SwarmCoordinator fails to build with UE 5.5

[jlsalmon]

When attempting to build UE 5.5.4, I encountered the following error at the Build Tools CS stage:

08:42:50 Running: C:\BuildTools\MSBuild\Current\Bin\MSBuild.exe "C:\UnrealEngine\Engine\Saved\CsTools\Engine\Source\Programs\UnrealSwarm\SwarmCoordinator\SwarmCoordinator.csproj" /property:"Platform"="AnyCPU" /property:"Configuration"="Development" /restore /verbosity:minimal /nologo
08:42:51 
08:42:52 C:\UnrealEngine\Engine\Binaries\ThirdParty\DotNet\8.0.300\win-x64\sdk\8.0.300\NuGet.targets(439,5): error MSB4062: The "NuGet.Build.Tasks.WarnForInvalidProjectsTask" task could not be loaded from the assembly C:\UnrealEngine\Engine\Binaries\ThirdParty\DotNet\8.0.300\win-x64\sdk\8.0.300\NuGet.Build.Tasks.dll. Could not load file or assembly 'NuGet.Build.Tasks, Version=6.10.0.97, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. An attempt was made to load a program with an incorrect format. Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask. [C:\UnrealEngine\Engine\Saved\CsTools\Engine\Source\Programs\UnrealSwarm\SwarmCoordinator\SwarmCoordinator.csproj]
08:42:52 Took 1.76s to run MSBuild.exe, ExitCode=1

Related forum post: https://forums.unrealengine.com/t/unable-to-build-ue-5-5-installedbuild/2243395

It seems that the UAT bundled dotnet version was upgraded to 8.0, but there are still some targets that rely on dotnet 6 e.g. SwarmCoordinator, but UAT does not install it automatically, and we are not installing dotnet 6 as part of the build prerequisites.

Installing dotnet-6.0-sdk via choco resolves the issue for me.

[slonopotamus]

No, installing dotnet-6.0-sdk does not resolve that issue. It was tried in 3c46153. And actually all that thread talks about dotnet runtime, not sdk.

[jlsalmon]

Hmm ok, I must have something confounding me in my setup then, because I somehow ended up with a successful build.

@slonopotamus So are you saying we don’t have a solution for this issue yet?

[slonopotamus]

So, my point is that SwarmCoordinator does not depend on .NET 6. It depends on 4.6.2.

And there is a workaround: cherry-pick https://github.com/EpicGames/UnrealEngine/commit/b86e244161df5bb32030818a48ffeccd3ed310ef in your 5.5 fork.

[slonopotamus]

I also want to notice that I experience the same issue when building installed UE-5.5 on a clean host machine (so neither Docker nor ue4-docker is involved). And on the first build this issue occurs. But it disappears on subsequent runs, without any modifications to installed software.

[jlsalmon]

That’s probably what confounded me then. I did the same thing. Clean host build failed -> installed dotnet 6 -> built again and it passed, so I assumed that was the fix. I guess the folks on the Unreal forum might be experiencing this “magical second run” syndrome as well.