MuJoCo 4 is out: a new viewer and renderer, mature soft bodies, and thin objects that no longer pass through
- Source: Google DeepMind
- Author: Open Source Robotics
- Published: 2026-11-06
- Category: research
Google DeepMind released MuJoCo 4.0 on 6 November. It replaces the old OpenGL viewer with MuJoCo Studio and the Filament renderer, makes soft bodies a stable built-in feature with plasticity and elastic rods, and adds speculative contacts so thin or fast objects stop passing through each other. Some soft-body and tendon models need migrating.
MuJoCo, Google DeepMind's open-source physics engine for robotics, released version 4.0.0 on 6 November, and Google DeepMind's robotics simulation team announced it the next day. Since version 3.5 the middle number only marks breaking changes, so the team kept 4.0 for a big step: 39 releases have come out since MuJoCo 3.0 in 2023. MuJoCo Playground 1.0, the team's library of simulated and real robot demos built on MuJoCo, came out alongside it.
Studio and Filament replace the old viewer
For a decade MuJoCo shipped a single fixed-function OpenGL 1.5 viewer. In 4.0, MuJoCo Studio leaves the experimental folder: a C++ and Python toolkit of building blocks for simulation, Dear ImGui interfaces and 3D rendering, with a ready-made Studio App as the new default viewer. Python plugins and model files reload while the simulation keeps running, so a custom debugging tool for one experiment takes little code. With --gfx=web, Studio runs on a remote or GPU-less machine and streams to a browser, where colleagues watch the same session and can take over control.
Rendering moves to Filament physically based rendering, with metal and rough materials, image-based lighting, soft shadows, and queues of colour, depth and segmentation frames from several cameras. live.mujoco.org opens any model in the browser with nothing installed. The old simulate viewer remains, in maintenance mode.
Soft bodies grow up
Deformables leave the experimental plugins and are solved together with rigid bodies, joints and tendons:
- A new implicit discrete integrator keeps stiff cables, thin shells and 3D solids stable at 5–10x larger timesteps. - Cloth can be made penetration-free with IPC contact. - Joints, tendons, shells and solids can now deform permanently: a wire stays bent, clay keeps a dent, metal foil keeps its dents and folded paper its creases (example models). - Elastic rods that bend and twist match the textbook cantilever deflection within 0.5% on CPU and on MuJoCo Warp. They replace <composite> and the cable plugin.
For a robot developer, cables, hoses, cloth and bags can now be part of a manipulation task without shrinking the timestep until training crawls.
Speculative contacts: thin and fast objects stop tunnelling
The "Speculative C…" in the announcement's title is speculative contacts. Turned on with <flag speculative="enable"/>, MuJoCo looks ahead along the path of fast-moving shapes within a step, keeps bodies on the separated side, and creates up to four contact points, so thin cards and plates land flat without sub-stepping. In the team's tests a robot arm caught a 0.7 mm playing card falling at over 2 m/s every time with a 16 ms timestep, where ordinary collision detection missed it above 8 ms. Humanoid stair climbing and thin-object grasping ran 6 to 9 times faster, because they could use larger timesteps. In 4.0 this covers convex shapes and meshes on CPU only; height fields, SDFs, flexes, MJX and MuJoCo Warp are to follow.
Also new
- The Python package is split: mujoco-core needs only NumPy, mujoco-studio adds Studio and Filament, and mujoco installs both. - New actuators and sensors, among them a PID actuator, a dcmotor built from a motor's datasheet, actuation delays, and tactile and contact sensors. - MuJoCo Menagerie now holds about 70 robot models and installs with pip install mujoco-menagerie. - A system identification toolbox, mujoco.sysid, helps fit a model to a real robot for sim-to-real work.
Before you upgrade
Most models load unchanged. The release notes have the full list and migration steps; the changes most likely to matter:
- Run pip uninstall -y mujoco before installing 4.0, or pip can remove files that moved into mujoco-core. - Python 3.10 is no longer supported; 3.11 to 3.15 are. - <composite>, the cable and shell elasticity plugins and flexvert equalities are gone. Move to <flexcomp> with <elasticity>; rods need the discrete, Euler or RK4 integrator. - Tendons wrap around a side site by a new rule, which can change existing models; the notes say where to move side sites. - RK4 is now fourth-order accurate for ball and free joints, so those simulations change. - The compiler now rejects some invalid models that used to load, such as a file included twice through different paths. - mujoco.experimental.studio is now mujoco.studio.
Libraries in this catalogue built on MuJoCo, such as mink, mjlab and MuJoCo MPC, may need their own releases before they work with 4.0, so check which MuJoCo version they ask for. MuJoCo stays under the Apache-2.0 licence.
Why it matters
MuJoCo is one of the physics engines most robot learning work runs on, from locomotion and manipulation research to the GPU-based tools built on it, so what it can simulate decides which tasks a lab can train in simulation. Version 4 removes two long-standing limits: soft objects such as cables and cloth were experimental and needed tiny timesteps, and thin or fast objects could pass through each other unless the timestep was shrunk. Both now work at larger timesteps, which means faster training. The new viewer also makes it easier to build custom debugging tools and to share a running simulation. The upgrade is not free: old soft-body models, some tendon setups and Python 3.10 users need changes, and speculative contacts are CPU-only for now.
Key takeaways
- MuJoCo 4.0.0 was released on 6 November, the first new major version since 3.0 in 2023.
- MuJoCo Studio and the Filament renderer replace the old OpenGL viewer, and Studio can stream a shared session to a browser.
- Soft bodies are now built in: implicit integration at 5–10x larger timesteps, penetration-free cloth, plasticity and elastic rods.
- Speculative contacts stop thin or fast objects passing through each other without sub-stepping, on CPU for now.
- Upgrading means uninstalling the old Python package first, dropping Python 3.10, and migrating composite and cable-plugin models.
Open-source angle
MuJoCo is Apache-2.0, and the 4.0 source, the Studio toolkit, the example models and the documentation are all in the public repository. MuJoCo Playground, MuJoCo Warp and MuJoCo Menagerie are separate public repositories from the same team.
Related technologies
- MuJoCo Studio
- Filament
- Deformable simulation
- Speculative contacts
- MuJoCo Warp
- MJX
Related projects
Links
- Source: https://github.com/google-deepmind/mujoco/discussions/3773