v25.12.2

What's Changed

• Adds experimental CompositeSolver that retries solvers until one succeeds and updated the ESOH solvers to use a composite algebraic solver by @MarcBerliner in (#5357)

v25.12.1

What's Changed

• ParameterSubstitutor bugfixes by @BradyPlanden in #5352
• Improve composite eSOH by @rtimms in #5347

Full Changelog: v25.12.0...v25.12.1

v25.12.0

What's Changed

• fix 2d concat bug by @aabills in #5310
• Restructure idaklu jax tests by @pipliggins in #5315
• Fix non-deterministic CI by @MarcBerliner in #5316
• #5299 add inverse linear kinetics by @rtimms in #5303
• Observe custom variables after solving + faster simulation build time by @MarcBerliner in #5308
• fix idaklu jax test issue by @aabills in #5319
• [Fix] Handle missing geometry keys in latexify for lithium plating mo… by @swastim01 in #5245
• fix np trapz bug by @valentinsulzer in #5324
• use zlib to make serialization smaller by @aabills in #5323
• Build(deps): bump the actions group across 1 directory with 12 updates by @dependabot[bot] in #5322
• Fix benchmarks by @pipliggins in #5318
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5195
• Fix a race in serialisation test_save_load_model test by @agriyakhetarpal in #5325
• Adds Python 3.13 by @BradyPlanden in #5330
• Coupled variable serialization by @valentinsulzer in #5326
• copyright should only include the year of creation by @Saransh-cpp in #5332
• Fix coupled variable by @aabills in #5334
• Fix bug with fixed_input_parameters missing input params in symbols by @MarcBerliner in #5321
• Removes automatic addition of constants to ParameterValue object by @BradyPlanden in #5336
• add tensors and basic operators by @aabills in #5335
• Add Solution.yp getter by @MarcBerliner in #5341
• Restructures ParameterValues class by @BradyPlanden in #5339
• SciPy 1.17.0 FutureWarning for explicit types by @BradyPlanden in #5344
• asymmetric stress-driven LAM by @rtimms in #5343
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5345
• broadcast to edges 2d by @aabills in #5340
• Build(deps): bump astral-sh/setup-uv from 7.1.6 to 7.2.0 in the actions group by @dependabot[bot] in #5346
• BasicDFN2D with variable porosity and active fractions in x and z directions by @DrSOKane in #5266
• Volume fractions and porosity that vary in y and z by @DrSOKane in #5214
• Cherry pick composite esoh fixes by @rtimms in #5349

Full Changelog: https://github.com/pybamm-team/PyBaMM/blob/main/CHANGELOG.md#v25120---2026-01-15

Breaking changes

• Deprecates check_already_exists argument for ParameterValues.update(). This removes the secondary checking for explicit keys in the ParameterValues object. If a key is not found, it will be added. Note, this includes typos in parameter names. This changes update to have update-insert functionality whereas previously it required the check_already_exists flag to be False. This also allows for direct dictionary update-insert through parameter_values["key"] = "value". (#5339)
• Removes default constants added to ParameterValues on construction. Only breaking if you rely on this functionality in custom models, parameters, etc. (#5336)

v25.10.2

What's Changed

Bugfix serialising InputParameter's. (#5289)
Bugfix BaseModels's initial_conditions_from scale evaluation (#5285)
Bugfix with bulk ocp lithiation (#5280)

Full Changelog: v25.10.1...v25.10.2

v25.10.1

What's Changed

• Sync develop with main by @BradyPlanden in #5252
• fix wsl links by @valentinsulzer in #5251
• chore: modernise uv usage in CI + fix cache by @Saransh-cpp in #5249
• Add docstrings to FuzzyDict methods by @swastim01 in #5242
• fix: Fixing non-deterministic CI issues by @Rishab87 in #5255
• Build(deps): bump the actions group with 2 updates by @dependabot[bot] in #5259
• Fix bug with initial conditions that violate events by @MarcBerliner in #5260
• Fix/discussion autoresponder 5186 by @swastim01 in #5261
• update to pytest 9.0, remove pytest-subtests by @valentinsulzer in #5271
• Set initial conditions from y slice by @rtimms in #5257
• Plot voltage components bugfix by @Daniel-Nicolae23 in #5270
• Composite electrode bug-fix for swelling submodel by @BradyPlanden in #5272

