Quick answer: a Fab plugin update added a new module (FabToolset) that the Launcher engine’s prebuilt build rules don’t know about, so every C++ build fails. Until Epic ships a fixed Fab build:
- Close Unreal and Visual Studio.
- In the Epic Games Launcher, go to Unreal Engine > Library, open Installed Plugins under 5.8 and Remove Fab.
- Build again. It succeeds.
- Get Fab assets from fab.com and the Launcher’s Fab Library meanwhile, and reinstall the Fab plugin once Epic updates it.
The error
The C++ build stops with:
Invalidating makefile for MyProjectEditor (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'.
Result: Failed (RulesError)
It happens in any C++ project on that engine, not just yours. Blueprint-only projects don’t build C++, so they don’t hit it.
Why it happens
Launcher engines ship with prebuilt build rules for every engine module (UE5Rules.dll). Fab 0.0.22 added a new module, FabToolset, to Engine/Plugins/Fab. The prebuilt rules don’t include it, so Unreal Build Tool can’t find it and stops.
Your code is fine. Nothing in your project needs changing.
What doesn’t help
Users who reported this tried these, and the error stayed:
- Verify on the engine in the Launcher.
- Reinstalling Fab (the same 0.0.22 comes back).
- Restarting the editor and the Launcher.
Don’t delete UE5Rules.dll or UE5RulesManifest.json from the engine. That makes the build break further until you run Verify.
Can I keep Fab and just disable it?
You can try: set Fab to disabled in your .uproject (see this tutorial for the lines). It’s quick, but we haven’t seen a confirmed report that it fixes this build error. If the error stays, remove Fab in the Launcher as in the quick answer.
FAQ
Will removing Fab affect my project’s assets? No. Assets you already added are normal files in your project. Only the in-editor Fab window goes away.
How do I know when it’s fixed? Check the Fab plugin’s version in the Launcher’s Fab Library. When it’s newer than 0.0.22, reinstall it and build once to check.
Does a source-built engine have this problem? A source build compiles its own rules, so it doesn’t use the prebuilt UE5Rules.dll. The Launcher’s Fab plugin isn’t made for source builds anyway.