreamingLevels directly. Do not add to UWorld::StreamingLevels directly. Use AddStreamingLevels , AddUniqueStreamingLevels , and RemoveStreamingLevels (from World.h ) which handle internal bookkeeping and StreamingLevelsToConsider . Forgetting to call Super::Initialize / Super::Deinitialize in UTickableWorldSubsystem . These calls enable and disable ticking respectively. Skipping them results in a subsystem that never ticks or never stops ticking. Quick Reference: Key APIs API Header Notes UGameplayStatics::LoadStreamLevel Kismet/GameplayStatics.h Async latent load of named sub level UGameplayStatics::UnloadStreamLevel Kismet/GameplayStatics.h Async latent unload UGameplayStatics::FlushLevelStreaming Kismet/GameplayStatics.h Blocking flush — use behind loading screens UGameplayStatics::OpenLevel Kismet/GameplayStatics.h Non seamless level travel ULevelStreamingDynamic::LoadLevelInstance Engine/LevelStreamingDynamic.h Runtime level instancing ULevelStreaming::GetLevelStreamingState Engine/LevelStreaming.h Query current stream state ULevelStreaming::SetShouldBeLoaded Engine/LevelStreaming.h Drive load state ULevelStreaming::SetShouldBeVisible Engine/LevelStreaming.h Drive visibility ULevelStreaming::SetIsRequestingUnloadAndRemoval Engine/LevelStreaming.h Remove level from world UWorld::ServerTravel Engine/World.h Multiplayer level transition UWorld::SeamlessTravel Engine/World.h Background seamless transition UWorld::GetStreamingLevels Engine/World.h Iterate all streaming levels UDataLayerManager::SetDataLayerRuntimeState WorldPartition/DataLayer/DataLayerManager.h World Partition data layer control (use UDataLayerManager::GetDataLayerManager(World) ) UWorldSubsystem::OnWorldBeginPlay Subsystems/WorldSubsystem.h Post BeginPlay init hook AGameModeBase::GetSeamlessTravelActorList GameFramework/GameModeBase.h Control actor persistence Related Skills ue gameplay framework — GameMode travel callbacks, PostSeamlessTravel , actor persistence rules. ue data assets tables — async asset loading patterns that complement level streaming. ue networking replication — net visibility transactions, server streaming authority. ue cpp foundations — subsystem patterns, UGameInstance lifetime.

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