Version 3.17.0 (October 22, 2026)

Significant new features

1. c00ed8153 Added a batch API in mujoco/mjbatch.h (in Python as
   mujoco.batch): many simulations of one model stepped on a thread pool of its own, each held as its
   integration state and loaded into an mjData for each call, so memory scales with threads and results are
   bit-identical to a plain loop at any thread count. It supports per-simulation model and option fields or models of
   the caller's, observations of the state after a step, recorded trajectories, persistent simulations (required for
   sleep), and arbitrary per-simulation functions. Follows the design of
   mjbatch by @kevinzakka.

Engine

2. Flex improvements:

   • 94483e166 Added bending elasticity for 1D flexes, using a twist-free isotropic
     discrete elastic rod model. The cross-section is selected with the new section
     attribute: a circle of the flex radius, or a box with half-sizes sectionsize
     that uses the stiffness of its weak axis. It is enabled by a positive young modulus,
     requires a straight reference configuration (curved initial poses can be set with qpos or keyframes), is
     compatible with edge equality constraints, and is integrated implicitly by the discrete
     integrator. Previously young was ignored for 1D flexes: existing models that set it now bend, and fail to
     compile if the cross-section is degenerate (zero radius with the default circle section), if a vertex joins a
     zero-length edge, or if the rod is not straight.

   • 36d0df9a5, 897e3fcb1 Interpolated flexes (trilinear or quadratic
     dofs) now support self-collision, excluding elements in
     the same or neighboring cells.

   • 39dd10f2d The default arena memory for flex models is now much smaller, sized from
     the flex mesh topology instead of nv^2. If the Newton solver runs out of arena memory it falls back to CG with
     a warning.

3. mj_setConst now computes only quantities derived from other fields of mjModel, while the compiler alone
   resolves spec-level conventions:

   • 55c1b585a, d6d41f324 The damping of position actuators from
     dampratio, the spring length of tendons from (-1, -1), and model statistics
     are now resolved only at compile time, not by mj_setConst. Set the model fields directly when editing a
     compiled model (mj_copyBack now copies tendon spring lengths back to the spec).

   • c8362756d Constraint solvers now scale their termination criteria by the trace of the current inertia
     matrix (of the model or island) instead of mjModel.stat.meaninertia. Models solved with PGS, noslip or islands
     disabled can stop at slightly different iterations. The meaninertia attribute is
     deprecated and now emits a compile warning; remove it to migrate.

   • abddec5b9 For body-based weld constraints without a specified
     relpose, the relative pose in qpos0 is now recomputed by mj_setConst when
     the model is modified after compilation (matching the second body's anchor in
     body-based connect constraints). The compiler marks such welds with a nonzero eq_data[11], and
     mjNEQDATA increased from 11 to 12.

• 865316f19 Added MJSETCONST_ARRAYS and MJSETCONST_SCALARS in mjxmacro.h, listing the
  mjModel arrays and scalars read and written by each phase of mj_setConst.

4. e15ecd842 Added single-shot multicontact for geoms with positive contact
   margins.

5. 7394a8672 mju_boxQP now updates its Cholesky factor incrementally with mju_cholUpdate, making
   cold-started solves 1.4-1.6x faster and warm-started solves 1.9-2.9x faster.

6. aa0f88e77 Trees marked sleep="init" which cannot be put to sleep now cause a warning
   rather than an error, and start awake. See Sleeping for details.

Compiler

9. 3576818f5, b0f206d04 Textures can be left by reference instead of being
   decoded into mjModel. With compiler/passthrough set to "textures", or
   texture/embed set to "false", a single-file texture is not read at compile time; the
   model holds its path instead of its pixels, so a scene that attaches many textured sub-models loads in a fraction of
   the memory and time. In addition, a compiled mjSpec no longer keeps the decoded pixels of its textures between
   compilations.

   • 3576818f5 What the model holds: tex_adr is -1, tex_width and tex_height are 0, ntexdata
     excludes the texture, the resolved path is at tex_pathadr, and any authored
     [content_...

Version 3.16.0 (October 14, 2026)

Significant new features

1. cf89a6375, 4316935b3, 6d98d9fab, 5f17e92eb, 89d59b11f,
   147dc67d0 Frames can now scale their contents at compile time—without modifying the
   mjSpec—via the new scale, scaling, and
   massscale attributes. The default similar policy scales all physical quantities so that
   dynamics are preserved under proportionally scaled gravity, whereas geometry scales only spatial dimensions and
   inertias (and supports anisotropic scaling). Masses scale with volume unless overridden by massscale. External
   references to scaled elements—including actuators, tendons, constraints, sensors, keyframes, and deformable/mesh
   assets—are scaled consistently, and mj_copyBack maps values back into unscaled spec coordinates. See
   frame/scale for details.

