MuJoCo 3.13.0 Release Notes
3.13.0
MuJoCo 3.13.0 introduces new integrator discrete, improved mesh support, and Python 3.15 compatibility. Key changes include enhanced stability and API updates.
Evidence and limits
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Source excerpts and review record
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Original source quotation: “Version 3.13.0 (September 8, 2026) General 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.”
Source E1
Original source quotation: “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.”
Source E2
Original source quotation: “b59b07fae Added support for Python 3.15 (GIL and Free-Threading).”
Source E3
Original source quotation: “Engine 5660353ec Added a new integrator discrete : the constraint solve and the implicit velocity update merge into one operation, performed in the effective metric $\widehat{M} = M + hD + h^2K$”
Source E4
Original source quotation: “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.”
Source E5
Original source quotation: “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 .”
Source E6
Original source quotation: “The actuator-gain treatment resolves the stiff-servo timestep limitation of #3443 (analysis contributed by @qiayuanl ).”
Source E7
Original source quotation: “See the integrator documentation for semantics and current limitations.”
Source E8
Original source quotation: “Warning Breaking API changes Removed the implicit flex effective-metric special case under implicit / implicitfast with the CG solver, introduced in 3.11.0.”
Source E9
Original source quotation: “This behavior now requires integrator="discrete" (with a primal solver: CG or Newton ), which additionally treats joint damping and stiffness implicitly inside the solve.”
Source E10
Original source quotation: “Models relying on the old behavior should set integrator to discrete ; models with flex elasticity or passive flex contact under implicit / implicitfast now raise a runtime error carrying this migration note.”
Source E11
Original source quotation: “0b4e17747 Restored clamping of non-positive pivots in the sparse inertia factorization, along with the associated mjWARN_INERTIA warning.”
Source E12
Original source quotation: “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.”
Source E13
Original source quotation: “c9d997610 Added single-shot multicontact for collisions with cylinder geoms.”
Source E14
Original source quotation: “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.”
Source E15
Original source quotation: “Scenes with many simultaneously sliding contacts speed up by 1.4-2x on average and 3-4x on the slowest steps.”
Source E16
Original source quotation: “Contribution by @kevinzakka .”
Source E17
Original source quotation: “Compiler 2a3957558 Custom text fields ( custom/text ) in MJCF now accept their values inside a <![CDATA[ ... ]]> block in addition to the data attribute.”
Source E18
Original source quotation: “When saving a model via mj_saveXML , custom text containing newlines or XML characters is exported as CDATA.”
Source E19
Original source quotation: “47ebfe48a Added cutoff attribute to sensor/tactile .”
Source E20
Original source quotation: “Samples 834983fbb Added render.cc : a headless offscreen rendering sample that saves an image to a PNG image.”
Source E21
Original source quotation: “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.”
Source E22
Original source quotation: “Bug fixes 67562d95f Rewrote the Plane-Mesh collider to fix bugs and improve performance.”
Source E23
Original source quotation: “Fixes #3524 .”
Source E24
Original source quotation: “Models f6c1004b7 Added house of cards example model and three free-standing arch models: roman, gothic, and hyperbolic.”
Source E25
Original source quotation: “Inspired by Song et al .”
Source E26
Original source quotation: “Rendering Warning Breaking API changes cb57b526f For materials with physically-based rendering parameters that do not provide a metallic texture the fallback is now white.”
Source E27
Original source quotation: “Previously the fallback was black, which caused scalar metallic values to be discarded when multiplied in the PBR shader.”
Source E28
Source：MuJoCo — Releases · github.com