Full Changelog: https://github.com/pybamm-team/PyBaMM/blob/develop/CHANGELOG.md#v25100---2025-10-29

v25.10.0

What's Changed

• Build(deps): bump the actions group with 2 updates by @dependabot[bot] in #5148
• Sync v25.8.0 with develop by @brosaplanella in #5149
• Fixed non-deterministic plotting CI issues by @Rishab87 in #5150
• 2D by @aabills in #5106
• Generalise initial state setting by @NicolaCourtier in #5129
• Attempt to Patch few non-deterministic CI issues. by @Rishab87 in #5156
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5147
• Build(deps): bump the actions group with 6 updates by @dependabot[bot] in #5157
• composite esoh by @aabills in #5160
• Total surface area to volume by @rtimms in #5165
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5166
• Build(deps): bump github/codeql-action from 3.29.8 to 3.29.10 in the actions group by @dependabot[bot] in #5167
• Added load_mesh_from_file function to support external 3D meshes. by @Rishab87 in #5162
• [GSoC 2025] Saving and Loading Custom Models by @medha-14 in #5056
• Fix drive cycle duration bug by @MarcBerliner in #5153
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5172
• Build(deps): bump the actions group with 3 updates by @dependabot[bot] in #5173
• fix 2d inner product by @aabills in #5175
• constant extrapolation in 2d by @aabills in #5178
• fix calculation of bulk ocp in hysteresis models by @rtimms in #5169
• Composite esoh half cell by @aabills in #5179
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5182
• Added an option for multiple initial conditions in IDAKLU solver by @Rishab87 in #4981
• chore: update pre-commit hooks by @pre-commit-ci[bot] in #5187
• Use T_init in initial_state by @rtimms in #5189
• Hysteresis ESOH by @aabills in #5163
• Build(deps): bump the actions group across 1 directory with 5 updates by @dependabot[bot] in #5188
• Update pybammsolvers to v0.2.0 by @MarcBerliner in #5199
• 1D+ sensitivities error with IDAKLU output_variables by @pipliggins in #5118
• Make time-based heaviside and modulo inputs explicit t_eval points with IDAKLUSolver by @MarcBerliner in #4994
• Add options to terminate IDAKLUSolver early when there's little progress by @MarcBerliner in #5201
• Computing Bruggeman coefficient from BPX porosity instead of hardcoding by @r0hansaxena in #5196
• Unpin numpy, bump pybammsolvers by @MarcBerliner in #5204
• Fix bug with out-of-bounds discontinuities in time by @MarcBerliner in #5205
• Fix doc links (make lychee happy) by @DavidMStraub in #5208
• Removes the default solver change warning by @BradyPlanden in #5209
• Added warning about installation through conda to docs and suggested installation via pip by @r0hansaxena in #5206
• [CI] Change MacOS runner from macos-13 to macos-15-intel by @DavidMStraub in #5213
• Remove deprecated JAX funtions by @DavidMStraub in #5203
• Build(deps): bump the actions group across 1 directory with 3 updates by @dependabot[bot] in #5216
• feat(EvaluateAt): add support for children evaluated at edges by @martinjrobins in #5190
• Build(deps): bump the actions group with 3 updates by @dependabot[bot] in #5220
• Allow sim to be pickled with output variables by @pipliggins in #5225
• Plot voltage components for composite electrodes by @Daniel-Nicolae23 in #5229
• Change fundamental SEI variable from thickness to concentration: attempt 2 by @DrSOKane in #4869
• #5198 update definition of hysteresis decay rate by @rtimms in #5217
• Serialise refactor by @aabills in #5235
• IDAKLU options property, fixes pickling issue with getstate by @BradyPlanden in #5234
• Serialise geometry varpts etc by @aabills in #5236
• [GSoC 2025] Serialising Parameter Sets by @medha-14 in #5177
• Update broken link by @valentinsulzer in #5246
• Build(deps): bump the actions group across 1 directory with 4 updates by @dependabot[bot] in #5243
• Set zip(..., strict=True) in key solver/expression tree files for len… by @swastim01 in #5241
• Fix: correct temperature initialisation in lumped and x_full thermal … by @swastim01 in #5248
• Enable contact overpotential display by @Jhah2025 in #5247
• Serialization refactor by @aabills in #5244
• Release v25.10.0 by @BradyPlanden in #5250

New Contributors

• @r0hansaxena made their first contribution in #5196
• @Daniel-Nicolae23 made their first contribution in #5229
• @swastim01 made their first contribution in #5241
• @Jhah2025 made their first contribution in #5247

