Unreal "UnrealBuildTool ... error" compiling in CI

UnrealBuildTool compiles your C++ target. When it reports an error, the cause is a compile failure, a missing module in the build dependencies, or a compiler/toolchain that does not match the engine version.

What this error means

The build fails with "UnrealBuildTool ... error" or "ERROR: UnrealBuildTool failed", with C++ errors or unresolved symbols earlier in the log.

unreal

MyGame.cpp(42): error: use of undeclared identifier 'FFoo'
ERROR: UnrealBuildTool exited with code 6
ERROR: Failed to build target.

Common causes

A C++ compile error in the target

Code that builds on a developer machine can fail in CI when an include, a module, or a platform define differs.

A toolchain or engine version mismatch

A compiler version the engine does not support, or building against the wrong engine version, makes UBT fail to compile.

How to fix it

Read the first compile error

Open the first error: or unresolved symbol above the UBT summary.
Add the missing include or module dependency in the .Build.cs.
Re-run; UBT exit codes are summaries, not causes.

Match the engine-supported toolchain

Use the compiler/toolchain version the engine documents (or the engine image) so UBT compiles cleanly.

Terminal

# Build a single target with UBT directly to isolate the error
./Engine/Build/BatchFiles/Linux/Build.sh MyGameEditor Linux Development \
  -project="$PWD/MyGame.uproject"

How to prevent it

Declare module dependencies in .Build.cs so headers resolve.
Use the engine-supported compiler/toolchain version.
Build the target locally with UBT before pushing.