diff --git a/dpgen2/op/prep_relax.py b/dpgen2/op/prep_relax.py
index 1ee2869a..40a25312 100644
--- a/dpgen2/op/prep_relax.py
+++ b/dpgen2/op/prep_relax.py
@@ -41,7 +41,11 @@ def execute(
         ncifs = len(ip["cifs"])
         config = ip["expl_config"]
         group_size = config["relax_group_size"]
-        ntasks = int(ncifs / group_size)
+        if group_size <= 0:
+            raise ValueError("relax_group_size must be greater than zero")
+        # Ceiling division keeps a final partial group instead of silently
+        # dropping CIFs when their count is not divisible by group_size.
+        ntasks = (ncifs + group_size - 1) // group_size
         task_paths = []
         for i in range(ntasks):
             task_dir = Path("task.%06d" % i)
diff --git a/tests/op/test_prep_relax.py b/tests/op/test_prep_relax.py
index 83dcc2b7..b8bd60a1 100644
--- a/tests/op/test_prep_relax.py
+++ b/tests/op/test_prep_relax.py
@@ -15,32 +15,64 @@
 
 
 class TestPrepRelax(unittest.TestCase):
-    def testPrepRelax(self):
+    def setUp(self):
+        self.cifs = []
+
+    def _make_cifs(self, count):
         cifs = []
-        for i in range(4):
+        for i in range(count):
             p = Path("%i.cif" % i)
             p.write_text("Mocked cif.")
             cifs.append(p)
+        self.cifs.extend(cifs)
+        return cifs
+
+    def _run_prep_relax(self, ncifs, group_size):
         op_in = OPIO(
             {
                 "expl_config": {
-                    "relax_group_size": 2,
+                    "relax_group_size": group_size,
                 },
-                "cifs": cifs,
+                "cifs": self._make_cifs(ncifs),
             }
         )
         op = PrepRelax()
-        op_out = op.execute(op_in)
+        return op.execute(op_in)
+
+    def test_prep_relax(self):
+        op_out = self._run_prep_relax(4, 2)
         self.assertEqual(op_out["ntasks"], 2)
         self.assertEqual(len(op_out["task_paths"]), 2)
         for i, task_path in enumerate(op_out["task_paths"]):
             self.assertEqual(str(task_path), "task.%06d" % i)
             self.assertEqual(len(list(task_path.iterdir())), 2)
 
+    def test_keeps_partial_final_group(self):
+        """Assign every CIF when the final task is not a full group."""
+        op_out = self._run_prep_relax(5, 2)
+
+        self.assertEqual(op_out["ntasks"], 3)
+        self.assertEqual(
+            [len(list(task_path.iterdir())) for task_path in op_out["task_paths"]],
+            [2, 2, 1],
+        )
+
+    def test_creates_task_when_group_is_larger_than_input(self):
+        op_out = self._run_prep_relax(1, 2)
+
+        self.assertEqual(op_out["ntasks"], 1)
+        self.assertEqual(len(list(op_out["task_paths"][0].iterdir())), 1)
+
+    def test_rejects_non_positive_group_size(self):
+        for group_size in (0, -1):
+            with self.subTest(group_size=group_size):
+                with self.assertRaisesRegex(ValueError, "greater than zero"):
+                    self._run_prep_relax(1, group_size)
+
     def tearDown(self):
-        for i in range(2):
-            if os.path.isdir("task.%06d" % i):
-                shutil.rmtree("task.%06d" % i)
-        for i in range(4):
-            if os.path.isfile("%s.cif" % i):
-                os.remove("%s.cif" % i)
+        for task_path in Path().glob("task.[0-9][0-9][0-9][0-9][0-9][0-9]"):
+            if task_path.is_dir():
+                shutil.rmtree(task_path)
+        for cif in self.cifs:
+            if cif.is_file():
+                os.remove(cif)