2. 592c54ad3, 92b8f4b27, d4595d020 Added compiler attributes
   savecompiled and savecanonical (both defaulting to
   "true") to preserve original authoring when saving models to XML. Setting savecompiled="false" saves the
   uncompiled mjSpec directly—retaining default classes, frames, and authored numeric values—so the saved XML
   reloads into an identical spec. Setting savecanonical="false" additionally preserves the original input
   conventions, such as orientation representations, degree/radian units, and actuator shortcuts; see
   Model Encoding & Saving. mj_copyBack (MjSpec.copy_back in Python) now updates only the
   modified fields in the spec while leaving all other authored values untouched.

3. 9d953b40c Added support for building MuJoCo with Bazel (initially covers core
   library, simulate, plugins, and samples).

General

Engine

6. 0998957cf The contact sensor now supports linvel and angvel
   data fields for reporting relative contact velocities, and its computation stage
   (mjtStage) is now inferred from the requested data and
   reduce attributes.

7. d823f9212 mj_setConst now computes the bounding volumes of geoms and bodies, the collision masks and
   margins of bodies, and the added mass of geoms in the ellipsoid fluid model. Changes to the size,
   pose, margin and gap of geoms, to the inertial frames of bodies and to the size of height fields are then safe;
   previously contacts could be missed. mj_setConst also computes the constants of sleeping bodies, which it
   skipped, and resolves the automatic sleep policy of trees anew.

8. 52824daf6 Corrected the shear contribution to solid and membrane stiffness for interpolated flexes, and the
   three-point Gauss quadrature used by quadratic flexes. Material parameters now produce the standard isotropic
   linear-elastic stiffness; existing models using these flexes can change behavior.

9. 3bc4b769a Improved numerical stability of sphere and cylinder spatial tendon
   geom wrapping near wrap onset and for inside wrapping. Thanks to @M-Colley for
   reporting and analyzing the issues in MJWarp issues
   1713 and
   1714.

10. 871e3725b Flex contacts are now reduced to mjMAXCONPAIR per geom only for rigid and interpolated
    flexes; other flexes keep all their contacts. Previously, cloth resting on a plane or on many static geoms could be
    held by at most 50 contacts and fall through.

11. efffd3a5f Fixed floating-point cancellation in capsule-capsule collision detection for parallel and
    near-parallel axes. Thanks to @ayonga for reporting the issue in MJWarp issue
    1758.

Compiler

Version 3.15.0 (October 5, 2026)

General

1. 1aa68e1ca .mjz archives written on Windows now always use / separators.

2. 1aa68e1ca Assets in a meshdir or texturedir now
   resolve correctly in .mjz archives when rewritten or when the directory path is absolute, uses
   .., or uses a URI scheme.

Engine

5. 1b973add4 Added experimental simplified Stable Neo-Hookean elasticity for non-interpolated 3D flexes,
   enabled only through mjsFlex.elastic3d = 1. It projects the material Hessian to positive semidefiniteness for the
   solver and Rayleigh damping, and preserves the Saint Venant-Kirchhoff (StVK) default (0).
   SNH requires the discrete integrator. The setting is not available in MJCF.

6. 2742de438 The cached flex bending factor now retains cross-coordinate couplings between differently
   oriented vertex bodies.

7. eed008905 Flex bending and stretching now include the motion and reaction forces of articulated vertex
   attachments. The discrete integrator supports these attachments with the CG solver; fixed and independent XYZ-slide
   attachments retain their optimized assembly. Elastic flexes attached to mocap bodies produce a compiler error.

8. 6224e95f6 Added single-shot multicontact for collisions with capsule geoms.

9. ebfb4662d Added the enclosed attribute to
   insidesite sensors, measuring how much an object juts out of a site (using directed
   Hausdorff distance) and reporting signed clearance/protrusion.

10. f80ef4244 Fixed 64-bit model size narrowing in binary MJB serialization: mj_saveModel and
    mj_loadModelBuffer now accept mjtSize for buffer_sz, supporting models larger than 2 GiB.

