Handling dependencies that don’t support free-threading

Build dependencies that may need special consideration

CFFI

CFFI added support for the free-threaded build of Python 3.14 in version 2.0.0. You can ensure it is installed by passing a version constraint to pip:

python -m pip install cffi>=2.0.0

If you want to force CFFI 2.0.0 to be used as a dependency of a project, you can use the following pyproject.toml snippet:

[build-system]
requires = [
  "cffi>=2.0.0",
]

You can also use the python_version environment marker to specify the constraint is only valid for Python 3.14 and newer:

[build-system]
requires = [
  "cffi>=2.0.0; python_version >= '3.14'",
  "cffi; python_version < '3.14'",
]

You can declare a runtime dependency in the project.dependencies section using the same syntax.

CFFI does not support the free-threaded build of Python 3.13.

mypyc

The mypyc bindings generator has preliminary support for the free-threaded build in the main branch of mypyc. If you maintain a package that ships binaries using mypyc, you should try building wheels using the development branch of mypyc. The maintainers of mypyc encourage users to ship wheels this way and report issues if they encounter any.

Usually mypyc is used with projects that can be straightforwardly used in a pure-python mode. If there is no compiled build available, we suggest using a pure-python build instead.

Other bindings generators

Cython, nanobind, pybind11, and PyO3 all fully support the free-threaded build. See the documentation of those projects for more details about using them with the free-threaded interpreter.

Runtime dependencies that don't support free-threading

Depending on PyYAML

PyYAML 6.0.3+ support free-threading Python starting with Python 3.14t+. It is unnecessary to use PyYAML-ft unless you must support Python 3.13t.

Other Runtime Dependencies

Zstandard

Python 3.14 includes the new compression.zstd module, backports are available under backports.zstd for Python 3.9-3.13 and can replace the zstandard package.