[adenzler-nvidia]

mujoco-warp declares mujoco>=3.13.0, but it no longer imports with MuJoCo 3.13 through 3.15: ConDataField references mjtConDataField.mjCONDATA_LINVEL at import time since #1813, which fails with AttributeError. This is the same failure #1777 fixed for 3.12, and it went unnoticed because CI never installs the declared minimum. MuJoCo 4.0 also split the Python distribution, so mujoco is now a metapackage that pulls in MuJoCo Studio, Filament, glfw and PyOpenGL on top of mujoco-core. The library uses none of these. Only mjwarp-viewer, mjwarp-record and the tutorial notebook's mujoco.Renderer cells need them.

This change depends on mujoco-core>=4.0.0 and adds a viewer extra (mujoco-core[rendering], pillow) for those tools; dev includes it. pillow was previously only in dev, so mjwarp-record failed on a plain install. benchmarks/run.py --view, the README and the tutorial notebook now use the extra, and the nightly workflow upgrades mujoco-core instead of mujoco. The lock moves to the nightly 4.0.1.dev1017127398, because py.mujoco.org has no final 4.0.0 and 4.0.0.devN sorts below 4.0.0.

Every check_version("mujoco>=...") in the package is satisfied by 4.0.0, so this removes them together with the code they guarded:

• In put_model and put_data: the eq_data padding, the plasticity and tendon_type defaults, and the host-side surfacevel computation.
• The rod record size probe, which compiled a model on every import.
• The kernel branches on ROD_HINGE_SIZE, FLEX_STIFFNESS_3D and FLEX_INTERP_PHYS_JAC. The first two are now plain constants.
• About 76 version-dependent skips and expected-value switches in the tests.

The flex_internal check in put_model goes too, because MuJoCo 4.0 has no such field. util_pkg.check_version stays, for features of MuJoCo nightlies that haven't been released yet.

The tradeoff is that MuJoCo older than 4.0 is no longer supported. 3.13–3.15 were already broken, and 3.16–3.17 were never tested by CI. The packaging change and the shim removal are independent, so I'm happy to split them if that's easier to review.

[adenzler-nvidia]

@thowell this is just a suggestion, feel free to close if you think there is a better way to do this.

[thowell]

Thanks @adenzler-nvidia! Great cleanup switching to mujoco-core>=4.0.0, adding the viewer extra, and removing the pre-4.0 shims — looks ready to merge once two minor mujoco-core packaging edge cases are addressed:

1. pyproject.toml (lines 49--52) & uv.lock

• Issue: mjviser in dev declares an unversioned mujoco dependency, which still pulls mujoco and mujoco-studio into uv.lock. Because mujoco was removed from [tool.uv.sources], running uv lock --upgrade-package mujoco-core or uv lock --upgrade (as in .github/workflows/nightly.yml) resolves mujoco from PyPI (mujoco==4.0.0 conflicts with mujoco-core==4.0.1.dev* and backtracks to mujoco==3.17.0), causing uv sync --all-extras to fail on import mujoco (OSError: libmujoco.so.3.17.0: cannot open shared object file).
• Recommendation: Override the transitive mujoco metapackage dependency under [tool.uv] (mjviser runs directly against mujoco-core[rendering]) and regenerate uv.lock (UV_NO_CONFIG=1 uv lock --default-index https://pypi.org/simple), which also drops mujoco and mujoco-studio from uv.lock.

[tool.uv]
override-dependencies = ["mujoco ; python_version < '0'"]

[tool.uv.sources]
warp-lang = {index = "nvidia"}
mujoco-core = {index = "mujoco"}

2. mujoco_warp/_src/util_pkg.py (lines 75--86)

• Issue: When only mujoco-core is installed (no mujoco dist-info), util_pkg.check_version("mujoco>=4.0.1.dev...") hits PackageNotFoundError and falls back to mujoco.__version__, which on nightly mujoco-core wheels reports "4.0.1" without the .dev* suffix and falsely evaluates to True against newer .dev* thresholds. Conversely, passing "mujoco-core>=..." without wheel metadata fails in the fallback because the importable module is mujoco.
• Recommendation: Check both "mujoco" and "mujoco-core" distribution metadata before falling back to importing mujoco.

  dist_names = (
    (package_name, "mujoco-core" if package_name == "mujoco" else "mujoco")
    if package_name in ("mujoco", "mujoco-core")
    else (package_name,)
  )
  for dist_name in dist_names:
    try:
      installed_str = importlib.metadata.version(dist_name)
      break
    except importlib.metadata.PackageNotFoundError:
      pass
  else:
    # Fallback: import the package and read __version__
    try:
      import importlib as _importlib  # noqa: F811

      mod_name = "mujoco" if package_name == "mujoco-core" else package_name
      mod = _importlib.import_module(mod_name)
      installed_str = mod.__version__
    except (ImportError, AttributeError) as err:
      raise importlib.metadata.PackageNotFoundError(package_name) from err

[adenzler-nvidia]

Thanks @thowell, both addressed.

1. Added the mujoco override under [tool.uv] and regenerated uv.lock (af4ca63). mujoco and mujoco-studio are gone from the lock, and uv lock --upgrade / --upgrade-package mujoco-core now give the same result with and without --default-index https://pypi.org/simple. One thing to be aware of: mujoco and mujoco-core both ship mujoco/__init__.py, so syncing an existing venv uninstalls the metapackage and takes that file with it. uv sync --reinstall-package mujoco-core or a fresh venv fixes it; CI is unaffected.

2. check_version now looks up both the mujoco and mujoco-core distribution metadata before falling back to mujoco.__version__, and the fallback imports mujoco for a mujoco-core spec (8cb5af4). Same behaviour as your snippet, with the name mapping pulled into two module-level dicts, plus tests.

[thowell]

thanks @adenzler-nvidia!