> ## Documentation Index > Fetch the complete documentation index at: https://mintlify.com/pybamm-team/PyBaMM/llms.txt > Use this file to discover all available pages before exploring further. # Changelog > Release history for PyBaMM, including new features, bug fixes, and breaking changes. PyBaMM uses **CalVer** versioning in the format `YY.MM.PATCH` (e.g., `25.12.0`). Major releases occur roughly every two months. **Deprecation policy**: breaking changes follow a deprecation window of at least one year or three releases (whichever is longer). Deprecated APIs emit a warning before removal so you have time to migrate. The entries below cover the most recent releases. View the [full changelog on GitHub](https://github.com/pybamm-team/PyBaMM/blob/main/CHANGELOG.md) for the complete history. ## Unreleased * Improved the performance of processed variables by replacing `casadi.vertcat` input stacking with numpy vectors. * Preserve custom variables and events in built-in model `to_config`. * Allow out-of-bounds initial state of charge to enable initialising a simulation at a voltage outside the voltage limits. * Added `cache_esoh` option to `Simulation` that caches the electrode SOH computation across repeated `solve` calls. * Eliminated the mass matrix inverse and temporary dense matrix objects when building the simulation. * Added the `hermite_reduction_factor` option to `IDAKLUSolver`, which dynamically compresses solution size by removing redundant points in the Hermite interpolant. * Added support for Python 3.14. * Added regularisation to the kinetics and OCPs for improved numerical stability. * DiffSL export for discretised PyBaMM models. * Fixed a bug in the exchange current density calculation for MSMR models. * Fixed a bug where converting `ExpressionFunctionParameter` to source code did not preserve `Interpolant` constructor calls with data arrays. * Fixed a bug where bounds of the particle size distribution were always taken from the primary phase when using 2 phases with particle size distribution. * The mass matrix inverse is no longer computed during discretisation; solvers now use sparse linear solves instead. * Dropped JAX support on macOS with Intel (x86\_64) processors. macOS users require Apple Silicon (M-series) for JAX features. * Added a small regularisation term to the exchange current density, slightly modifying kinetics as stoichiometry approaches 0 or 1. * The default OCP barrier as stoichiometry approaches 0 or 1 is now smooth instead of asymptotically approaching infinity. * Migrated `docs` and `dev` dependencies from `project.optional-dependencies` to `dependency-groups` per PEP 735. ## v25.12 — January 2026 **Features** * Added experimental `CompositeSolver` that tries multiple solvers until one succeeds, and updated the ESOH solvers to use a composite algebraic solver. **Features** * Improved the robustness of the `ElectrodeSOHComposite` model and made the calculation of equilibrium stoichiometries consistent for models with hysteresis. **Bug fixes** * Fixed a bug where `ExpressionFunctionParameter` children were not properly substituted after JSON deserialisation, causing `ElectrodeSOHSolver` to fail with "Variable not implemented" errors. **Features** * Porosity and active material fractions are now `FunctionParameters` of y and z, as well as x. * Added support for asymmetric loss of active material. * Added `Solution.yp` property to return the time derivatives of the solution. * Added `TensorField` for rank-1 and rank-2 tensors, `TensorProduct` for outer products, and tensor divergence support in 2D finite volume. * Added support for Python 3.13. * Reduced the time to build `Simulation`s by creating a post-processing step for variables. * Added the ability to observe custom variables from a `Solution` object using `Solution.observe(symbol)`. * Added inverse kinetics for linear kinetics. **Bug fixes** * Fixed a bug where `fixed_input_parameters` ignored `InputParameter` values within expressions in `ParameterValues`. * Fixed a bug with domain shape evaluation. * Fixed a bug where `IDAKLUSolver` errors were not raised correctly. **Breaking changes** * Removed default constants added to `ParameterValues` on construction. Only breaking if you rely on this in custom models or parameters. ## v25.10 — October–November 2025 **Bug fixes** * Fixed a bug with serialising `InputParameter`s. * Fixed a bug with missing inputs for `initial_conditions_from` scale evaluation. **Features** * Allow setting initial conditions from `y_slices` of a `Solution` object. **Bug fixes** * Fixed a bug where simulations whose initial conditions violated the events could continue solving. * Fixed an issue with composite electrode and "swelling only" mechanics sub-models not creating the cell thickness variable. **Features** * Added