Full Changelog: v25.8.0...v25.10.0

v25.8.0

Features

• Added plot_3d_cross_section & plot_3d_heatmap functions to support plotting for 3D thermal simulations. (#5130)
• Added a Basic3DThermalSPM with two way coupling. (#5112)
• Enables the passing of inputs throughout set_initial_soc. (#5122)
• Adds on_failure option to BaseSolver with options for "warn", "ignore", and "raise" to change behaviour on solver failure. Defaults to "raise" to retain historic functionality. (#5105)
• Creates a boundary mesh size object that returns the distance from the center of the leftmost/rightmost control volume to the boundary of the domain (#5108)
• Introduced entry points for models, similar to parameter sets, and moved entry point handling to pybamm.dispatch.entry_points. There is now experimental support for loading third-party models outside of the PyBaMM framework via pybamm.Model("model_name"). This API is currently unstable until further notice and may be subject to change without warning. (#4490)
• Allow for overriding the spatial method's extrapolation and for using constant extrapolation of boundary values (#5107)
• Creates BaseProcessedVariable to enable object combination when adding solutions together (#5076)
• Added a Constant symbol for named constants. This is a subclass of Scalar and is used to represent named constants such as the gas constant. This avoids constants being simplified out when constructing expressions. (#5070)
• Generalise pybamm.DiscreteTimeSum to allow it to be embedded in other expressions (#5044)
• Adds all key-value pair to output_variables sensitivity dictionaries, accessible through solution[var].sensitivities['all']. Aligns shape with conventional solution sensitivities object. (#5067)
• Added a new BaseHysteresisOpenCircuitPotential class that sets variables for the lithiation and delithiation OCP and the hysteresis voltage (H = U_lith - U_delith). Allow the initial hysteresis state to be a function of position through the electrode. Allow the hysteresis decay rates of the Axen and Wycisk models to be a function of stoichiometry and temperature. Added a heat source term in each active material phase Q_hys = i_vol * (U - U_eq) where i_vol is the volumetric interfacial current density, U is the OCP (i.e. includes hysteresis), and U_eq is the "equilibrium OCP". Renamed the open-circuit potential models to be more descriptive. The options "Axen" and "Wycisk" are now "one-state hysteresis" and "one-state differential capacity hysteresis". The old option names still work but will raise a warning. (#4893)
• Add support for output_variables to pybamm.DiscreteTimeSum and pybamm.ExplicitTimeIntegral expressions. (#5071)
• Added 3D FEM and meshes supporting rectangular and cylindrical geometries (#5009)

Bug fixes

• Fix non-deterministic ShapeError in 3D FEM gradient method (#5143)
• Fixes negative electrode boundary values for half-cell voltage contributions. (#5139)
• Makes A_cc L_z * L_y * number of layers (#5138)
• Fixes TimeIntegral expression node summation when dependent on an input parameter. (#5119)
• Fixed a bug that ignored the default duration of drive cycles for CRate steps and a bug that overwrote custom period arguments for drive cycles. (#5090)
• Converts sensitivities to numpy objects, fixing bug in DiscreteTimeSum sensitivity calculation (#5037)
• Raises error if pybamm.Interpolant given 1D x values that are not strictly increasing (#5061)
• Fixes inconsistency of the returned shape of a pybamm.DiscreteTimeSum variable depending on output_variables being set or not. (#5098)
• Fixed a bug where simplifications cause heavisides to evaluate as booleans (#4893)
• Fixed a bug in the WyciskOpenCircuitPotential model where the differential capacity was not being evaluated correctly. (#4893)

Breaking changes

• Changed behavior of drive cycle steps in pybamm.Experiments to treat each time point as a discontinuity, consistent with how input interpolants work. This ensures more accurate simulation of drive cycles with rapid changes. (#5141)
• Removed the IREE code from the IDAKLU solver (#5080)
• Removed support for Python 3.9 (#5052)
• In OCP hysteresis models, users need to explicitly give the equilibrium, delithiation, and lithiation OCPs when using a hysteresis model. E.g., you must provide all three of "Negative electrode OCP [V]", "Negative electrode delithiation OCP [V]", and "Negative electrode lithiation OCP [V]". (#4893)

v25.6.0

Features

• Renamed MSMR parameters from symbols to written out names with units as a non-breaking change with a deprecation warning. (#5027)

Optimizations

• Update docs, examples and tests to use IDAKLUSolver. (#4996)
• Add a solver option to change on_extrapolation behavior to "error", "warn", or "ignore" on extrapolation events. (#4993)
• Improve reliability of CasadiAlgebraicSolver and added an option for the step_tol of the Newton iteration. (#4985)
• Speed up calculation of variable sensitivities in ProcessedVariable (#5000)

Bug fixes

• Fixed a bug in the QuickPlot which would return empty values for 1D variables at the beginning and end of a timespan. (#4991)
• Fixed a bug in the Exponential1DSubMesh where the mesh was not being created correctly for non-zero minimum values. (#4989)
• Fixed sensitivity calculation for pybamm.DiscreteTimeSum. (#5007)

Breaking changes

• Remove sensitivity functionality for Casadi and Scipy solvers, only pybamm.IDAKLU solver can calculate sensitivities. (#4975)

v25.4.2

Bug fixes

• Improve reliability of AlgebraicSolver and change ElectrodeSOHHalfCell solver to a Trust-Region method. (#4982)

v25.4.1

Bug fixes

• Remove a regularization term in the harmonic mean. (#4977)

Breaking changes

• Changed default solver to pybamm.IDAKLUSolver. (#4915)