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Summary

This PR publishes MPI-enabled monoprop wheels that work with the MPI library already installed on the user's machine. They don't bundle MPI, and they don't require the MPI-5 standard ABI. The approach follows mpi4py's wheels (cibw/, _mpiabi.py). We build one extension per MPI ABI family, identified by the library's soname:

• mpich (libmpi.so.12): MPICH, Intel MPI, MVAPICH and Cray MPICH.
• openmpi (libmpi.so.40).
• mpiabi (libmpi_abi.so.1): built and tested locally, not shipped yet. It is waiting for an mpi4py release that uses it.

monoprop loads the variant matching the family mpi4py is using.

How it works at runtime. Every build ships a serial monoprop._core. MPI builds add monoprop._core_mpi.<abi>, in a separate nanobind domain.

• A propagator built with comm=None always runs serially. Importing monoprop never imports mpi4py or initializes MPI.
• Passing an mpi4py communicator loads the matching MPI module on first use. Before loading, monoprop checks the ABI family (from mpi4py's wheel file name, or from the library otherwise) and the MPI version.
• Each implementation has a minimum version: Open MPI 4.1, MPICH 4.0, Intel MPI 2021.0, MVAPICH 3.0, Cray MPICH 8.1. Configure-time and runtime checks both enforce it. An implementation not in that list only gets a warning.

C++ SDK. The SDK in the wheel gains libmonoprop_mpi_<abi>. find_package(monoprop CONFIG COMPONENTS MPI) selects it to match the consumer's MPI.

Design spec and implementation plan: docs/superpowers/specs/2026-10-23-mpi-wheels-design.md and docs/superpowers/plans/2026-10-23-mpi-wheels.md. They are not part of this PR and can be shared on request.

Stacked on #421. The base branch is feat/scikit-build-config-settings.

Breaking changes:

• monoprop.has_mpi is removed. Use bool(monoprop.MPI_ABIS).
• comm=None now always runs serially. MPI builds used to fall back to MPI_COMM_WORLD.
• monoprop::monoprop is serial in every build. MPI C++ consumers use COMPONENTS MPI and monoprop::monoprop_mpi.

Changes

• CMake
  • Core sources are compiled once per variant (cmake/custom/monopropCore.cmake).
  • cmake/custom/monopropMPIABI.cmake reads the ABI family and vendor version from the mpi.h macros (-E -dM) and enforces the minimum versions. It is installed for consumers too.
  • cmake/custom/monopropMPI.cmake finds MPI either with FindMPI (dev builds) or from monoprop.mpi-abis <abi>=<prefix> entries (wheel builds).
  • Under CIBUILDWHEEL=1, the wheel keeps no RPATHs from the build machine.
• Bindings: the comm bridge is split into comm_serial.cpp and comm_mpi.cpp, so bindings.cpp.in no longer needs #ifdefs. monoprop_add_python_core builds _core and _core_mpi.<abi>.
• Python
  • New monoprop._backend (core_module, detect_mpi4py_abi, check_mpi_version) and the public monoprop.MPI_ABIS.
  • New errors: MPIUnavailableError, MPIAbiMismatchError, MPIVersionError.
  • Dispatch now goes through core_module(comm).
• Wheels
  • tools/cibw/install-mpi.sh installs PyPI mpich==5.0.2 and openmpi==5.0.11 to build against.
  • repair-wheel.sh excludes MPI from auditwheel/delocate, then runs check_wheel.py. The audit checks MPI linkage per variant, RPATHs (including empty entries), that no MPI library is bundled, and that _mpi_abis.py lists exactly the shipped modules.
  • run-tests.sh / test-wheel-mpi.sh run the serial suite, then the MPI suite against each MPI from PyPI.
• CI
  • New test-wheels deploy job, which also gates publishing. It covers:
    • the version floors: Ubuntu 22.04's Open MPI 4.1.2 and MPICH 4.0, each with source-built and PyPI mpi4py, and impi-rt==2021.1.1;
    • the latest Intel MPI;
    • Homebrew Open MPI, including the C++ consumer.
  • Deploy PR triggers now also cover the wheel-only files.
  • The setup action gains mpi: mpich and build-deps inputs.
• Dev:
  • New recipes just test-wheel and just inspect-mpi.
  • just test-find-package adds an MPI leg and a check that an ABI mismatch fails at configure time.
  • The Nix dev shell puts Open MPI on the loader path.
• Docs: README, getting-started, building (COMPONENTS MPI migration, release-wheel reproduction), parallelism (backend selection, supported versions), and testing (testing release wheels).

Verification so far

• Locally (Nix, Open MPI 5.0.11):
  • serial and MPI Python and C++ suites;
  • the mpi marker suite at 1, 2 and 4 ranks;
  • the find_package consumer, serial and MPI on 2 ranks.
• Local docker cibuildwheel (cp312, manylinux x86_64):
  • both variants built and the audit found 0 issues;
  • the serial suite and both PyPI-MPI suites passed.
• Only this PR's CI can show: macOS (delocate and @rpath), linux-arm, abi3audit, and the test-wheels floor jobs.

Checklist

• Tests added or updated to cover the changes
• Documentation updated (docstrings, docs/, CONTRIBUTING.md) if needed
• CHANGELOG / release notes updated if applicable (breaking changes are in the commit footers, for the release notes)

AI/LLM disclosure

• I did not use LLM tooling, or used it only privately for ideation
• I used the following tool to help write this PR description: Pi (claude-opus-5-5)
• I used the following tool to generate or modify code: Pi (claude-opus-5-5 orchestrating claude-sonnet-5-5 / claude-haiku-5-5 subagents). Every commit carries an Assisted-by trailer.

Important

By opening this PR I confirm that I have read CONTRIBUTING.md and I agree to the terms of the Contributor License Agreement.

Warning

If you're contributing on behalf of your employer, contact cla@algorithmiq.fi to arrange a Corporate CLA.

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Docs preview: https://pr-426.monoprop-docs.pages.dev

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.45%. Comparing base (3cbd80b) to head (57c36e4).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/monoprop/_backend.py	90.47%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           feat/scikit-build-config-settings     #426      +/-   ##
=====================================================================
- Coverage                              97.99%   97.45%   -0.54%     
=====================================================================
  Files                                     14       15       +1     
  Lines                                    797      865      +68     
  Branches                                 111      120       +9     
=====================================================================
+ Hits                                     781      843      +62     
- Misses                                    11       16       +5     
- Partials                                   5        6       +1     

Flag	Coverage Δ
cpp	97.45% <91.30%> (-0.54%)	⬇️

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
89.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud