diff --git a/src/CelerisLab/lbm/block_stepper.py b/src/CelerisLab/lbm/block_stepper.py
index 8389bd8..4c32bad 100644
--- a/src/CelerisLab/lbm/block_stepper.py
+++ b/src/CelerisLab/lbm/block_stepper.py
@@ -207,8 +207,11 @@ class PeriodicBlockStepper:
         """Advance n steps: halo refresh, then core pull/collide, per step.
 
         All n steps are enqueued in order on the one stream and the host waits
-        once before returning (same launches and bits as n step(1) calls). On a
-        failure the instance is failed and step_count keeps its entry value.
+        once before returning (same launches and bits as n step(1) calls).
+        Asynchronous device errors surface at that single wait, so they are not
+        attributed to the step that caused them; call step(1) repeatedly to
+        localize one. On any failure the instance is failed and step_count keeps
+        its entry value.
         """
         if isinstance(n, bool) or not isinstance(n, Integral) or n < 0:
             raise ValueError("n must be a nonnegative integer")
diff --git a/tests/unit/test_block_step_sync.py b/tests/unit/test_block_step_sync.py
index 5779a70..51b76ca 100644
--- a/tests/unit/test_block_step_sync.py
+++ b/tests/unit/test_block_step_sync.py
@@ -78,6 +78,25 @@ class BlockStepSync(unittest.TestCase):
             run.step(2)
         self.assertEqual((run.step_count, run._status), (1, "failed"))
 
+    def test_launch_failure_mid_batch_fails_closed_without_waiting(self):
+        run, log = backend()
+        calls = []
+        step = run._step
+
+        def failing(*args, **kwargs):
+            calls.append(1)
+            if len(calls) == 2:
+                raise RuntimeError("launch failure")
+            return step(*args, **kwargs)
+
+        run._step = failing
+        with self.assertRaises(RuntimeError):
+            run.step(3)
+        self.assertNotIn("sync", [name for name, _ in log])
+        self.assertEqual((run.step_count, run._status), (0, "failed"))
+        with self.assertRaises(RuntimeError):
+            run.download()
+
 
 if __name__ == "__main__":
     unittest.main()