# Interchange GLTF importer truncates skeletal mesh bone influences to 4 per vertex **URL:** \ **Category:** Issues and Bug Reporting (Unreal Engine)\ **Tags:** unreal-engine, open, bug-report\ **Created:** [August 20, 2026, 5:57pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391 "2026-08-20T17:57:58Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Amogus2389434](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Amogus2389434](https://forums.unrealengine.com/u/Amogus2389434)\ **Post date:** [August 20, 2026, 5:57pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/1 "2026-08-20T17:57:58Z")
### Summary The UE5 Interchange GLTF importer appears to truncate skeletal mesh bone influences to a maximum of 4 influences per vertex, even when the source glTF asset contains additional valid JOINTS\_n / WEIGHTS\_n attribute sets representing more than four influences. As a result, importing a glTF skeletal mesh through the Interchange GLTF pipeline can produce a skeletal mesh whose skinning data differs from the source asset, with additional bone influences being silently discarded. ### What Type of Bug are you experiencing? Other ### Steps to Reproduce 1. Create a skeletal glTF asset with one or more vertices influenced by more than 4 bones. 2. Import the glTF asset into UE5. 3. Inspect the imported skeletal mesh’s skin weights. ### Expected Result The importer should preserve all supported JOINTS\_n / WEIGHTS\_n attribute sets from the source glTF ### Observed Result Only the first 4 bone influences per vertex are imported. Additional influences present in the source glTF are discarded, resulting in incorrect skinning compared to the original asset. ### Affects Versions 5.8 ### Platform(s) Windows ### Additional Notes These are the relevant sections of the UE source that appear to be involved: [https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Import/Private/Gltf/InterchangeGltfMesh.cpp#L185](https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Import/Private/Gltf/InterchangeGltfMesh.cpp#L185) [https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Private/GLTF/GLTFMesh.cpp#L437](https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Private/GLTF/GLTFMesh.cpp#L437) [https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Public/GLTFMesh.h#L26](https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Public/GLTFMesh.h#L26) [https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Public/GLTFAccessor.h#L145](https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Public/GLTFAccessor.h#L145) ---
**Author:** ![UE\_FlavienP](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/ue_flavienp/32/1194546_2.png) [@UE\_FlavienP](https://forums.unrealengine.com/u/UE_FlavienP)\ **Post date:** [August 21, 2026, 10:40am UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/2 "2026-08-21T10:40:09Z")
It is the glTF specification that limits mesh bone influences to 4 per vertex. UE is aligned on the format spec. Where do you get your glTF from? Are you exporting it from a specific DCC? [https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#skinned-mesh-attributes](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#skinned-mesh-attributes) ![image](https://d3kjluh73b9h9o.cloudfront.net/original/4X/6/f/2/6f2593199443e87a33f5a8b556be13d2cf40fed6.png) ---
**Author:** ![Amogus2389434](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Amogus2389434](https://forums.unrealengine.com/u/Amogus2389434)\ **Post date:** [August 21, 2026, 11:23am UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/3 "2026-08-21T11:23:24Z")
The glTF spec limits each `JOINTS_n` / `WEIGHTS_n` attribute to 4 joints/weights, but it does not necessarily limit a vertex to 4 total influences. Additional influences can be stored in additional attribute sets, for example: - `JOINTS_0` / `WEIGHTS_0` → influences 0–3 - `JOINTS_1` / `WEIGHTS_1` → influences 4–7 - and so on… The spec explicitly describes this mechanism for vertices influenced by more than four joints. The asset was exported from Blender 5.2 using its built-in glTF 2.0 exporter. ---
**Author:** ![UE\_FlavienP](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/ue_flavienp/32/1194546_2.png) [@UE\_FlavienP](https://forums.unrealengine.com/u/UE_FlavienP)\ **Post date:** [August 21, 2026, 12:53pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/4 "2026-08-21T12:53:05Z")
Thanks, that is pass my knowledge of the glTF importer implementation, I created a ticket so a dev look into it. By any chance, do you have a model already made you could share with me in DM? ---
**Author:** ![Amogus2389434](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Amogus2389434](https://forums.unrealengine.com/u/Amogus2389434)\ **Post date:** [August 21, 2026, 1:11pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/5 "2026-08-21T13:11:11Z")
[test.glb](https://forums.unrealengine.com/uploads/short-url/iQKPG8vXH6Fcgrxx0kxy61lWo8z.glb) (91.6 KB) Here’s a model created in Blender to demonstrate If you import the model into Blender and switch to edit mode, you’ll notice that some vertices are influenced by 10 bones. However, in the Unreal Editor, under the “Edit Skin Weights” menu, no more than 4 bones influence any given vertex. ---
**Author:** ![Bug-Reporter](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/bug-reporter/32/997076_2.png) [@Bug-Reporter](https://forums.unrealengine.com/u/Bug-Reporter)\ **Post date:** [September 23, 2026, 3:01pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/9 "2026-09-23T15:01:24Z")
UE-397404 has been ‘Closed’ as a duplicate of an existing known issue. Origin Issue: UE-392966 ---
**Author:** ![Bug-Reporter](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/bug-reporter/32/997076_2.png) [@Bug-Reporter](https://forums.unrealengine.com/u/Bug-Reporter)\ **Post date:** [September 23, 2026, 3:01pm UTC](https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/10 "2026-09-23T15:01:24Z")
UE-392966 has been added to our ‘To Do’ list. Someone’s been assigned this task.