Problem playing animations in Isaac Sim 5.1

[squirantes]

I am trying to create a simulation of a bird in isaac Sim 5.1. I have the algorithms and behaviours programmed, and I only need an animated model now.
I search for the internet a free model and get it, but is not rigged. So I import it directly to blender to rig it. It follows the XYZ Euler axis. I just add bones, add weights to the mesh, check the bones movement deforms it correctly to make it look like the bird moves its wings and then create an animation that lasts 40 frames of the bird moving its wings and up and down. I export it to ".usd" format.
I open it in Isaac Sim 5.1 and everything's normal. The bird loads in its correct position and when hitting play, the animation plays in loop. But if I import it as a reference in an empty scene, it plays one and stays still. I understand this is because the Endtime Code for the bird is 40 (the last frame) and for the scene is way bigger, but I cannot modify the end time code of every scene, so I am forced to use Anim Graph.
So, I enable AnimGraph, then I create the graph, add the skeleton, put the animation clip and add the graph to the skelroot.
When hitting play, it happens two things:

• The model rotates
• If trying to move the bird root xform (that envelopes everything), the xform moves but the model remains where it was when hitting play. The animgraph had no root motion option.
  I don't know why it happens and I'm desperate.

[zwdoescode]

@squirantes could you please share your files and detailed steps for us to replicate the issue?

[squirantes]

that's strange, I thought the Anim Graph was inside the prim.

Well, since it seems it cannot be export, I'll explain the process.

1- I'd open the Prim in Isaac Sim.

2- In the Hierachy on "stage" tab, I'd do Right click > Create > Animation > Anim Graph

3- I'd select the skeleton "Armature" of the prim

4- Then in the graph, I'd drag and drop an Animation Clip node and connect its pose to the final node, then, in the animationClip, where it says "Animation Source", I add the animation of the prim

5- I go to the SkelRoot called "Armature" too and do Right click > Add > Animation > Anim Graph and select the animgraph we created.

And there you can see it will happen exactly like that

[zwdoescode]

@squirantes My bad. I forgot to enable animagraph extensions. With extensions enabled, I can see the anima graph. Here is what I see. I am not really seeing the two issues you mentioned. Did I miss something?

Screen.Recording.2026-03-14.at.6.01.43.PM.mov

[squirantes]

it is most likely you forgot to attach the animgraph to the skelroot (I supose it because the animation is just playing once). You should do right click in the skelRoot > Add > Animation > AnimGraph.

[zwdoescode]

Thanks! I am able to reproduce the issue.

Screen.Recording.2026-03-21.at.10.37.54.PM.mov

Both problems originate from how the AnimGraph system (omni.anim.graph.core) evaluates and writes skeleton poses:

Bug 1 — Model rotates on Play:
Your crow's root bone ("Bone") has a non-trivial rest transform (rotated ~72° from world Z-up, which is normal for Blender exports). When AnimGraph evaluates the skeleton, it appears to apply the pose in a space that doesn't properly account for this bone orientation relative to the SkelRoot, causing the visible rotation.

Bug 2 — Moving parent xform doesn't move the mesh:
AnimGraph writes the skeleton pose directly to the Fabric rendering layer in what appears to be a fixed coordinate space. It does not compose the full ancestor transform chain — so if you move /World/Crow (the parent of the SkelRoot), the AnimGraph output stays pinned to the original world position.

Workarounds

Since both bugs are in the AnimGraph evaluation pipeline (compiled C++), they need an NVIDIA fix. In the meantime:

Option 1: Skip AnimGraph — use USD timeline looping
Your animation works perfectly without AnimGraph. To get looping:

1. Set your scene's endTimeCode to 40 (matching your animation)
2. In the Timeline panel at the bottom, click the loop icon (🔁)
3. Press Play — the SkelAnimation loops natively via USD evaluation, no AnimGraph needed

This also means the mesh follows parent transforms correctly.

Option 2: Script-based looping
If you need different assets with different animation lengths, you can create a simple Action Graph or Python callback that resets the timeline when it reaches the end of your animation.

Here is the video showing that with the workaround, it works well

Screen.Recording.2026-03-21.at.10.40.16.PM.mov

[zwdoescode]

I will also create an internal bug for the team to fix the animation graph! Thanks for reporting this.

[squirantes]

Thank you for the answer, it is absolutely great!
I hope they check this, though, but in the meantime your answer is a great workaround.

[zwdoescode]

@squirantes One more question... Will you be migrating to Isaac Sim 6.0? There will be some extension changes for animation graph for Isaac Sim 6.0+. The internal team will focus on targeting these changes.

[squirantes]

I understand.
I highly doubt it because this is for an specific project. But I'll keep an eye on the changes and talk to my team about them for future upgrades