Occasional issue where background worker thread crashes in FD3D12RayTracingPipelineState constructor.

Development Cinematics & Media
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We are using the command line to run an Unreal editor instance to render specific scenes using MRQ. We’re experiencing an issue where a worker thread occasionally crashes before reaching BeginPlay, creating empty render folders. This crash happens rarely, and when it does, rerunning the exact same command line resolves it.

Crash Detail

All of our crashes show the same error in the log:

LogLevel: ActivateLevel /Game/(ProjectName)/Levels/Sublevels/UEDPIE_0_L_LevelLoadNotify 0 0 1
[2026.03.10-02.00.45:243][  2]LogThreadingWindows: Error: Runnable thread Background Worker #3 crashed.
[2026.03.10-02.00.45:247][  2]LogWindows: Error: === Critical error: ===
[2026.03.10-02.00.45:248][  2]LogWindows: Error:
[2026.03.10-02.00.45:248][  2]LogWindows: Error: Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0x0000017249f4a310
[2026.03.10-02.00.45:248][  2]LogWindows: Error:
[2026.03.10-02.00.45:248][  2]LogWindows: Error: [Callstack] 0x00007ffa30525f8f UnrealEditor-D3D12RHI.dll!FD3D12RayTracingPipelineState::FD3D12RayTracingPipelineState() [D:\\build\\++UE5\\Sync\\Engine\\Source\\Runtime\\D3D12RHI\\Private\\D3D12RayTracing.cpp:2765]

We have multiple levels that can be rendered, and this specific crash happens regardless of which level we’re trying to render.

Crash Frequency

This crash happens rarely, and when it does, rerunning the exact same command resolves it. In our efforts to reproduce it, we’ve observed a probability of around 2%–3%.

Command and Project Details

The Command we use is:

<Engine Location> <Project Location> <LevelToRender> -skipcompile -Multiprocess -NoLoadingScreen -FixedSeed -log -nohmd -Unattended -MRQInstance -deterministicaudio -audiomixer

Repro

This bug happens rarely, and we haven’t found a way to reproduce it consistently. However, we’ve found that limiting the number of cores used to load the project with -corelimit increases the probability of this crash occurring.

Hypothesis

From our testing and attempts to reproduce the bug, the problem appears related to level loading. Specifically, the raytracing shader worker thread is accessing resources that aren’t ready yet. Our experiments with limiting core counts increase the crash probability, which suggests a race condition between the main loading thread and the graphics worker thread.

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Steps to Reproduce
This bug happens rarely (1 in 50 estimated), and we haven’t found a way to reproduce it consistently. However, we have found that limiting the number of cores used to load the project with -corelimit increases the probability of this crash occurring.

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Hi Claude,

Can you confirm this happens in 5.7 (three most recent versions are supported)? UE 5.3 is outside of the dev support range, and a lot of work has gone into ray tracing since then.

If it does, can you make sure that editor debug symbols are installed and attach the entire call stack/log?

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Yes, please reach out if you are still having issues. That said if its not all happening on the same box it could be a driver issue. Also it could be a bad cache issue. Can you try and delete things in the /Saved and/or the /Intermediate fodlers. Also try and rebuild your DDC.

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Hi Shaun,

I was afraid that this would be the response :slight_smile: But thanks for the very prompt reply.

To answer your question; no, currently I can’t confirm that this happens in any version later than 5.3. This is probably a good reason to get around to that upgrade. We have a window in a couple of weeks where that should be possible. I will update this thread when I know more.

regards
Claude[Attachment Removed]