diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index de35339b..bb2be7d2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,17 +41,18 @@ jobs:
           fi
 
   wheels:
-    name: Build / ${{ matrix.os }} / Python 3.${{ matrix.python.minor }}
+    name: Build / ${{ matrix.os }} / Python 3.${{ matrix.python.minor }} / FT${{ matrix.python.ft }}
     needs: matrix_config
     strategy:
       fail-fast: false
       matrix:
         os: ${{ fromJson(needs.matrix_config.outputs.matrix_os) }}
         python:
-            - {minor: 10, req_build: 'requirements-build-3_11.txt', req_test: 'requirements-dev-3_11.txt'}
-            - {minor: 11, req_build: 'requirements-build-3_11.txt', req_test: 'requirements-dev-3_11.txt'}
-            - {minor: 12, req_build: 'requirements-build-3_12.txt', req_test: 'requirements-dev-3_12.txt'}
-            - {minor: 13, req_build: 'requirements-build-3_13.txt', req_test: 'requirements-dev-3_13.txt'}
+            - {minor: 10, req_build: 'requirements-build-3_11.txt', req_test: 'requirements-dev-3_11.txt', ft: '0'}
+            - {minor: 11, req_build: 'requirements-build-3_11.txt', req_test: 'requirements-dev-3_11.txt', ft: '0'}
+            - {minor: 12, req_build: 'requirements-build-3_12.txt', req_test: 'requirements-dev-3_12.txt', ft: '0'}
+            - {minor: 13, req_build: 'requirements-build-3_13.txt', req_test: 'requirements-dev-3_13.txt', ft: '0'}
+            - {minor: 13t, req_build: 'requirements-build-3_13.txt', req_test: 'requirements-dev-3_13.txt', ft: '1'}
 
     runs-on: ${{ matrix.os }}
     outputs:
@@ -75,6 +76,7 @@ jobs:
           CIBW_BEFORE_BUILD: pip install -r {project}/${{ matrix.python.req_build }}
           CIBW_BEFORE_TEST: pip install -r {project}/${{ matrix.python.req_test }}
           CIBW_TEST_COMMAND: pytest {project}/test
+          CIBW_ENABLE: ${{ matrix.python.ft == '1' && 'cpython-freethreading' || '' }}
 
       - run: pip install pipx
         if: matrix.os == 'macos-13-xlarge'
@@ -87,10 +89,11 @@ jobs:
           CIBW_BEFORE_BUILD: pip install -r {project}/${{ matrix.python.req_build }}
           CIBW_BEFORE_TEST: pip install -r {project}/${{ matrix.python.req_test }}
           CIBW_TEST_COMMAND: pytest {project}/test
+          CIBW_ENABLE: ${{ matrix.python.ft == '1' && 'cpython-freethreading' || '' }}
 
       - uses: actions/upload-artifact@v4
         with:
-          name: dist-wheels-${{ matrix.os }}-py3${{ matrix.python.minor }}  # Unique artifact name
+          name: dist-wheels-${{ matrix.os }}-py3${{ matrix.python.minor }}-t${{ matrix.python.ft }}  # Unique artifact name
           path: dist/*
 
   upload:
@@ -105,11 +108,6 @@ jobs:
           path: dist
           merge-multiple: true
 
-      # - name: Flatten dist directory
-      #   run: |
-      #     find dist -mindepth 2 -type f \( -name '*.whl' -o -name '*.tar.gz' \) \
-      #       -exec mv {} dist/ \;
-
       - uses: pypa/gh-action-pypi-publish@release/v1
         with:
           password: ${{ secrets.PYPI_TOKEN }}
diff --git a/requirements-build-3_13.txt b/requirements-build-3_13.txt
index dd8af061..467a95b6 100644
--- a/requirements-build-3_13.txt
+++ b/requirements-build-3_13.txt
@@ -1,2 +1,2 @@
-numpy==2.2.5
+numpy==2.3.1
 setuptools==80.*
diff --git a/requirements-dev-3_13.txt b/requirements-dev-3_13.txt
index c0f0285b..e029979c 100644
--- a/requirements-dev-3_13.txt
+++ b/requirements-dev-3_13.txt
@@ -1,4 +1,4 @@
-numpy==2.2.5
+numpy==2.3.1
 pytest==8.3.3
 invoke==2.2.0
 hypothesis==6.131.16
diff --git a/src/_arraykit.c b/src/_arraykit.c
index 1646a0ca..7ce767ad 100644
--- a/src/_arraykit.c
+++ b/src/_arraykit.c
@@ -72,7 +72,7 @@ static struct PyModuleDef arraykit_module = {
     .m_methods = arraykit_methods,
 };
 
-PyObject *
+PyObject*
 PyInit__arraykit(void)
 {
     import_array();
@@ -134,6 +134,9 @@ PyInit__arraykit(void)
         Py_XDECREF(m);
         return NULL;
     }
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
     return m;
 }
 
diff --git a/src/auto_map.c b/src/auto_map.c
index 64b65fb5..cc05795a 100644
--- a/src/auto_map.c
+++ b/src/auto_map.c
@@ -426,8 +426,13 @@ fami_iternext(FAMIObject *self)
             else {
                 PyObject* t = PyTuple_New(2);
                 if (!t) { return NULL; }
+#if PY_VERSION_HEX >= 0x030D0000  // Python 3.13+
+                PyObject* k = PyList_GetItemRef(self->fam->keys, index);
+#else
                 PyObject* k = PyList_GET_ITEM(self->fam->keys, index);
-                Py_INCREF(k);
+                Py_XINCREF(k);
+#endif
+                if (!k) { return NULL; }
                 PyTuple_SET_ITEM(t, 0, k);
                 PyTuple_SET_ITEM(t, 1, PyLong_FromSsize_t(index));
                 return t;
@@ -438,8 +443,13 @@ fami_iternext(FAMIObject *self)
                 return PyArray_ToScalar(PyArray_GETPTR1(self->keys_array, index), self->keys_array);
             }
             else {
+#if PY_VERSION_HEX >= 0x030D0000  // Python 3.13+
+                PyObject* yield = PyList_GetItemRef(self->fam->keys, index);
+#else
                 PyObject* yield = PyList_GET_ITEM(self->fam->keys, index);
-                Py_INCREF(yield);
+                Py_XINCREF(yield);
+#endif
+                if (!yield) { return NULL; }
                 return yield;
             }
         }
@@ -1302,11 +1312,11 @@ lookup(FAMObject *self, PyObject *key) {
     return self->table[table_pos].keys_pos;
 }
 
-// Insert a key_pos, hash pair into the table. Assumes table already has appropriate size. When inserting a new itme, `hash` is -1, forcing a fresh hash to be computed here. Return 0 on success, -1 on error.
+// Insert a key_pos, hash pair into the table. Assumes table already has appropriate size. When inserting a new item, `hash` is -1, forcing a fresh hash to be computed here. Return 0 on success, -1 on error.
 static int
 insert_obj(
         FAMObject *self,
-        PyObject *key,
+        PyObject *key,  // NOTE: a borrowed reference
         Py_ssize_t keys_pos,
         Py_hash_t hash)
 {