Fix UE5 Lighting Build Fails Instantly 🔦
Lighting build stuck at 99% or failing? Here is how to fix common Swarm Agent errors and get your static lighting baked.
There is nothing worse than waiting for a lighting build only to have it fail or hang indefinitely. Usually, the culprit is the Swarm Agent. Here is the quick fix! âš¡
Method 1: Clean the Swarm Cache
Look in your Windows System Tray (bottom right) for the Swarm Agent icon (yellow/orange 'S').
Double-click it to open the window.
Go to the Menu bar → Cache → Clean.
Click Yes to confirm.
This clears out old, potentially corrupted lighting data. Try building again!
Method 2: Force Validate
If cleaning the cache didn't work, we need to force Swarm to validate the data.
In the Swarm Agent window, click the Settings tab.
Set CacheFolder to a path with plenty of space.
Change CleanOldCache to True.
Pro Tip: Firewall Issues
If Swarm won't even start, your firewall might be blocking it. Ensure this executable has permission:
Engine\Binaries\DotNET\UnrealBuildTool\SwarmAgent.exe