Intro

We are working on the mujoco-usd-converter.

My setup

MuJoCo 3.5.0, Python, Windows and Linux

What's happening? What did you expect?

Segfault when accessing tendons through the python bindings on a model that is included using body/attach.

Accessing the MjsWrap objects from MjsTendonPath in the Python bindings causes a segfault when the model was loaded via <attach>.

I found this when trying to iterate over the tendon path in the softfoot model from within scene.xml from https://github.com/google-deepmind/mujoco_menagerie/tree/main/iit_softfoot

Steps for reproduction

I've attached a shell script and python program that will segfault:
mujoco_tendon_repro.zip

setup_and_run.sh -- the entry point:

1. Creates a .venv virtual environment using uv venv
2. Installs mujoco>=3.5.0 via uv pip install
3. Downloads only the iit_softfoot directory from mujoco_menagerie using a sparse git checkout (avoids cloning the full repo)
4. Runs the Python repro script

repro_tendon_bug.py -- the reproduction:

1. Loads softfoot.xml directly via MjSpec.from_file(), compiles it, and iterates over all tendons/path waypoints -- proving the model itself is valid
2. Loads scene.xml (which includes the softfoot via <attach/>), then does the same iteration
3. For each waypoint, attempts to access wp.target.name -- this is where the crash should occur due to the Python bindings bug

To run it:

cd /home/lrohan/dev/mujoco_tendon_repro
./setup_and_run.sh

Minimal model for reproduction

The repro zip/script will download the softfoot scene from the MuJoCo Menagerie.

Code required for reproduction

The repro zip/script contains the Python code.

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.