Lightmass fails to build on Apple Silicon Macs in UE 5.6+ when Embree is enabled.

General Issues and Bug Reporting (Unreal Engine)
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Summary

On Apple Silicon Macs, static lighting builds consistently fail in Unreal Engine 5.6, 5.7, and 5.8 when Embree is enabled. The same project builds successfully in Unreal Engine 5.5.4.

Disabling Embree by adding the following setting to DefaultLightmass.ini restores normal Lightmass functionality:

[DevOptions.StaticLighting]
bUseEmbree=false

This suggests the regression is related to the Embree integration introduced or modified after Unreal Engine 5.5.4.

What Type of Bug are you experiencing?

Editor

Steps to Reproduce

1.- Use an Apple Silicon Mac.
2.- Install Unreal Engine 5.6, 5.7, or 5.8.
3.- Create a new Blank project (or open an existing project).
4.- Add a Static Directional Light and any static mesh.
5.- Build Lighting.

Expected Result

Lightmass successfully computes the static lighting and completes the lighting build.

Observed Result

The lighting build fails immediately with the message: “Lighting build failed”.

The Output Log contains:
LogStaticLightingSystem: Running Lightmass w/ ImmediateImport mode ENABLED
LogStaticLightingSystem: Running Lightmass w/ ImmediateProcess mode ENABLED
LogStaticLightingSystem: Running Lightmass w/ Sorting mode ENABLED
LogEditorBuildUtils: Build time 0:00
LogStaticLightingSystem: Warning: Failed to build lighting!!! Lighting build failed.

Running UnrealLightmass manually also shows:
Failed to open scene channel with GUID …
Failed to import scene file
Segmentation fault

Affects Versions

5.8
5.7
5.6

Platform(s)

Mac

Additional Notes

  • Tested on an M3 Pro running macOS 26.5.2
  • Unreal Engine 5.5.4 builds lighting successfully on the same machine using the same project.
  • The issue has been reproduced in Unreal Engine 5.6, 5.7, and 5.8.
  • Firewall settings do not affect the issue.
  • UDP Messaging and TCP Messaging plugins are enabled.
  • UnrealLightmass starts correctly and successfully initializes its networking components.

[DevOptions.StaticLighting]
bUseEmbree=false

Completely resolves the issue and allows Lightmass to build static lighting normally.
This strongly suggests that the regression is related to Embree rather than the messaging or networking infrastructure.