Version 3.14.0 (September 22, 2026)

General

1. 458189298 Added the ipc flag, an experimental contact mode of the discrete
   integrator for penetration-free flex contact. Each step minimizes an incremental potential subject to linearized
   contact constraints, using a barrier-free augmented Lagrangian whose subproblems are the discrete solve. Every
   committed position update is verified intersection-free by continuous collision detection, so flex contact cannot
   tunnel. Supported for dim-2 flexes: a flex with edge equality constraints keeps its elasticity in the constraint
   solver, while elastic2d elasticity is integrated implicitly through the effective metric. The contacts the mode
   resolves are frictionless. The mode keeps contact multipliers across steps that no state specification covers, so
   mj_getState/mj_setState do not capture its full state and exact replay is not supported.

2. 2fec92237 Introduced archive resource providers
   (mjp_registerArchiveResourceProvider). Archive providers use the
   mjpResourceProvider interface to mount and read archive containers (such as
   .mjz/.zip), decoupling container handling from format decoders and enabling
   on-demand asset extraction without requiring a pre-allocated VFS.

3. e312ce82d Frames are now preserved when saving MJCF: <frame> elements are written with
   their pos and quat and their contents in frame-relative coordinates, so a saved model reloads with the same
   frames. Previously the frame transformation was accumulated into the children on save and only the frame's name was
   kept. Changes to the pos or quat of a frame in an mjSpec now also take effect when the spec is
   recompiled.

Engine

5. 9e0c6b577 The mjWARN_INERTIA warning is now also raised by the modified-inertia
   factorizations of the implicitfast and damped-Euler integrators (previously silent) and of the
   implicit integrator (previously a fatal error).

6. 35c0631de The sparse Newton solver no longer aborts with a "rank-deficient sparse Hessian" error when
   rounding loses a pivot of its Hessian, as can happen in single precision with ill-conditioned inertia. The pivot is
   now clamped and its row decoupled, as in the dense factorization.

Bug fixes

7. 2a3f1f5a4 Procedural model editing is no longer quadratic in the number of elements. mjSpec previously
   recomputed the compilation signature on every mjs_addGeom-like call, and mjs_setName rescanned every
   name of the element's type. Both are now incremental. Building a spec with 8000 geoms is roughly 10x faster.

8. ac329bd17 Fixed the torque applied by weld constraints in
   mj_rnePostConstraint. The rotational constraint force was used directly as a world-frame torque, missing the
   scaling by 0.5 * torquescale and the rotation into the world frame. Force and
   torque sensors on bodies held by a weld that carries torque were wrong (#2533).

9. 263c638c0 mj_rnePostConstraint now accounts for the forces of spatial tendons (spring, damper,
   actuator, limit, friction loss, equality and armature), so force and
   torque sensors register the forces that tendons apply to bodies (#832).

10. eb662fcaf Tendon actuatorfrclimited now defaults to "auto" as
    documented. Previously the default was "false" and actuatorfrcrange was silently ignored unless
    actuatorfrclimited was set explicitly.

11. 54be9cce6 mjd_transitionFD and mjd_inverseFD now raise an error when
    sleeping is enabled. Previously, their repeated evaluations changed the sleep state, leading to
    internal errors or wrong derivatives.

12. e38573c64 Fixed a memory leak in mjs_delete where deleted elements and subtrees were retained in
    mjSpec until mj_deleteSpec (#2882).

13. 65ea54c3c Fixed an out-of-bounds read when parsing the header of a GMSH file loaded
    by flexcomp. Truncated headers are now reported as an error.

Version 3.13.0 (September 8, 2026)

General

1. eb18d77ca The .mjz encoder now writes the root file as model.xml in the archive as
   this is less susceptible to breakage due to file renaming.

2. 4affbb64a Site geometries can now also be associated with meshes (type="mesh" with
   mesh="name"), supporting visualization, the insidesite
   sensor, and the new mj_insideSite function.

3. b59b07fae Added support for Python 3.15 (GIL and Free-Threading).

Engine

4. 5660353ec Added a new integrator discrete: the constraint solve and the implicit velocity update merge
   into one operation, performed in the effective metric $\widehat{M} = M + hD + h^2K$, which incorporates both
   implicit damping $hD$ and implicit position stiffness $h^2 K$. Under this integrator mjData.qacc is
   the discrete step map $(v^+ - v)/h$, and joint, tendon and actuator stiffness and damping join the solver's
   metric, making passive springs and actuator position gains stable at timesteps far beyond the explicit stability
   limit. Constraint rows are treated implicitly as well: solref spring--dampers are evaluated at the end of the
   step, so constraints are stable at any timeconst; under this integrator the refsafe
   flag replaces contact and limit rows stiffer than the timestep can resolve by the stiffest zero-restitution row
   instead of clamping timeconst. The actuator-gain treatment resolves the stiff-servo timestep limitation of
   #3443 (analysis contributed by @qiayuanl). See the
   integrator documentation for semantics and current limitations.

