Hello Epic,
Building C++ projects fails on my UE 5.8.4 (Epic Games Launcher install) whenever the Fab plugin (version 0.0.22) is installed. Without the plugin, the build works.
ENVIRONMENT
- Unreal Engine 5.8.4 (Build.version: Major 5, Minor 8, Patch 4, Changelist 58902052, IsPromotedBuild 1, branch ++UE5+Release-5.8), installed with the Epic Games Launcher on 2026-10-23.
- Windows 11 Pro, Visual Studio 2026 (MSVC 14.50.35717), Windows SDK 10.0.22621.0.
- Project: a C++ project (module “TheGameProject”) that does not use Fab in code.
THE ERROR
Running Engine\Build\BatchFiles\Build.bat for the editor target ends with:
Invalidating makefile for TheGameProjectEditor (Fab.uplugin has been added)
Expecting to find a type to be declared in a module rules named ‘FabToolset’ in ‘UE5Rules, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null’. This type must derive from the ‘ModuleRules’ type defined by UnrealBuildTool.
Result: Failed (RulesError)
It fails before any of my own code is compiled.
WHAT I FOUND
- Engine\Plugins\Fab\Fab.uplugin (VersionName 0.0.22, EngineVersion 5.8.0) lists two modules: “Fab” and “FabToolset” (the latter with LoadingPhase None).
- Engine\Plugins\Fab\Source\FabToolset\FabToolset.Build.cs exists and declares “public class FabToolset : ModuleRules”.
- The generated rules project Engine\Intermediate\Build\BuildRulesProjects\UE5Rules\UE5Rules.csproj lists Fab.Build.cs but has no entry for FabToolset.Build.cs, and the precompiled Engine\Intermediate\Build\BuildRules\UE5Rules.dll (dated 2026-10-23, from the engine install) therefore does not contain a rules type for FabToolset.
- The Fab plugin folder was changed on 2026-10-28 at 22:45, on a machine where builds had worked until then. The old Source\Fab folder dates from 2026-09-21, and the Source\FabToolset folder is new from that moment. I did not install anything by hand at that time, so I assume the Launcher or the editor updated the plugin.
WHAT I TRIED
- Epic Games Launcher: Verify of UE 5.8. This restored UE5Rules.dll but did not change the Fab plugin, and the error stayed.
- Restarting the editor and the Launcher: no change.
- Uninstalling the Fab plugin in the Launcher: the build then succeeds (Result: Succeeded).
- Reinstalling the Fab plugin from the Launcher: the same version (0.0.22) is installed and the same RulesError returns.
(While troubleshooting I also removed UE5Rules.dll and UE5RulesManifest.json once, which broke the build further; Verify restored them. I mention it so you know the full history.)
QUESTIONS
1. Is Fab plugin 0.0.22 meant to work with the 5.8.4 Launcher build, or should the Launcher ship a matching update (UE5Rules/the rules project) for FabToolset?
2. Is there an official fix, or a plugin/engine version to install, so that C++ projects build with Fab installed?
3. Is disabling the Fab plugin in the .uproject a supported workaround for C++ builds?
I can send the full UnrealBuildTool log (C:\Users\\AppData\Local\UnrealBuildTool\Log.txt) if you need it.
Thank you,
Akarh