diff --git a/pyproject.toml b/pyproject.toml
index 8ecdc47..69072f0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,7 @@ test = [
 source = "vcs"
 
 [tool.hatch.envs.test]
+python = "3.12"
 features = ["test"]
 [tool.hatch.envs.test.scripts]
 run = "pytest -vv {args}"
diff --git a/tests/data/notebooks/example.ipynb b/tests/data/notebooks/example.ipynb
index 2755585..b41c331 100644
--- a/tests/data/notebooks/example.ipynb
+++ b/tests/data/notebooks/example.ipynb
@@ -7,7 +7,10 @@
    "outputs": [],
    "source": [
     "import random\n",
-    "random.randint(0, 10)"
+    "from pathlib import Path\n",
+    "\n",
+    "r = random.randint(0, 10)\n",
+    "Path(\"r.txt\").write_text(str(r))\n"
    ]
   }
  ],
diff --git a/tests/test_runner.py b/tests/test_runner.py
index ba39cb1..b486284 100644
--- a/tests/test_runner.py
+++ b/tests/test_runner.py
@@ -1,3 +1,5 @@
+import shutil
+from contextlib import chdir
 from pathlib import Path
 from typing import TYPE_CHECKING
 
@@ -54,5 +56,8 @@ def mk_nb() -> None:
     assert exc_rr.value.traceback == exc_orig.value.traceback
 
 
-def test_cli() -> None:
-    main([f"--preludes={preludes_dir}", str(nbs_dir)])
+def test_cli(tmp_path: Path) -> None:
+    with chdir(tmp_path):
+        shutil.copytree(nbs_dir, "notebooks")
+        main([f"--preludes={preludes_dir}", "notebooks"])
+        assert Path("notebooks/r.txt").read_text() == "6"