JustSleightly
As of June 24th 2025, VRChat 2025.2.3, build 1664, Avatar previews that I have spent hours fixing have been rebroken.
In the latest particular case, my suspicion is the patch note "Avatars will now update constraints before they are scaled to fit in the preview UI."
I was intentionally utilizing constraints in the preview pose animation to manipulate the scaling in the preview UI by not updating the constraint until after it was calculated. For example, in the second attached image, I was intentionally uploading the model with the arm at origin because somehow the mesh of the right arm would cause the preview to calculate the scale incorrectly, so I used constraints to restore the arm after calculations.
The only remaining method I believe to resolve this Avatar Preview would be to pre-pose the resting pose of the .fbx in blender to have the arms down at her sides, but although the humanoid rig still functions, applying that armature modifier then causes the shoulders and so on to behave fundamentally different with IK as the arms are lifted again via locomotion/IK.
I have gone back and forth with Fax (who recommended opening this canny) and other internal members for insight, and here's an overview of everything I know about avatar menu previews and what I've done to workaround them over the last few months to shed some light on what we've tried.
-----
The menu preview is based on not just the unity bounds of the avatar, but more specifically the bounds that VRChat calculates during the first frame including your animators and any logic they perform (supposedly).
Therefore things that may contribute to this (and multiple things may be contributing)
  1. Unity Bounds of an Avatar (like its meshes)
  2. FBX Bounds of an Avatar (like out of blender, including "exploding shapekeys" used for programs like Substance Painter)
  3. Positions of Gameobjects (such as like empty gameobjects that are floating around your avatar too far)
  4. And whether those things may or may not be animated on or off at runtime during the first frame
-----
For example on our other AVM model, it had 3 underlying issues that each individually caused the preview to break so I had to find and fix each one
  1. The FBX uploaded literally by itself had a small preview.
  • Solution: Posing the arms straight down like a pencil pose image.png
  1. The Blade in the left hand even existing broke the bounds. Possibly because in blender it was sideways at origin. I didn't want to go edit it in blender.
  • Solution: I disabled for a few frames in the preview clip at the beginning before reenabling it for it to dissolve in
  1. I had a series of arbitrary empty gameobjects used as constraint sources that hovered around my body in a large area
  • Solution: I scaled the root of the objects to 0, so all of its children that were scattered around your body all condensed into a single point, and i rescaled it back to 1 at runtime via animation
-----
For the current problem avatar at hand (attached images), the primary issue we had was that the right cyberarm (which uses a duplicate armature) for some reason its mesh at tpose triggers the menu preview to shrink because I guess VRC thinks its horizontal bound was too big. Since this arm is still the same size of an arm and same mesh, but just on a duplicate armature, it seems that VRC treats parts of the mesh weightpainted to non-humanoid bones differently than to humanoid rig bones.
The previous solution: I had to upload the avatar like this and use constraint animations to repair the arm at runtime.
Latest VRChat update breaks this solution, as it seems to calculate the constraint regardless of when it is animated (even beyond the 1st frame bounds calculation) prior to applying the preview scaling.
-----
Overall, please advise what can be done to work around these avatar preview menu issues, or implement some fixes/offer some alternatives to manipulate these avatar previews more consistently. It can be frustrating enough to constantly upload and iterate to test these previews alongside another current avatar preview bug where previewing the same avatar at different times can yield inconsistent sizes (which I'll open a separate canny about and link here)