MuJoCo 3.15.0 Release Notes

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

Warning

Breaking API changes

1. b3dd9e617 Actuator dampratio now computes the reflected inertia
   $m$ at mjModel.qpos0 as the operational-space inertia $(J M^{-1} J^T)^{-1}$ (averaged across force
   outputs for multi-output actuators such as orientation), rather than summing
   $\text{dof\_M0}_j / J_j^2$ across degrees of freedom. This accounts for tendon and actuator armature as well
   as off-diagonal inertial coupling, and avoids extreme damping values when a multi-DOF transmission has small
   Jacobian entries. In practice, actuators using dampratio on multi-link kinematic chains may exhibit lower
   damping than before, while actuators driving tendons or sites that span multiple joints will be damped much more
   accurately instead of being overdamped.

2. 5023a4a50 Removed the deprecated internal flex collision option and associated evpair
   structures.

1. 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.

2. 2742de438 The cached flex bending factor now retains cross-coordinate couplings between differently
   oriented vertex bodies.

3. 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.

4. 6224e95f6 Added single-shot multicontact for collisions with capsule geoms.

5. 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.

6. 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.