PyBaMM 26.7.0.0

Breaking changes

• The minimum supported NumPy version is now 2.0.0 for both pybamm and pybammsolvers (previously unpinned). NumPy 1.x is no longer supported. (#5647)
• The "voltage as a state" model option now defaults to "true": voltage is solved as an algebraic state, making standard models DAEs. Reading Voltage [V] from a solution is now an O(1) state lookup instead of a post-solve expression evaluation (up to 73% faster end-to-end for dense output; 8-24% faster experiment cycling across SPM/SPMe/DFN; up to 48% faster with in-solver output_variables=["Voltage [V]"]). The default IDAKLUSolver, CasadiSolver (all modes), and JaxSolver(method="BDF") handle DAEs; ODE-only solvers (ScipySolver, JaxSolver(method="RK45")) require {"voltage as a state": "false", "surface form": "false"} for SPM/SPMe, which remains a supported configuration. Known trade-off: continuous solves of rapidly alternating current profiles (e.g. interpolant drive cycles with more than ~25 current reversals in one solve) can be slower, up to ~2x for SPM in the worst measured case; the legacy configuration above restores previous performance for these workloads. Experiment-driven cycling is unaffected. (#5573)
• CasadiSolver, ScipySolver, and CasadiAlgebraicSolver are deprecated; use IDAKLUSolver (or NonlinearSolver for algebraic systems) instead. It is now the default for every model. (#5624)

Features

• pybammsolvers now ships Linux aarch64 (arm64) wheels, built on manylinux_2_34 native ARM runners. The libstdc++ std::string ABI (_GLIBCXX_USE_CXX11_ABI) is now detected from the linked CasADi library instead of hard-coded, resolving the unresolved-symbol failure against CasADi's C++11-ABI aarch64 wheel. (#5653)
• Added skip_surface_form_check option to EISSimulation to bypass the surface form validation. (#5632)
• Added ability to export pybamm.Simulation with experiments to DiffSL format (#5557)
• PyBaMM and pybammsolvers now develop in a single repository — a UV workspace under packages/ — while continuing to release independently to PyPI. Release tags are namespaced (pybamm-v* and pybammsolvers-v*), and PyBaMM's CI now tests against the in-repo solver on every platform. The published pybamm package and its dependency on pybammsolvers are unchanged for users. See RELEASE.md for the release model. (#5512)
• Legacy BPX v0.x files/objects now load again: bpx itself detects and converts them to the v1.x schema on a best-effort basis (with a UserWarning), so ParameterValues.create_from_bpx/create_from_bpx_obj no longer raise a ValidationError. PyBaMM officially supports bpx>=1. (#5574)
• create_from_bpx/create_from_bpx_obj now also accept BPX files that omit State fields (or the whole State section): the ambient/initial temperatures default to the reference temperature and the initial electrolyte concentration to 1000 mol.m-3 (logged), while opt-in fields (initial hysteresis state, heat transfer coefficient) are left for the model to default. (#5574)

Bug fixes

• Fixed ParameterValues.to_json (and Serialise of parameter values) raising TypeError: ... got multiple values for argument on create_from_bpx outputs, whose functional parameters are functools.partial objects with keyword-bound arguments (e.g. constant diffusivity, tabular OCPs, exchange-current density). The serialiser now only creates symbols for a partial's remaining required arguments, leaving bound and defaulted arguments in place. (#5641)
• Fixed the Dependabot uv update job failing to resolve the workspace (ModuleNotFoundError: No module named 'scikit_build_core'): pybammsolvers now declares a static version in pyproject.toml, so build-tool-less resolvers read its metadata without invoking the scikit-build-core backend. version.py remains the single source of truth, mirrored into pyproject.toml and vcpkg.json by a new sync-pybammsolvers-version pre-commit hook. (#5655)
• Fixed the pybammsolvers Windows wheels failing to build: vcpkg's transitive reference-LAPACK/Fortran toolchain chain broke on a pruned upstream MSYS2 download (a 404). The Windows build now avoids that chain, using a suitesparse-klu-only sundials port and vcpkg 2026.06.24 on a windows-2025 runner. (#5650)
• Fixed the deprecated "<X> electrode diffusivity [m2.s-1]" alias silently overriding the current "<X> particle diffusivity [m2.s-1]": the current name now takes precedence in ParameterValues (instead of being overwritten by the deprecated alias), and a warning is raised when both are set, so setting or fitting particle diffusivity is no longer a silent no-op. The deprecated key is retained for backward compatibility, and create_from_bpx now emits only the current particle diffusivity name. (#5642)
• Fixed the pybammsolvers source distribution bundling the SUNDIALS/SuiteSparse submodule trees (~291 MB, over PyPI's per-file limit) when built from a checkout with the submodules initialised; the sdist now excludes them. (#5623)
• Fixed the pybammsolvers editable auto-rebuild failing to configure when the SUNDIALS/SuiteSparse submodules are absent even though the libraries were already built in .idaklu; the from-source bootstrap (and its submodule requirement) is now skipped once the libraries exist. (#5623)
• Fixed the Read the Docs documentation build, which broke once pybammsolvers became a workspace member with no published wheel during the release: RTD now fetches the SUNDIALS/SuiteSparse submodules and installs gfortran/libopenblas so the solver builds from source. Link checking was also restructured so transient external-link failures no longer block builds: Sphinx linkcheck is advisory, and the CI lychee check runs offline on PRs with a weekly external sweep that files a tracking issue. (#5659)