UE5.6+ - Mutable - IDPrefix crash when combining Child Objects, Add to Mesh Component and Extend Mesh Section.

So the issue seems to be a combination of Add To Mesh Component and Extend Mesh Section.

I've tried this in Epic Launcher 5.6, 5.6 in-house modified engine as well as ue5-main 5.7 from git (editor built 14th of July) and they all produce the same crash.

This seemed to work perfectly fine for us in 5.5.x

The repro-steps I provided does stack 2x of LOD0 Section 0 of Manny but that isn't important. In a normal use-case, you'd use multiple meshes sharing the core skeleton but for simplicity of the repro I stuck to only using Manny.

Steps to Reproduce

Should only need Third-Party Template, base Mutable plugin loaded and no specific changes to the editor.

You will need at least 2 Customizable Object (CO) graphs, with one being attached to the other.

In my example setup, I got the following COs:

• CO_Parent (aka. Parent)
• CO_Child (aka. Child)

For skeletal mesh, any should do but for this example I use SKM_Manny_Simple as the Skeleton Reference as well as the Skeletal Mesh.

 

Setup Parent graph:

1. Add and wire up the following nodes:

1. Mesh Component

1. Name: MC_Parent
2. Reference Skeletal Mesh: SKM_Manny_Simple
2. Mesh Section

1. Material: Any (e.g. WorldGridMaterial)
2. Enable Tags: MyTag
3. Skeletal Mesh

1. Mesh: SKM_Manny_Simple - Mesh section: Any (e.g. LOD0 Section 0)
4. Object Group

1. Name: IN_Child
2. Type: All Options

 

Setup Child graph:

1. Add and wire up the following nodes:

1. Add To Mesh Component

1. Component Name: MC_Parent
2. Mesh Section

1. Material: Any (e.g. WorldGridMaterial)
3. Extend Mesh Section

1. Required Tags: MyTag
4. Skeletal Mesh

1. Mesh: SKM_Manny_Simple
2. LOD0 Section 0 to Mesh Section
3. LOD0 Section 1 to Extend Mesh Section
5. Compile
6. Crash

Assertion failed: IDPrefix [Link Removed] [Line: 101]

UnrealEditor_Core!FDebug::CheckVerifyFailedImpl2() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:728]
UnrealEditor_MutableRuntime!mu::MeshFormatBuffer() [D:\build\++UE5\Sync\Engine\Plugins\Mutable\Source\MutableRuntime\Private\MuR\OpMeshFormat.cpp:101]
UnrealEditor_MutableRuntime!mu::MeshMerge() [D:\build\++UE5\Sync\Engine\Plugins\Mutable\Source\MutableRuntime\Internal\MuR\OpMeshMerge.h:964]
UnrealEditor_MutableRuntime!mu::CodeRunner::RunCode_Mesh() [D:\build\++UE5\Sync\Engine\Plugins\Mutable\Source\MutableRuntime\Private\MuR\CodeRunner.cpp:1503]
UnrealEditor_MutableRuntime!mu::CodeRunner::RunCode() [D:\build\++UE5\Sync\Engine\Plugins\Mutable\Source\MutableRuntime\Private\MuR\CodeRunner.cpp:6260]
UnrealEditor_MutableRuntime!mu::CodeRunner::Run() [D:\build\++UE5\Sync\Engine\Plugins\Mutable\Source\MutableRuntime\Private\MuR\CodeRunnerTasked.cpp:308]
UnrealEditor_MutableRuntime!UE::Tasks::Private::TExecutableTaskBase<`mu::CodeRunner::Run'::`122'::<lambda_1>,void,void>::ExecuteTask() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Public\Tasks\TaskPrivate.h:907]
UnrealEditor_MutableRuntime!UE::Tasks::Private::FTaskBase::TryExecuteTask() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Public\Tasks\TaskPrivate.h:527]
UnrealEditor_MutableRuntime!LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48>::TTaskDelegateImpl<`LowLevelTasks::FTask::Init<`UE::Tasks::Private::FTaskBase::Init'::`2'::<lambda_1> >'::`13'::<lambda_1>,0>::CallAndMove() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Public\Async\Fundamental\TaskDelegate.h:171]
UnrealEditor_Core!LowLevelTasks::FTask::ExecuteTask() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Public\Async\Fundamental\Task.h:627]
UnrealEditor_Core!LowLevelTasks::FScheduler::ExecuteTask() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Async\Fundamental\Scheduler.cpp:387]
UnrealEditor_Core!LowLevelTasks::FScheduler::WorkerLoop() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Async\Fundamental\Scheduler.cpp:724]
UnrealEditor_Core!`LowLevelTasks::FScheduler::CreateWorker'::`2'::<lambda_1>::operator()() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Async\Fundamental\Scheduler.cpp:188]
UnrealEditor_Core!FThreadImpl::Run() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\HAL\Thread.cpp:69]
UnrealEditor_Core!FRunnableThreadWin::Run() [D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Windows\WindowsRunnableThread.cpp:159]