Live Coding enters infinite recompile loop after header (.h) modification due to CppDependencyCache partition error in UE 5.8

General Issues and Bug Reporting (Unreal Engine)
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Summary

A severe regression exists in the Unreal Build Tool (UBT) and Unreal Build Accelerator (UBA) dependency tracking system during Live Coding sessions in UE 5.8.

When a project is clean, Live Coding correctly reports “Target is up to date”. However, if any header file (.h) is modified, UHT runs and UBT hits a cache partition error: CppDependencyCache - There is no partition covering path to…[File].dep.json!.

Although the initial compilation patch compiles and links successfully, the dependency cache is permanently corrupted for those files. On every single subsequent Live Coding trigger with absolutely zero code changes UBT forces a full recompile of the module files instead of skipping them, resulting in an infinite recompile loop in this session.

This results in an infinite recompile loop during the session, leading to a massive waste of development time on redundant recompilations

What Type of Bug are you experiencing?

Foundation (C++ Tools, Profiling, & Pipeline)

Steps to Reproduce

  1. Create a fresh, clean Blank C++ project in UE 5.8.

  2. Create a new C++ Actor class (e.g., MyActor).

  3. Close the Unreal Editor and perform a clean build of the Editor target from your IDE (Visual Studio 2022 or Rider).

  4. Trigger Live Coding (Ctrl+Alt+F11) → Result: Target is up to date (Expected behavior).

  5. Modify MyActor.h (e.g., add a dummy variable) and trigger Live Coding.

  6. Notice the CppDependencyCache errors regarding missing partitions for .dep.json files in the log. The patch compiles and links successfully.

  7. Without touching or modifying any files, trigger Live Coding again and again (and a fourth time).

Expected Result

On step 7 and onward, UBT should recognize that no files were modified since the last patch and output Target is up to date.

Observed Result

UBT ignores the up-to-date state and forces a recompile of MyActor.cpp and Module.Project.gen.cpp on every single execution, wasting compilation time.

Affects Versions

5.8
5.7

Platform(s)

Windows

Additional Notes

Run 1 log:

Using bundled DotNet SDK version: 10.0 win-x64
Running UnrealBuildTool: dotnet “....\Engine\Binaries\DotNET\UnrealBuildTool\UnrealBuildTool.dll” -Target=“BugTestEditor Win64 Development -Project=”“C:/Users/eis-a/Documents/Unreal Projects/BugTest/BugTest.uproject”“” -LiveCoding -LiveCodingModules=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCodingModules.json” -LiveCodingManifest=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCoding.json” -WaitMutex -LiveCodingLimit=100
Log file: C:\Users\eis-a\AppData\Local\UnrealBuildTool\Log.txt
Determining max actions to execute in parallel (6 physical cores, 12 logical cores)
Executing up to 6 processes, one per physical core
Requested 1.5 GB memory per action, 646.04 MB available: limiting max parallel actions to 1
UbaServer - Listening on 0.0.0.0:1345
Target is up to date
Using Unreal Build Accelerator local executor to run 0 action(s)
Total time in Unreal Build Accelerator local executor: 0.32 seconds
Output binary: C:\Program Files\Epic Games\UE_5.8\Engine\Binaries\Win64\UnrealEditor.exe

Result: Succeeded
Total execution time: 9.82 seconds
Trace written to file C:\Users\eis-a\AppData\Local\UnrealBuildTool\Trace.uba with size 3.5kb

Run 2 log:

Running C:\Program Files\Epic Games\UE_5.8\Engine\Build\BatchFiles\Build.bat -Target=“BugTestEditor Win64 Development -Project=”“C:/Users/eis-a/Documents/Unreal Projects/BugTest/BugTest.uproject”“” -LiveCoding -LiveCodingModules=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCodingModules.json” -LiveCodingManifest=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCoding.json” -WaitMutex -LiveCodingLimit=100
Using bundled DotNet SDK version: 10.0 win-x64
Running UnrealBuildTool: dotnet “....\Engine\Binaries\DotNET\UnrealBuildTool\UnrealBuildTool.dll” -Target=“BugTestEditor Win64 Development -Project=”“C:/Users/eis-a/Documents/Unreal Projects/BugTest/BugTest.uproject”“” -LiveCoding -LiveCodingModules=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCodingModules.json” -LiveCodingManifest=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCoding.json” -WaitMutex -LiveCodingLimit=100
Log file: C:\Users\eis-a\AppData\Local\UnrealBuildTool\Log.txt
Determining max actions to execute in parallel (6 physical cores, 12 logical cores)
Executing up to 6 processes, one per physical core
Requested 1.5 GB memory per action, 1.26 GB available: limiting max parallel actions to 1
UbaServer - Listening on 0.0.0.0:1345
UHT compiled-in object format Default
UHT processed BugTestEditor in 11.9421664 seconds (0 generated files written)
CppDependencyCache - There is no partition covering path to C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\MyActor.cpp.dep.json!
CppDependencyCache - There is no partition covering path to C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\Module.BugTest.gen.cpp.dep.json!
Building BugTestEditor…

===== Toolchain Information =====
Using ISPC compiler (C:\Program Files\Epic Games\UE_5.8\Engine\Source\ThirdParty\Intel\ISPC\bin\Windows\ispc.exe)
Intel(r) Implicit SPMD Program Compiler (Intel(r) ISPC), 1.24.0 (build commit @ 20250404, LLVM 18.1.2)
Using Visual Studio 14.44.35228 toolchain (C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207) and Windows 10.0.22621.0 SDK (C:\Program Files (x86)\Windows Kits\10).

