How To Fix Player Location Desync In UEFN And Unreal Engine Multiplayer
The Nightmare of Multiplayer Transform Desync
Every multiplayer game developer eventually faces the moment their netcode starts lying to them. You script a sequence where a player sits in a chair, the chair moves across the map, and everything looks flawless on the server. But when you load up a second client, the illusion shatters. Client A sees themselves riding the chair perfectly. Client B sees the chair moving while Client A floats frozen in mid-air at the starting line.
This specific phenomenon—the uefn player location desync—is a well-documented pain point when using player inputs to trigger MoveTo commands on props holding attached players. The server registers the correct absolute location, but the simulated proxies (the representations of the player on other clients) fail to inherit the updated transform from their parent prop.
Whether you are building a custom experience in Unreal Editor for Fortnite (UEFN) or architecting a standalone dedicated server in Unreal Engine 5, understanding why attachment replication fails is critical. In this tutorial, we will break down the exact mechanics of network dormancy, why attachments break client-side prediction, and how to force the replication graph to respect your world state.
Why Attachments Break Network Updates
To fix the desync, you first need to understand how Unreal Engine handles actor replication hierarchically.
When a character actor attaches to a prop (like sitting in a chair device), its transform becomes relative to the parent actor. In a networked environment, the server must decide how to broadcast this data to 100 different clients efficiently. To save bandwidth, Unreal Engine aggressively utilizes a concept called Network Dormancy.
When a player sits in a chair and stops providing direct movement input, the server's replication graph may flag the player actor as dormant. The server assumes: "The player isn't moving independently, so I don't need to send updates for them. I will only send updates for the moving chair."
The Math Behind the Desync
Let's look at the concrete numbers that cause this failure:
- Server Tick Rate: Typically 30Hz in UEFN.
- Prop NetUpdateFrequency: Often defaults to 100 updates per second.
- Character MinNetUpdateFrequency: Can drop to as low as 2.0 (2 updates per second) when no input is detected or dormancy kicks in.
When you call MoveTo on the chair, the chair's transform updates at 30Hz. However, because the attached player is dormant or updating at a vastly reduced frequency, the other clients never receive the RPC (Remote Procedure Call) telling them to update the player's relative position. The result? A massive visual desync.
If you are dealing with broader variables failing to replicate alongside movement, you might also be running into The Unreal Engine Multiplayer Sync Bug Ruining Your World States And How To Fix It.
Step 1: The UEFN Verse Workaround
In the official UEFN bug reports, developers noted a vital clue: exiting the chair fixes the issue.
Why does exiting fix it? Because changing the player's movement mode from Custom (seated) back to Walking forces the server to flush network dormancy and broadcast a hard, absolute transform update to all clients.
We can recreate this "flush" programmatically in Verse without forcing the player to actually leave the vehicle. By rapidly manipulating the player's state or forcing a micro-teleport, we can force the replication graph to wake up.
Here is a Verse implementation that handles moving a prop while ensuring the attached player's transform stays synced across all clients:
using { /Fortnite.com/Devices }
using { /Fortnite.com/Characters }
using { /Verse.org/Simulation }
using { /UnrealEngine.com/Temporary/SpatialMath }
prop_mover_device := class(creative_device):
@editable TargetProp : creative_prop = creative_prop{}
@editable ChairSeat : chair_device = chair_device{}
# Moves the prop and forces a network update for any attached players
SyncMoveProp(NewLocation : vector3, NewRotation : rotation, Duration: float)<suspends>:void=
# 1. Execute the movement on the server
MoveResult := TargetProp.MoveTo(NewLocation, NewRotation, Duration)
# 2. Workaround for the UEFN Player Location Desync
# Force a state refresh to wake up the replication graph
ForceNetworkUpdate()
ForceNetworkUpdate():void=
# Check if a player is currently seated in the attached chair
if (SeatedAgent := ChairSeat.GetSeatedPlayer[]):
if (FortCharacter := SeatedAgent.GetFortCharacter[]):
# Grabbing the current absolute transform
CurrentTransform := FortCharacter.GetTransform()
# Teleporting the character to their exact current location
# This action triggers an immediate absolute transform broadcast
# to all proxy clients, resolving the visual desync.
FortCharacter.TeleportTo[CurrentTransform.Translation, CurrentTransform.Rotation]
Print("Forced replication update for seated player.")
By calling TeleportTo to the exact same coordinates, we aren't changing the gameplay state, but we are tricking the underlying C++ engine into firing a TeleportPhysics flag, which completely resets the client-side prediction for that actor.