5. 0b4e17747 Restored clamping of non-positive pivots in the sparse inertia factorization, along with the
   associated mjWARN_INERTIA warning. The guard was inadvertently dropped in the 3.3.0 conversion of qLD to CSR
   format; since then, models with singular mass matrices silently produced non-finite accelerations, typically
   surfacing as divergence warnings and automatic resets.

6. c9d997610 Added single-shot multicontact for collisions with cylinder geoms.

7. 10eeb8289 The Newton solver with elliptic cones now rebuilds the cone-augmented Hessian
   factor with a single refactorization instead of per-contact rank-1 updates when a flop-count model predicts this is
   faster. Scenes with many simultaneously sliding contacts speed up by 1.4-2x on average and 3-4x on the slowest steps.
   Contribution by @kevinzakka.

Compiler

8. 2a3957558 Custom text fields (custom/text) in MJCF now accept their values inside a
   <![CDATA[ ... ]]> block in addition to the data attribute. When saving a model via mj_saveXML, custom
   text containing newlines or XML characters is exported as CDATA.

9. 47ebfe48a Added cutoff attribute to sensor/tactile.

Samples

10. 834983fbb Added render.cc: a headless offscreen rendering sample that saves an image to a
    PNG image. Supports both the Filament and classic OpenGL backends, with command-line options for camera, resolution,
    keyframe, simulation steps, geom/site groups, visualization and rendering flags, labels, and frames.

Bug fixes

11. 67562d95f Rewrote the Plane-Mesh collider to fix bugs and improve performance. Fixes #3524.

Models

12. f6c1004b7 Added house of cards
    example model and three free-standing arch
    models: roman, gothic, and hyperbolic. Inspired by Song et al.

Rendering

Version 3.12.0 (August 20, 2026)

General

1. 3f8db4c1 The MJCF grammar is now defined in a single source of truth schema file, src/xml/mjcf.schema. The parser's grammar table, presence constraints, keyword maps, typed attribute bindings and save policies are generated from it and gated by tests, as are the schema's enum keywords and declared defaults against the C headers and default-constructors.

Actuation

3. 279df98c Added the pid actuator: a PID controller with real position and velocity setpoint inputs, optional integral action (ki, integrating the position error with imax anti-windup), setpoint rate limiting (slewmax), and an optional feedforward input. This subsumes the functionality of the mujoco.pid plugin with proper activation state: correct under all integrators and visible to keyframes and sensors. With a zero velocity setpoint it is identical to position. The input signature is any subset of [pos, vel, ff], selected by input; absent setpoint inputs are fixed at zero, so the control vector contains no inert entries.
4. 2f1843f4 The dcmotor on-board controller is redesigned: the input attribute selects any subset of [pos, vel, ff, voltage], where pos and vel are setpoints for the controller, ff is a torque feedforward, and voltage is the raw terminal voltage (the default, a plain voltage-commanded motor). Controller gains are in torque space, as for pid, and the drive voltage compensates back-EMF as in a current-controlled driver: commanded torque is delivered exactly until a limit is reached. The keyword input="none" selects the empty signature: the actuator has no control inputs and is purely passive, so friction, cogging and back-EMF braking can be used as passive joint forces.

Engine

6. 83e621d7 Optimized large-mesh convex collision detection with up to 2x speedup in certain cases.
7. 55d13aec Replaced the per-step sparse Cholesky factorization of the flex block of the implicit effective metric M + K with its prefactored per-vertex 3x3 diagonal blocks. The blocks precondition the CG constraint solver and drive an iterative solve for qacc_smooth, which now converges on tolerance rather than a fixed threshold. Flexes with elastic2d stretch stiffness step roughly twice as fast; bending-only flexes keep the exact constant factor and are unchanged.
8. 86e98601 Rewrote cleaner box-box SAT collider.

Models

13. 2a3554c8 Added drape example model: three cloths draped over a sphere, demonstrating passive collisions. It replaces the sphere_passive model, which has been removed.
14. 55d13aec Added bag example model: a cloth bag, held open by pinning the ring of vertices around its mouth, catching the standard humanoid dropped in from above. Unlike the poncho models, which are bending-only, this model exercises the 2D stretch elasticity of a flex.

