pip "Project ... does not support editable installs" (PEP 660)

pip install -e . needs the project’s build backend to implement the PEP 660 build_editable hook. An old backend (or one that never added editable support) cannot produce an editable wheel, so pip refuses.

What this error means

pip install -e . fails with ERROR: Project file:///path does not support editable installs or a Backend does not support build_editable message. A regular pip install . of the same project works.

pip output

ERROR: Project file:///home/runner/work/app does not support editable installs.
Hint: the build backend does not implement the optional 'build_editable' hook.

Common causes

Build backend predates PEP 660

Editable installs were standardized in PEP 660. A backend pinned to an old version - or one that never implemented build_editable - cannot build an editable wheel.

Old setuptools without editable_wheel

setuptools gained PEP 660 support in 64.0. On an image with older setuptools, pip install -e . falls back and fails instead of using the modern editable path.

How to fix it

Upgrade the build backend

For setuptools projects, a current setuptools implements PEP 660. Upgrade the build frontend and backend first.

Terminal

python -m pip install --upgrade pip "setuptools>=64" wheel
pip install -e .

Declare a PEP 660-capable backend

Make sure [build-system] names a backend version that supports editable builds.

pyproject.toml

[build-system]
requires = ["setuptools>=64", "wheel"]
build-backend = "setuptools.build_meta"

Fall back to a non-editable install

If you cannot upgrade the backend, a plain install still works for CI that only needs the package importable (you lose live source editing).