Step 2: The Native Unreal Engine C++ Fix
If you are building outside of UEFN and have access to source code or standard Unreal Engine C++, you don't need to rely on the teleport hack. You can directly interact with the networking API to manage dormancy and force updates.
When you move an actor that has other actors attached to it, you need to explicitly tell the server to wake up those attached actors.
// Example of forcing a net update on an attached character in UE5 C++
void AMyVehicle::MoveVehicleAndSync(FVector NewLocation, FRotator NewRotation)
{
// 1. Move the parent actor
SetActorLocationAndRotation(NewLocation, NewRotation);
// 2. Iterate through all attached actors (e.g., seated players)
TArray<AActor*> AttachedActors;
GetAttachedActors(AttachedActors);
for (AActor* AttachedActor : AttachedActors)
{
if (AMyGameCharacter* SeatedPlayer = Cast<AMyGameCharacter>(AttachedActor))
{
if (SeatedPlayer->HasAuthority())
{
// Wake up the actor if it went dormant during the ride
SeatedPlayer->FlushNetDormancy();
// Force an immediate replication update for the next tick
SeatedPlayer->ForceNetUpdate();
// For extreme latency scenarios, fire a targeted client RPC
SeatedPlayer->Client_ForceSetLocation(SeatedPlayer->GetActorLocation(), SeatedPlayer->GetActorRotation());
}
}
}
}
// In MyGameCharacter.cpp
void AMyGameCharacter::Client_ForceSetLocation_Implementation(FVector NewLoc, FRotator NewRot)
{
// ETeleportType::TeleportPhysics ensures client prediction is overridden
SetActorLocationAndRotation(NewLoc, NewRot, false, nullptr, ETeleportType::TeleportPhysics);
}
This C++ approach is significantly more performant than the Verse workaround because it directly manipulates the NetDriver rather than relying on physics teleports to trigger side effects.
Step 3: Persisting the Correct State to Your Backend
Fixing the live replication desync is only half the battle. If your game features a persistent world, housing, or saved locations, you must ensure that the correct absolute coordinates are saved to your database.
If a player disconnects while the server and client are temporarily desynced, saving the client's predicted location will result in the player spawning inside walls or falling through the map upon reconnecting. Always ensure your save logic queries the server-authoritative transform after a forced net update.
Building a custom backend to handle high-frequency location saves requires setting up load balancers, database sharding, and SSL cert management—easily 4-6 weeks of dedicated infrastructure work. If you are looking to bypass that grind, horizOn provides a Backend-as-a-Service explicitly designed for game developers. With horizOn, real-time state persistence and database management come pre-configured out of the box, letting you ship your game instead of your infrastructure.
For more advanced techniques on shifting away from slow REST APIs for real-time state saving, check out Ditch Http Polling An Unreal Engine Websockets Tutorial For Real Time Backends.
5 Best Practices for Multiplayer Movement & Attachments
To prevent location desyncs from plaguing your project in the future, adopt these architectural best practices:
1. Never Trust the Client's Attachment State Clients will often predict that they have successfully attached to a vehicle or prop before the server confirms it. Always use server-authoritative RPCs to validate attachments. If the server denies the attachment, smoothly interpolate the client back to their server-known location.
2. Manage NetDormancy Manually for Vehicles
Don't let the engine automatically put players to sleep just because they are seated. If a prop is actively moving via MoveTo or a timeline, write logic to explicitly call FlushNetDormancy on all attached children every few ticks until the movement completes.
3. Keep Attachment Hierarchies Shallow Unreal Engine's replication graph struggles with deep hierarchies (e.g., Player attached to a Chair, attached to a Train, attached to a Moving Platform). Keep attachments to a maximum of one level deep. Instead of attaching the chair to the train, make the chair a static mesh component of the train actor itself.
4. Use Hard RPCs for Critical State Changes
When a player mounts or dismounts a prop, do not rely on standard property replication (bReplicates). Property replication is queued and can be dropped if bandwidth is tight. Use reliable NetMulticast or Client RPCs to broadcast the exact dismount coordinates.
5. Validate Transforms on Dismount When a player exits a chair, implement a server-side check that compares the player's new absolute location against the prop's location. If the distance exceeds a logical threshold (e.g., > 500 units), snap the player back to the prop's exit socket before giving them back movement control.
Conclusion
Multiplayer desyncs are rarely random; they are almost always the result of the engine aggressively optimizing network traffic at the expense of visual accuracy. By understanding how network dormancy affects attached actors, you can proactively force updates using Verse teleports in UEFN or FlushNetDormancy in C++.
Stop letting the engine guess where your players are. Take explicit control of your replication graph, enforce server authority, and ensure your absolute transforms are synced across every connected client.
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