autosummary fails and does not read modules

[aparmendariz]

Describe the bug

When building the documentation for our python package (https://mcfpy.github.io/mcf/) using Sphinx (sphinx-build -b html docs/source docs/build/html), the build fails during the builder-inited event with the following error:

ExtensionError: Handler <function process_generate_options at ...> for event 'builder-inited' threw an exception 
(exception: no module named optpolicy_main.OptimalPolicy)

This issue occurs even though the same configuration worked with earlier Sphinx versions. The failure happens before page generation, at the autosummary pre-processing step.

The failing import refers to the class:

class OptimalPolicy:
    ...

which is part of the public API and previously built without issues.

How to Reproduce

minimal index.rst:

License
-------

**mcf** is distributed under the `MIT License <https://github.com/MCFpy/mcf?tab=MIT-1-ov-file#readme>`__.

.. toctree::
   :hidden:

   getting_started.rst
   user_guide.rst
   algorithm_reference.rst
   python_api.rst
   FAQ.rst
   changelog.rst

Module that is not parsing with autosummary :
optpolicy_main.py

Environment Information

Run sphinx-build -b html docs/source docs/build/html
Running Sphinx v8.2.3
loading translations [en]... done
Converting `source_suffix = ['.rst', '.md']` to `source_suffix = {'.rst': 'restructuredtext', '.md': 'restructuredtext'}`.
[autosummary] generating autosummary for: FAQ.rst, algorithm_reference.rst, algorithm_reference/QIATEs.rst, algorithm_reference/bgates_cbgates.rst, algorithm_reference/fairscores.rst, algorithm_reference/inference.rst, algorithm_reference/local_centering.rst, algorithm_reference/optimal-policy_algorithm.rst, algorithm_reference/training.rst, changelog.rst, ..., user_guide.rst, user_guide/common_support.rst, user_guide/computational_speed.rst, user_guide/data_cleaning.rst, user_guide/estimation.rst, user_guide/experimental_features.rst, user_guide/feature_selection.rst, user_guide/optimal-policy_example.rst, user_guide/post_estimation_diagnostics.rst, user_guide/weights_clusters.rst

Extension error (sphinx.ext.autosummary)!

Versions
========

* Platform:         linux; (Linux-6.11.0-1018-azure-x86_64-with-glibc2.39)
* Python version:   3.12.12 (CPython)
* Sphinx version:   8.2.3
* Docutils version: 0.21.2
* Jinja2 version:   3.1.6
* Pygments version: 2.19.2

Last Messages
=============

None.

Loaded Extensions
=================

None.

Traceback
=========

      File "/opt/hostedtoolcache/Python/3.12.12/x64/lib/python3.12/site-packages/sphinx/events.py", line 415, in emit
        raise ExtensionError(
    sphinx.errors.ExtensionError: Handler <function process_generate_options at 0x7fbc6ecb84a0> for event 'builder-inited' threw an exception (exception: no module named optpolicy_main.OptimalPolicy)


The full traceback has been saved in:
/tmp/sphinx-err-66l48k9j.log

To report this error to the developers, please open an issue at <https://github.com/sphinx-doc/sphinx/issues/>. Thanks!
Please also report this if it was a user error, so that a better error message can be provided next time.
Error: Process completed with exit code 2.

Sphinx extensions

extensions = [
    'sphinx.ext.autodoc',
    'sphinx.ext.autosummary',
    'sphinx.ext.coverage',
    'sphinx.ext.intersphinx',
    'sphinx.ext.mathjax',
    'sphinx.ext.napoleon',
    'sphinx_copybutton',
    'sphinx.ext.githubpages',
    'sphinx.ext.doctest',
    'sphinx_design', 
    'myst_parser' 
]

Additional context

This is the link to github: https://github.com/MCFpy/mcf

conf.py:
conf.py

[AA-Turner]

@aparmendariz we only provide free support for the latest version of Sphinx. Please can you verify the error occurs for Sphinx 9? Also please verify that you can import optpolicy_main.OptimalPolicy in the venv used for Sphinx.

A