"Increase Windows stack" Patch in 3.4.0 breaks the mujoco python binding in Windows

[Chuanfang-Neptune]

Intro

Hi!

I am a phd student using MuJoCo for my research on soft animals/robots with long spines.

My setup

mujoco == 3.4.0
python = 3.10
This bugs is specific to Windows OS (Win11 in my case). On Linux everything is good.

What's happening? What did you expect?

In short, I suspect that commit b6f25ca that fixed #2898 broke the mujoco Python bindings on Windows OS.

In the #2898, OP mentioned a bug that Windows executables of mujoco (simulate.exe, etc.) had problems loading long nested models. Thanks for fixing this in 3.4.0.

Indeed, I had similar issues as OP that simulate.exe could not load a long nested model for mujoco<3.4.0 (crashed without a log). However, this bug was trivial to me as I was mainly working with my model through mujoco or dm_control python bindings for coding controllers/sensors/data analysis, etc. Although in mujoco<3.4.0, executables on Windows OS could not load a long nested model, the Python bindings worked normally.

After the fix in 3.4.0, executables on Windows OS could load a long nested model, but the Python bindings did not work anymore (crashed without a log).

It would be optimal if both executable and Python bindings work on Windows OS. But if only one of the two works, I would vote to keep the original version where the Python bindings work.
Because from the Python binding, a user could invoke the viewer. But if only the executable works, the user cannot use the Python bindings to do detailed programming and control of the model.

Steps for reproduction

1. Download the model Body_REV02_76_body.xml.

2. Install mujoco==3.3.7

3. Run the test script below. (You will get the print message "Model loaded successfully" => python binding works)

4. Use simulate.exe to load the model (Crashes without a log => the executable doesn't work)

5. Install mujoco==3.4.0 (After the patch)

6. Run the test script below again. (You will not get the print message and crash without a long => python binding doesn't work)

7. Use simulate.exe to load the model (It loads. => the executable works )

(The same thing goes for dm_control.mjcf.Physics.from_mjcf_model('Body_REV02_76_body.xml') I guess it calls the same buggy thing in mujoco)

Minimal model for reproduction

For reproducing the bug, I use OP's model with 4 more bodies.
Body_REV02_76_body.xml

Code required for reproduction

import mujoco

model = mujoco.MjModel.from_xml_path('Body_REV02_76_body.xml')
data  = mujoco.MjData(model)
print('Model loaded successfully.')

Confirmations

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

[Purnavu-12]

Workaround confirmation + reproduction details (Windows)

Hi team! Just confirming this is still broken in 3.4.0 on Windows 11 (Python bindings crash on deep/nested models, while the standalone simulate.exe now works fine).

Reproduction (crashes in 3.4.0, works in 3.3.7):

import mujoco
model = mujoco.MjModel.from_xml_path('Body_REV02_76_body.xml')  # or any very deep nested model
data = mujoco.MjData(model)
print('Model loaded successfully.')

What works as a workaround:

Downgrade to mujoco==3.3.7 (pip install mujoco==3.3.7)
Use Python 3.13 (or lower) — Python 3.14 has no wheels yet, so it tries to build from source and fails without MUJOCO_PATH set
Pre-built wheels for cp313-win_amd64 install instantly and load deep models without crash

Tested successfully today (Jan 11, 2026) on Windows 11:

Python 3.13.2 (via pyenv-win)
mujoco 3.3.7
Output: 3.3.7 + "Model loaded successfully! ✓" (no segfault)

For most Python/ML users (RL, dm_control, etc.), the Python bindings are way more important than the increased stack in the executable, so reverting the stack size bump (or making it conditional for pythonmodule vs executable) would be ideal.
Happy to test patches or provide more details if helpful. Thanks for the great simulator!
Environment:

Windows 11
Python 3.13.2
mujoco 3.3.7 → works
mujoco 3.4.0 → crashes on load