setuptools v78 no longer accepts options containing uppercase or dash characters in setup.cfg. This means if your package uses this feature, or if any of your dependencies need to be built from source (and are using the invalid syntax) then an error will be raised.

This can be worked around in the uv CLI by excluding new versions:

$ uv sync --exclude-newer 2025-03-24

This fix can be persisted in the pyproject.toml as:

[tool.uv]
exclude-newer = "2025-03-24T00:00:00Z"

Note this will prevent upgrading other packages.

If you are using uv pip, you can set a build constraint instead:

echo "setuptools<78" | uv pip install -b - <package>

This fix can be persisted in the pyproject.toml as:

[tool.uv]
build-constraint-dependencies = ["setuptools<78"]

There is an unrelated uv bug that does not propagate build-constraint-dependencies in uv sync (#12441). We will be releasing a fix for that soon.

I've asked for the change to be reverted upstream at pypa/setuptools#4870 (comment)

We're also adding a dedicated warning for this error case in #12438

See also

• Invalid dash-separated key 'description-file' on aiosonic, pytest-watch appeared in last hour #12434
• Issue with installing fasttext Using uv despite Fix #1446 #12437