diff --git a/.github/workflows/CI.yaml b/.github/workflows/CI.yaml
index e99b9c5..0f25020 100644
--- a/.github/workflows/CI.yaml
+++ b/.github/workflows/CI.yaml
@@ -16,8 +16,17 @@ jobs:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
         runs-on: ["ubuntu-24.04"]
         include:
-          - runs-on: "macos-15"
+          - runs-on: "ubuntu-24.04"
             python-version: "3.13"
+            freethreaded: true
+          - runs-on: "ubuntu-24.04"
+            python-version: "3.14"
+            freethreaded: true
+          - runs-on: "macos-15"
+            python-version: "3.14"
+            freethreaded: true
+          - runs-on: "macos-15"
+            python-version: "3.14"
           - runs-on: "macos-15"
             python-version: "3.9"
     # Pull in the test script from run_tests and distribute python from matrix versions
@@ -26,6 +35,7 @@ jobs:
       python-version: ${{ matrix.python-version }}
       runs-on: ${{ matrix.runs-on }}
       requirements-file: ".github/requirements-${{ matrix.python-version }}.txt"
+      freethreaded: ${{ matrix.freethreaded }}
 
   # run-tests-legacy-python:
   #   needs: run-tests-modern-python # Wait until tests pass on python 3.9+
@@ -60,6 +70,6 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-    - run: jq --exit-status 'all(.result == "success")' <<< '${{ toJson(needs) }}'
-    - name: Done
-      run: exit 0
+      - run: jq --exit-status 'all(.result == "success")' <<< '${{ toJson(needs) }}'
+      - name: Done
+        run: exit 0
diff --git a/.github/workflows/run_tests.yaml b/.github/workflows/run_tests.yaml
index 5bce01f..f267b7f 100644
--- a/.github/workflows/run_tests.yaml
+++ b/.github/workflows/run_tests.yaml
@@ -12,6 +12,10 @@ on:
       requirements-file:
         required: true
         type: string
+      freethreaded:
+        required: false
+        type: boolean
+        default: false
 
 jobs:
   run_test:
@@ -33,5 +37,7 @@ jobs:
         uv pip install . -r ${{ inputs.requirements-file }} -v --reinstall --system
         python -c "import parsnip; print('parsnip', parsnip.__version__)"
 
+        python -c "import sys; print('GIL:', getattr(sys, '_is_gil_enabled', lambda: 'unknown')())"
+
     - name: Test with pytest
       run: python -m ${{ runner.os != 'Windows' && 'pytest' || 'pytest tests' }} -v -n auto