Rendering

Warning

Breaking API changes

15. cc7fb98c Added explicit texture coordinates to built-in geometries (Plane, Box, Sphere, Ellipsoid, Capsule, Cylinder) in both the Classic renderer and Filament. 2D textures applied to primitive shapes will look different as textures are mapped using canonical UV parameterizations rather than projecting onto the $x,y$ plane.

    For finite planes, textures are now anchored to the bottom-left corner instead of the center. This will cause the most common visual breakage, as common procedural checker textures will be phase shifted. Infinite planes continue to be anchored at the origin with no visual changes.

    

16. f9a00bd5 Added light/softness: edge softness for spotlights under physically-based lighting models, given as the fraction of the cone over which intensity falls to zero. The default of 0.2 is a semi-soft cone which delivers the full intensity everywhere inside it, ...

Version 3.11.0 (July 27, 2026)

Engine

1. 4787c809 Added geom/surfacevel: the velocity of a geom's surface as seen by contacts, given as a velocity field with a constant component and a rotational component about the geom frame origin. This allows conveyor belts, treadmills and turntables to be modeled with static geoms and no degrees of freedom: friction drives touching bodies along the motion of the surface, with the field projected onto each contact's tangent plane. Surface velocities compose correctly with each other and with body motion. Note that the contact rows of mjData.efc_vel, and the constraint-state sensors that read them, report the velocity relative to the moving surface rather than to the geom, since that is the quantity the constraint acts on; for geoms without surfacevel the two are identical. Contact-point visualization draws an arrow along the surface velocity at contacts with moving surfaces.

   

2. a264d0bc Added geom/adhesion and pair/adhesion: an adhesive force associated with a contact, useful for modeling sticky materials. Contacts can pull with up to the given force before breaking, and the friction budget becomes $\mu(f_N + \text{adhesion})$. Combined with gap, adhesive contacts apply "adhesion at a distance", useful for modeling magnets. Resting penetration is unaffected by adhesion. mj_contactForce reports the net interface force, whose normal component can now be negative.

   

3. f0fa3d82 Replaced midpoint integration of free bodies with gyroscopic derivatives in the implicitfast integrator: the bias-force derivative of every standalone free body is applied via a local unsymmetric solve of its decoupled block, making implicitfast identical to implicit for such bodies. Unlike midpoint integration, which required vacuum and no constraints, this applies in all environments (contacts, fluid, constraints), and is compatible with discrete-time inverse dynamics. Spinning free bodies no longer gain energy, but tumbling motion is now mildly damped; models requiring long-horizon energy conservation of tumbling bodies in vacuum should use RK4. The invdiscrete flag no longer has any effect on forward dynamics.

4. 5618666a Added body/simple attribute ("false"/"auto") to disable the simple body mass matrix optimization. This is useful for domain randomization, where model parameters may change post-compilation.

5. 14c0b0c9 mj_setConst now recomputes the mjModel.{body,geom,site}_sameframe flags, to account for changes in body/geom/site frames after compilation.

6. 2444defc Added support for multiccd with arbitrarily large meshes.

7. a04b0c5b Added flg_gravcomp and flg_surfacevel boolean flags to mjModel. These flags replace the fast-path checks as originally guarded by ngravcomp. Since the engine uses these integers as flags (zero vs. non-zero), the new flags are honest boolean properties, writeable from the Python bindings at runtime. The field ngravcomp is deprecated and will be removed in a future release.

8. a1f38c8e Replaced quadratic scratch in DFS flood-fill island discovery with a linear-memory Union-Find (disjoint set). Contribution by @teerthsharma.

Actuation

20. d507e921 Refactored actuator infrastructure in preparation for MIMO (multi-input multi-output) actuator support. Each actuator now has ctrlnum (number of controls) and outnum (number of force outputs). The total counts nu = sum(ctrlnum) and nout = sum(outnum) dimension mjData.ctrl and mjData.actuator_force, respectively, nactuator is the number of actuators. For existing actuators ctrnum = outnum = 1, so nactuator == nu == nout and existing code is unaffected.

21. 56a93979 Setpoints of position and intvelocity servos acting on 3D rotational transmissions (ball joints, or site transmissions with a refsite and purely rotational gear) are now interpreted on the circle: the force uses the setpoint representative nearest the current angle, so targets winding beyond half a turn are tracked continuously instead of slipping by full turns. Behavior is identical whenever the error does not exceed π. Relatedly, intvelocity actuators now expose actlimited, which was previously hardcoded to "true": as for [general](https:...

See the changelog

See the changelog

See the changelog