diff --git a/.github/workflows/tests_mr.yml b/.github/workflows/tests_mr.yml
index ea17b2f2..2a65e925 100644
--- a/.github/workflows/tests_mr.yml
+++ b/.github/workflows/tests_mr.yml
@@ -22,11 +22,14 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install dependencies
+    - name: Configure python
       run: |
         python -m pip install --upgrade pip
+        python -m pip install poetry
         python -m pip install flake8 pytest
-        pip install -e .
+    - name: Install dependencies
+      run: |
+        python -m poetry install
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names
@@ -35,4 +38,4 @@ jobs:
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        pytest
+        python -m poetry run pytest
diff --git a/pyproject.toml b/pyproject.toml
index 9cbfcde0..9fd2f542 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -46,7 +46,12 @@ scikit-image = [
 
 [tool.poetry.group.dev.dependencies]
 pytest = ">=8.1.1"
-vtk = "*"
+# vtk 9.7.0 dropped cp39 wheels (and publishes no requires-python metadata,
+# so Poetry cannot exclude it automatically). Pin the last cp39 release there.
+vtk = [
+  { version = ">=9.0,<9.7", python = "<3.10" },
+  { version = "*", python = ">=3.10" },
+]
 pre-commit = "*"
 pyvista = "^0.43.2"
 coverage = ">=7.0.1"