Tox 'HandledError| SetupCfg .. tox:tox not found' in projects with setup.cfg and pyproject.toml

[Dantali0n]

Issue

Tox should run when an environment is specified tox -e lint in a project with a setup.cfg file for the [metadata] and all [tool.tox] configuration in pyproject.toml. But instead it gives an error about the section tox:tox not being found in setup.cfg.

Environment

Provide at least:

• OS: Arch Linux
• Python: 3.13.7
• Tox: 4.30.1

Output of pip list of the host Python, where tox is installed

Package               Version
--------------------- --------------
arrow                 1.3.0
binaryornot           0.4.4
cachetools            6.1.0
certifi               2025.8.3
cfgv                  3.4.0
chardet               5.2.0
charset-normalizer    3.4.3
click                 8.2.1
colorama              0.4.6
cookiecutter          2.6.0
cruft                 2.16.0
distlib               0.3.9
filelock              3.18.0
gitdb                 4.0.12
GitPython             3.1.45
identify              2.6.12
idna                  3.10
Jinja2                3.1.6
markdown-it-py        4.0.0
MarkupSafe            3.0.2
mdurl                 0.1.2
nodeenv               1.9.1
packaging             25.0
pip                   25.2
platformdirs          4.3.8
pluggy                1.6.0
pre_commit            4.2.0
Pygments              2.19.2
pyproject-api         1.9.1
python-dateutil       2.9.0.post0
python-slugify        8.0.4
PyYAML                6.0.2
requests              2.32.5
rich                  14.1.0
shellingham           1.5.4
six                   1.17.0
smmap                 5.0.2
text-unidecode        1.3
tox                   4.30.1
typer                 0.16.1
types-python-dateutil 2.9.0.20250809
typing_extensions     4.14.1
urllib3               2.5.0
virtualenv            20.31.2

Output of running tox

Output of tox -rvv

ROOT: HandledError| SetupCfg failed loading /home/lukken/projects/astron/lofar2/stingray/setup.cfg due to section tox:tox not found

Minimal example

setup.cfg

[metadata]
name = lofar-stingray
description = LOFAR station statistics gather and dump utility, writes station statistics continuously to the local S3 based object storage.
long_description = file: README.md
long_description_content_type = text/markdown
url = https://git.astron.nl/lofar2.0/stingray
license = Apache License 2.0
classifiers =
    Development Status :: 3 - Alpha
    Environment :: No Input/Output (Daemon)
    Intended Audience :: Developers
    Intended Audience :: Science/Research
    License :: OSI Approved :: Apache Software License
    Operating System :: OS Independent
    Programming Language :: Python
    Programming Language :: Python :: 3
    Programming Language :: Python :: 3 :: Only
    Programming Language :: Python :: 3.11
    Programming Language :: Python :: 3.12
    Topic :: Utilities
    Topic :: System :: Archiving :: Compression
    Topic :: Scientific/Engineering
    Topic :: Scientific/Engineering :: Astronomy

[options]
include_package_data = true
packages = find:
python_requires = >=3.11
install_requires = file: requirements.txt

pyproject.toml

[build-system]
requires = [
    "setuptools>=70.0",
    "setuptools_scm[toml]>=8.0",
    "wheel"
]
build-backend = "setuptools.build_meta"

[tool.tox]
# Generative environment list to test all supported Python versions
requires = ["tox>=4.21"]
env_list = ["lint"]

[tool.tox.env_run_base]
package = "editable"
deps = [
    "-r{toxinidir}/requirements.txt",
    "-r{toxinidir}/tests/requirements.txt"]
commands = [["python", "--version"], ["python", "-m", "pytest", "tests/{posargs}"]]

# Command prefixes to reuse the same virtualenv for all linting jobs.
[tool.tox.env.lint]
deps = [
    "ruff",
    "-r{toxinidir}/tests/requirements.txt"]
commands = [
    ["python", "-m", "ruff", "--version"],
]

[Daverball]

I believe this regression was caused by #3578

[webknjaz]

cc @ssbarnea

[webknjaz]

@Dantali0n a bit unrelated, but you shouldn't have wheel in your [build-system].

[ThomasWaldmann]

Can confirm, just stumbled over same issue. "tox<4.30" helps (I also switched from the legacy hack to a native pyproject.toml tox config).

https://github.com/nsupdate-info/nsupdate.info/actions/runs/17476454410/job/49637437038?pr=586

[ssbarnea]

@Daverball PRs are welcomed, tbh I ditched setup.cfg years ago and these days I am also dropping the tox.ini file from last projects.

[hroncok]

Similarly, a project that has no tox configuration at all previously said No tox.ini or setup.cfg or pyproject.toml or tox.toml found, assuming empty tox.ini but continued successfully. With a valid setup.cfg file without a tox config, it now fails with the same error as reported here.

[mtelka]

Maybe it would be enough (at least as a short term solution) to prefer pyproject.toml over setup.cfg, e.g. by moving SetupCfg a bit below in https://github.com/tox-dev/tox/blob/main/src/tox/config/source/discover.py

[cpburnz]

Tox (v4.32.0) similarly fails if you use tox.toml but also have a setup.cfg without tox:tox.

[webknjaz]

@cpburnz it's normally useful to contribute a regression test with your expectations in a PR. So if somebody starts working on fixing it, there would be something to check against.

[cpburnz]

I can provide a reproducible example. I don't know where to start with a regression test.

-------- Original Message -------- On Saturday, 11/15/25 at 06:30 🇺🇦 Sviatoslav Sydorenko @.***> wrote:

webknjaz left a comment (tox-dev/tox#3602)

@.***(https://github.com/cpburnz) it's normally useful to contribute a regression test with your expectations in a PR. So if somebody starts working on fixing it, there would be something to check against.

— Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you were mentioned.Message ID: @.***>

[CyberTailor]

A workaround is to use tox -c pyproject.toml explicitly.