Flit Migration Check: setup.py to flit_core 4
Open a setup.py, setup.cfg or pyproject.toml and this extension lists every line that still belongs to setuptools, Hatch, Poetry or the old Flit 3 layout, with its line number and the exact flit_core 4 line that replaces it. It runs offline, inside the editor, on your own file.
Free web version (same engine): https://getreadystack.com/tools/flit-migration-check
Why now
- flit_core 4.0 (on PyPI 2026-08-04) no longer reads [tool.flit.metadata]. A package whose
[build-system] asks for flit_core >=3.2 with no <4 pin and still keeps [tool.flit.metadata] now gets flit_core 4 from pip, and its sdist no longer builds.
- flit_core 4.1 (on PyPI 2026-09-16) accepts license expressions with WITH. A
license = "Apache-2.0 WITH LLVM-exception" line needs flit_core >=4.1,<5 in requires.
license = "MIT" (an SPDX expression) needs flit_core >=3.11; import-names needs flit_core >=4.- Flit builds pure Python only: "If your package needs a build step, you won't be able to use Flit". Compiled extensions (
ext_modules, Extension(, cythonize() and custom cmdclass are reported as BLOCKER lines: keep setuptools for those packages.
- Before you upload: PyPI rejects an upload file over 100.0 MiB by default, and it refuses invalid Trove classifiers and a name too close to an existing project.
What you see
Each finding is one line of your file and one line of the target:
| Your line |
flit_core 4 line |
install_requires=[...] |
[project] dependencies = [...] |
extras_require={...} |
[project.optional-dependencies] |
"console_scripts": [...] |
[project.scripts] name = "pkg.cli:main" |
data_files=[...] |
[tool.flit.external-data] directory = "data" |
python_requires=">=3.8" |
requires-python = ">=3.8" |
[tool.flit.metadata] |
[project], or pin flit_core >=2,<4 |
[project.entry-points.console_scripts] |
[project.scripts] |
dynamic = ["dependencies"] |
write the field out: Flit fills only version, description, import-names, import-namespaces |
On the bundled sample, the bundled sample setup.py gives 22 findings on 22 lines (10 warnings, 12 notes, 0 blockers). A finished flit_core 4 pyproject.toml gives 0 findings.
Rules
38 rules across setup.py, setup.cfg and pyproject.toml. Every rule names its source page on flit.pypa.io (pyproject.toml config, Why use Flit?, Release history), read on 2026-09-30.
Yardstick: every replacement line is copied from the Flit 4.1.0 documentation, not guessed.
Free and paid
Also searched as
flit, flit_core, hatch, hatchling, setuptools to flit, setup.py to pyproject.toml, twine upload checks.
Limits
The checker reads text; it does not run setup.py. Values computed at run time (a version read from a file, a list built in a loop) are shown as the line that sets them, with the flit replacement to write by hand.
