What does "Light build failed. Swarm Agent failed to kick off" mean, and How can I fix it?


I would like to ask that when I want to unbuild my project, it says “Light build failed. Swarm Agent failed to kick off”. How can I fix the problem? I have built my UE4 project in Blueprint (Epic Games Installer) and it is quite advanced… I tried to restart the editor, reinstalled it. I enabled Unreal Engine (verify), but nothing. Could someone please help me?

Thanks in advance!

Swarm is a background process that builds the light for you. If you’re not using static lighting you don’t need it. If you’re just pressing the ‘build’ button, one of the defaults, is that it also builds light.

If you are using static lighting, then you need to find out what’s stopping swarm from running. It could be something like anti-virus, or another security process you have on your PC.

Thanks for the reply!

I turned off every anti-malvare and anti-virus software (btw i only have windows defenders from the beginning), and still i got this problem. I can’t completely finish my game until I build up the lights. Is it so annoying that I don’t know what to do with it, maybe some other solution?

As far as I can tell, there are many reasons for this. Have you looked in the output log? ( windows → developer tools → … ). I’m assuming this is a vanilla install, or did you build the engine?

Thanks for the reply again! :slight_smile:

I download the unreal engine from the Epic Games Launcher, and i develope the game with blueprints…

This is the Swarm Agen log (the error message):

[Interface:OpenChannel] Error: System.IO.IOException: process cannot reach the file “C:\Users\balaz\AppData\Local\UnrealEngine\4.26\Saved\Swarm\SwarmCache\AgentStagingArea\v1.DA24ACD8274453AD26E158832695FF65ACE1853B.mtrlgz” because it is being used by another process.
in the following location: System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
in the following location: System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
in the following location: System.IO.FileStream…ctor(String path, FileMode mode, FileAccess access, FileShare share)
in the following location: NSwarm.FSwarmInterface.OpenChannel(String ChannelName, EChannelFlags ChannelFlags) location: D:\Build++UE4\Sync\Engine\Saved\CsTools\Engine\Source\Editor\SwarmInterface\DotNET\SwarmInterface.cs, line: 1604