PEP 703 free-threading landed in Python 3.13 (October 2024) as an experimental, separately compiled build. Installing 3.13 does not give you free-threading: the default build still has the GIL, and the free-threaded interpreter is a distinct binary, python3.13t on Unix and python3.13t.exe on Windows, produced by --disable-gil.
Check at runtime with sys._is_gil_enabled(), which returns False only on the free-threaded build with the GIL actually off. sysconfig.get_config_var("Py_GIL_DISABLED") reports whether the interpreter was compiled for it.
Extension modules must opt in by defining Py_mod_gil as Py_MOD_GIL_NOT_USED; if they do not, importing them re-enables the GIL at runtime and your parallelism silently disappears. Setting PYTHON_GIL=0 or passing -X gil=0 forces the GIL to stay off and makes such imports emit a warning instead. Wheels for this ABI carry the cp313t tag, so a package with only cp313 wheels will be built from source.