uniform grid sizing across subdomains in the x-dimension. * Added `electrode_phases` kwarg to `plot_voltage_components()` to choose between primary or secondary phase overpotentials. * Added `num_steps_no_progress` and `t_no_progress` options to `IDAKLUSolver` for early termination when little progress is detected. * Added helper functions to import external 3D meshes. * Added support for algebraic and differential surface form in composite models. * Generalised `set_initial_soc` to `set_initial_state` and added Thevenin initial state setting. **Bug fixes** * Fixed `KeyError: 'min'` in `latexify()` for lithium plating models when certain geometry keys are missing. * Fixed a bug where no error was raised when a list of input sets was provided to the solver while using `Experiment`. * Fixed a bug where simulations using `output_variables` in `IDAKLUSolver` couldn't be pickled. * Fixed a bug where time-based Heaviside or modulo discontinuities could trigger out-of-bounds errors in time arrays. **Breaking changes** * Updated hysteresis decay rate parameters to a "true" hysteresis decay rate, changing the unit interpretation. * Changed fundamental variable for all SEI models from thickness to concentration. ## v25.8 — August 2025 **Features** * Added `plot_3d_cross_section` and `plot_3d_heatmap` functions for 3D thermal simulation plots. * Added `Basic3DThermalSPM` model with two-way coupling. * Added `on_failure` option to `BaseSolver` with choices `"warn"`, `"ignore"`, and `"raise"`. * Introduced entry points for models (similar to parameter sets), enabling experimental support for loading third-party models via `pybamm.Model("model_name")`. * Added 3D FEM and meshes supporting rectangular and cylindrical geometries. * Added a `Constant` symbol for named constants (subclass of `Scalar`) to prevent constants from being simplified away. * Added an option for multiple initial conditions in the IDAKLU solver. **Bug fixes** * Fixed domain handling and temperature initialisation in the lumped thermal model. * Fixed non-deterministic `ShapeError` in the 3D FEM gradient method. * Fixed a bug that ignored the default duration of drive cycles for `CRate` steps. * Fixed inconsistency in the returned shape of `pybamm.DiscreteTimeSum` depending on whether `output_variables` was set. **Breaking changes** * Changed behaviour of drive cycle steps in `pybamm.Experiment` to treat each time point as a discontinuity. * Changed `A_cc = L_z * L_y * number of layers`, altering the interpretation of electrode capacity variables. * Removed IREE code from the IDAKLU solver. * Removed support for Python 3.9. * OCP hysteresis models now require explicit equilibrium, delithiation, and lithiation OCPs. ## v25.6 — May 2025 **Features** * Renamed MSMR parameters from symbols to written-out names with units (non-breaking, with deprecation warning). **Optimisations** * Updated docs, examples, and tests to use `IDAKLUSolver` as the recommended solver. * Added `on_extrapolation` solver option (`"error"`, `"warn"`, or `"ignore"`). * Improved reliability of `CasadiAlgebraicSolver` with a `step_tol` option for the Newton iteration. * Sped up calculation of variable sensitivities in `ProcessedVariable`. **Bug fixes** * Fixed `QuickPlot` returning empty values for 1D variables at the beginning and end of a timespan. * Fixed `Exponential1DSubMesh` not being created correctly for non-zero minimum values. **Breaking changes** * Removed sensitivity functionality from CasADi and SciPy solvers; only `pybamm.IDAKLUSolver` can calculate sensitivities. ## v25.4 — April 2025 **Bug fixes** * Improved reliability of `AlgebraicSolver` and changed `ElectrodeSOHHalfCell` solver to a Trust-Region method. **Features** * Added "use lumped thermal capacity" option in the lumped thermal model. **Bug fixes** * Removed an incorrect regularisation term in the harmonic mean. **Breaking changes** * Changed default solver to `pybamm.IDAKLUSolver`. **Features** * Added `axen_ocp` module for OCP with hysteresis within `interface.open_circuit_potential`. * Added `get_summary_variables` to return a dictionary of computed summary variables. * Added support for particle size distributions combined with particle mechanics. * Added `InputParameter` support in PyBaMM experiments. * Added support for the `"pchip"` interpolator using the CasADI backend. **Breaking changes** * Added `skip_ok` option to `step` to allow infeasible steps to be skipped at initial conditions. * Deprecated `CrateTermination` and renamed it to `CRateTermination`. **Bug fixes** * Fixed a bug with observing outputs of 2D FEM simulations. * Fixed a bug in simulating FEM models with `IDAKLUSolver`. *** For older releases (v25.1, v24.x, and earlier), see the [full CHANGELOG on GitHub](https://github.com/pybamm-team/PyBaMM/blob/main/CHANGELOG.md).