Fix UE5 Lighting Build Fails Instantly 🔦

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! ⚡

Method 1: Clean the Swarm Cache

1. 1Look in your Windows System Tray (bottom right) for the Swarm Agent icon (yellow/orange 'S').
2. 2Double-click it to open the window.
3. 3Go to the Menu bar → Cache → Clean.
4. 4Click 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.

1. 1In the Swarm Agent window, click the Settings tab.
2. 2Set CacheFolder to a path with plenty of space.
3. 3Change 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