Using Unreal Build Accelerator local executor to run 2 action(s)
CPU 6 physical cores, 12 logical cores
Memory 15.77 GB physical, 30.94 GB/41.61 GB committed
UBA Storage capacity 40 GB
[1/2] Compile [x64] Module.BugTest.gen.cpp
[2/2] Compile [x64] MyActor.cpp

Total time in Unreal Build Accelerator local executor: 45.10 seconds
Output binary: C:\Program Files\Epic Games\UE_5.8\Engine\Binaries\Win64\UnrealEditor.exe

Result: Succeeded
Total execution time: 65.44 seconds
Trace written to file C:\Users\eis-a\AppData\Local\UnrealBuildTool\Trace.uba with size 9.6kb
File C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\Module.BugTest.gen.cpp.obj was modified or is new
File C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\MyActor.cpp.obj was modified or is new
Building patch from 2 file(s) for Live coding module C:\Users\eis-a\Documents\Unreal Projects\BugTest\Binaries\Win64\UnrealEditor-BugTest.dll
UbaCli v5.8.0-Uba_v1.0.0-52526909 (Rootdir: “C:\ProgramData\Epic\UbaCli”, StoreCapacity: 0Gb)

Starting trace: Debug59862b11-f5d4-42e5-be8c-04cba1ebe7e7
Running C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\bin\Hostx64\x64\link.exe @C:\Users\eis-a\AppData\Local\Temp\7B07.tmp

Detoured run took 1.4s
Successfully linked patch (0.000s)
Patch creation for module C:\Users\eis-a\Documents\Unreal Projects\BugTest\Binaries\Win64\UnrealEditor-BugTest.dll successful (0.000s)

Run 3 log

Running C:\Program Files\Epic Games\UE_5.8\Engine\Build\BatchFiles\Build.bat -Target=“BugTestEditor Win64 Development -Project=”“C:/Users/eis-a/Documents/Unreal Projects/BugTest/BugTest.uproject”“” -LiveCoding -LiveCodingModules=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCodingModules.json” -LiveCodingManifest=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCoding.json” -WaitMutex -LiveCodingLimit=100
Using bundled DotNet SDK version: 10.0 win-x64
Running UnrealBuildTool: dotnet “....\Engine\Binaries\DotNET\UnrealBuildTool\UnrealBuildTool.dll” -Target=“BugTestEditor Win64 Development -Project=”“C:/Users/eis-a/Documents/Unreal Projects/BugTest/BugTest.uproject”“” -LiveCoding -LiveCodingModules=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCodingModules.json” -LiveCodingManifest=“C:/Program Files/Epic Games/UE_5.8/Engine/Intermediate/LiveCoding.json” -WaitMutex -LiveCodingLimit=100
Log file: C:\Users\eis-a\AppData\Local\UnrealBuildTool\Log.txt
Determining max actions to execute in parallel (6 physical cores, 12 logical cores)
Executing up to 6 processes, one per physical core
Requested 1.5 GB memory per action, 1.73 GB available: limiting max parallel actions to 1
UbaServer - Listening on 0.0.0.0:1345
Building BugTestEditor…

===== Toolchain Information =====
Using ISPC compiler (C:\Program Files\Epic Games\UE_5.8\Engine\Source\ThirdParty\Intel\ISPC\bin\Windows\ispc.exe)
Intel(r) Implicit SPMD Program Compiler (Intel(r) ISPC), 1.24.0 (build commit @ 20250404, LLVM 18.1.2)
Using Visual Studio 14.44.35228 toolchain (C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207) and Windows 10.0.22621.0 SDK (C:\Program Files (x86)\Windows Kits\10).

Using Unreal Build Accelerator local executor to run 2 action(s)
CPU 6 physical cores, 12 logical cores
Memory 15.77 GB physical, 30.08 GB/41.61 GB committed
UBA Storage capacity 40 GB
[1/2] Compile [x64] Module.BugTest.gen.cpp
[2/2] Compile [x64] MyActor.cpp

Total time in Unreal Build Accelerator local executor: 11.17 seconds
Output binary: C:\Program Files\Epic Games\UE_5.8\Engine\Binaries\Win64\UnrealEditor.exe

Result: Succeeded
Total execution time: 14.18 seconds
Trace written to file C:\Users\eis-a\AppData\Local\UnrealBuildTool\Trace.uba with size 5.7kb
File C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\Module.BugTest.gen.cpp.obj was modified or is new
File C:\Users\eis-a\Documents\Unreal Projects\BugTest\Intermediate\Build\Win64\x64\UnrealEditor\Development\BugTest\MyActor.cpp.obj was modified or is new
Building patch from 2 file(s) for Live coding module C:\Users\eis-a\Documents\Unreal Projects\BugTest\Binaries\Win64\UnrealEditor-BugTest.dll
UbaCli v5.8.0-Uba_v1.0.0-52526909 (Rootdir: “C:\ProgramData\Epic\UbaCli”, StoreCapacity: 0Gb)

---- Starting trace: Debug0ddc8f5b-2bbf-4cc3-aed9-9329dd1c505d ----
Running C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\bin\Hostx64\x64\link.exe @C:\Users\eis-a\AppData\Local\Temp\811E.tmp

Detoured run took 1.1s
Successfully linked patch (0.000s)
Patch creation for module C:\Users\eis-a\Documents\Unreal Projects\BugTest\Binaries\Win64\UnrealEditor-BugTest.dll successful (0.000s)