gh-621 addresses part of this. As of today, it's not yet very easy to test with a free-threaded CPython build and make the test suite pass. The hiccups are:

pip does not yet have support. Two issues: it incorrectly installs incompatible abi3 wheels, and it refuses to install wheels with the cp313t/cp313td ABI tag. The former doesn't affect meson-python, the latter was a little painful to work around (see below for how).
cython does not yet have full support. The latest release (3.0.10) fails to build one of our test packages (the complex one), however the branch from Update module setup code for free-threaded CPython cython/cython#6137 does work.
CPython needs to be built from source (./configure --with-pydebug --disable-gil)

In order to make pip install wheels with the t tag, the _fail_if_link_is_unsupported_wheel function in pip/_internal/resolution/resolvelib/factory.py needs to be patched - simply replacing raise UnsupportedWheel(msg) with return is enough.

Then the venv fixture in our test suite needs patching, because it installs a fresh copy of pip in the venv's it creates. This hacky patch does that (replace path to site-packages as needed):

diff --git a/tests/conftest.py b/tests/conftest.py
index 80303a14f..68df9e2ef 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -101,7 +101,14 @@ class VEnv(EnvBuilder):
 @pytest.fixture()
 def venv(tmp_path_factory):
     path = pathlib.Path(tmp_path_factory.mktemp('mesonpy-test-venv'))
-    return VEnv(path)
+    fixture = VEnv(path)
+
+    # Apply fix to `pip` installed into venv to avoid unknown tag issue
+    sitepkg_dir = '/home/rgommers/mambaforge/envs/nogil/lib/python3.13/site-packages'
+    sitepkg_venv = str(path) + '/lib/python3.13/site-packages'
+    _file = '/pip/_internal/resolution/resolvelib/factory.py'
+    subprocess.run(['cp', sitepkg_dir + _file, sitepkg_venv + _file])
+    return fixture
 
 
 def generate_package_fixture(package):

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