Description

A licensee has reported that the following repro steps can result in a "Failed to merge bones" error:

  • One of our Skeletal Mesh assets was imported into engine version 5.4 using an older version of the FBX importer.
  • The engine was subsequently upgraded to version 5.5.
  • The art team then modified the original FBX file, adding new bones.
  • Attempting to reimport it using the reimport function resulted in the following error: "Failed to merge bones"

Internal only: a repro project has been provided. See repro steps for a faster repro.

Known workaround

Set the console variable "Interchange.FeatureFlags.Import.FBX" to false to use legacy FBX import, instead of Interchange.

Steps to Reproduce

Steps to Reproduce

  • Launch editor
  • Set "Interchange.FeatureFlags.Import.FBX" console variable to false
  • Drag and drop attached UI_Test.fbx file into content browser.
  • Check 'Skeletal mesh' when dialog pops up
  • Import
  • Set "Interchange.FeatureFlags.Import.FBX" console variable to true
  • Reimport newly created skeletal mesh with new file using attached UI_test2fbx.fbx file

Actual Result: Dialog pops up stating "Failed to merge bones". Selecting 'Yes' or 'No' does not result in the same hierarchy as a reimport done with Legacy.

Expected result: No dialog popping up. Asset is reimported as with legacy.

Have Comments or More Details?

There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-325794 in the post.

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Fixed
Fix Commit46318776
CreatedSep 15, 2025
ResolvedSep 29, 2025
UpdatedOct 6, 2025
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