1. Maintainer effort is expensive, and we collectively have limited bandwidth -
   hence platform support is strongly influenced by the willingness of one or
   more maintainers to put in that effort.

   - Maintainers are trusted by the whole team. We generally do not question
     *why* a maintainer is motivated to put in the effort. If they are being
     paid for their effort or doing it as part of their job, that is fine -
     however they should disclose this to the Steering Council, and indicate
     whether long-term support is conditional on their employment or contractor
     status for the support tiers that include releasing wheels to PyPI.

     *Rationale: releasing wheels to PyPI is a long-term commitment by the
     project as a whole, see the backwards compatibility section below.*

2. CI support for the platform is required, preferably with native runners.
   Free is best, however decisions on paid CI are up to the Steering Council.
   Emulation for running the test suite (e.g., under QEMU) or self-hosted
   buildbots are slower and less reliable, hence not preferred.

3. There should be broad enough demand for support for the platform for the
   tiers that include releasing wheels to PyPI.

   - A previously used rule of thumb: >=0.5% of the user base should be on this
     platform. There may be reasons to deviate from this rule of thumb.

     *Note: finding clean data sources isn't always easy. If wheels are already
     being shipped, for NumPy or for a comparable project, then download data
     from PyPI may be obtained through BigQuery. For new platforms, sources
     like the*
     `Steam Hardware & Software Survey <https://store.steampowered.com/hwsurvey/?platform=combined>`__
     *may have to be used.*

4. Adding a regular CI job (i.e., not aimed at uploading wheels to PyPI) for a
   platform to the NumPy CI matrix is much cheaper, and easily reverted in case
   of problems. The bar for adding such jobs is low, and assessed on a
   case-by-case basis.

5. For all platforms in any supported tier: the relevant prerequisites in our
   dependencies must be met. E.g., build tools have support, and for wheels
   there is support in CPython, PyPI, cibuildwheel, manylinux, and
   ``scipy-openblas64`` or another easily-integrated BLAS library.

6. Decision making:

   - Moving a platform to a lower support tier must be discussed on the mailing list.
     The circumstances for each platform are unique so the community will
     evaluate each proposal to demote a platform on a case-by-case basis.
   - Moving a platform to a higher support tier, if that higher tier includes
     releasing wheels on PyPI for that platform, must be discussed on the
     mailing list.
   - Adding an entry to a support tier in this NEP for (a) an unsupported
     platform or (b) a tier which does not include uploading wheels to PyPI can
     be done on GitHub through a regular pull request (assuming it's clear from
     the discussion that the relevant maintainers agree it doesn't need to hit
     the mailing list).


Releasing wheels to PyPI
''''''''''''''''''''''''

The wheels that the NumPy team releases on PyPI for the ``numpy`` package get
hundreds of millions of downloads a month. We therefore highly value both
reliability and supply chain security of those release artifacts. Compromising
on those aspects is unlikely to be acceptable for the NumPy team.

The details of how wheels are produced, tested and distributed can be found in
the `numpy/numpy-release <https://github.com/numpy/numpy-release>`__
repository. Some key requirements of the current setup, which aren't likely to
change soon, are:

1. Must be buildable on publicly-visible CI infrastructure (i.e., GitHub).
2. Must be tested well enough (meaning native runners are preferred; QEMU is quite slow).
3. Must be publishable to PyPI automatically, through PyPI's trusted publishing
   mechanism.


Tier 1
~~~~~~

- Must have regular CI support on GitHub or (exceptionally) through another
  well-integrated CI platform that the release team and Steering Council deem
  acceptable.
- The NumPy team releases wheels on PyPI for this platform.
- CI failures (either regular CI or wheel build CI) block releases.
- All maintainers are responsible to keep the ``main`` branch and wheel builds
  working.

Tier 1 platforms:

+---------------------------+--------------------------------------------------------------------------+
| Platform                  | Notes                                                                    |
+===========================+==========================================================================+
| Windows x86-64            |                                                                          |
+---------------------------+--------------------------------------------------------------------------+
| Windows arm64             |                                                                          |
+---------------------------+--------------------------------------------------------------------------+
| Windows x86               | 32-bit Python: note this is shipped without BLAS, it's legacy            |
+---------------------------+--------------------------------------------------------------------------+
| Linux x86-64 (manylinux)  |                                                                          |
+---------------------------+--------------------------------------------------------------------------+
| Linux aarch64 (manylinux) |                                                                          |
+---------------------------+--------------------------------------------------------------------------+
| macOS arm64               |                                                                          |
+---------------------------+--------------------------------------------------------------------------+
| macOS x86-64              | Expected to move to unsupported by 2027/28 once the platform is dropped  |
|                           | by GitHub                                                                |
+---------------------------+--------------------------------------------------------------------------+


Tier 2
~~~~~~

- Must have regular CI support, either as defined for Tier 1 or through a
  reliable self-hosted service.
- The NumPy team releases wheels on PyPI for this platform.
- CI failures block releases.
- Must have at least one maintainer who commits to take primary and long-term
  responsibility for keeping the ``main`` branch and wheel builds working.

Tier 2 platforms:

+---------------------------+-------+------------------------------------------+
| Platform                  | Notes | Contacts                                 |
+===========================+=======+==========================================+
| Linux x86-64 (musllinux)  |       | Ralf Gommers                             |
+---------------------------+-------+------------------------------------------+
| Linux aarch64 (musllinux) |       | Ralf Gommers                             |
+---------------------------+-------+------------------------------------------+
| Free-threaded CPython     |       | Nathan Goldbaum, Kumar Aditya,           |
|                           |       | Ralf Gommers                             |
+---------------------------+-------+------------------------------------------+


Tier 3
~~~~~~

- Is supported as part of NumPy's regular CI setup for the ``main`` branch. CI
  support as defined for Tier 2.
- No wheels are released on PyPI for this platform.
- CI failures block releases (skips may be applied when the failure is clearly
  platform-specific and does not indicate a regression in core functionality).
- Must have at least one maintainer or a regular contributor trusted by the
  NumPy maintainers who commits to take responsibility for CI on the ``main``
  branch working.

Tier 3 platforms:

+--------------------+----------------------------------------+----------------------------------+
| Platform           | Notes                                  | Contacts                         |
+====================+========================================+==================================+
| FreeBSD            | Runs on Cirrus CI                      | Ralf Gommers                     |
+--------------------+----------------------------------------+----------------------------------+
| Linux ppc64le      | Runs on IBM-provided self-hosted       | Sandeep Gupta                    |
|                    | runners, see gh-22318_                 |                                  |
+--------------------+----------------------------------------+----------------------------------+
| Emscripten/Pyodide | We currently provide nightly wheels,   | Agriya Khetarpal, Gyeongjae Choi |
|                    | used for interactive docs              |                                  |
+--------------------+----------------------------------------+----------------------------------+


Unsupported platforms