diff --git a/tests/algorithm/test_dapo_algorithm.py b/tests/algorithm/test_dapo_algorithm.py
index e4c143f55c2..74260b57723 100644
--- a/tests/algorithm/test_dapo_algorithm.py
+++ b/tests/algorithm/test_dapo_algorithm.py
@@ -108,6 +108,32 @@ def test_symmetric_accuracy(self, mock_compute_score):
         self.assertEqual(good["accuracy"], 1.0)
         self.assertEqual(bad["accuracy"], -1.0)
 
+    def test_overlong_penalty_piecewise(self):
+        fn = MathDAPORewardFn(
+            enable_overlong_penalty=True,
+            penalty_factor=1.0,
+            max_response_length=100,
+            cache_length=20,
+        )
+        # DAPO paper (Sec. 2.4): zero until `max_response_length - cache_length`,
+        # then linear, then -penalty_factor beyond `max_response_length`.
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(79)), 0.0)
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(80)), 0.0)
+        self.assertAlmostEqual(fn.compute_overlong_penalty(torch.zeros(90)), -0.5)
+        self.assertAlmostEqual(fn.compute_overlong_penalty(torch.zeros(100)), -1.0)
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(101)), -1.0)
+
+    def test_overlong_penalty_without_soft_window(self):
+        fn = MathDAPORewardFn(
+            enable_overlong_penalty=True,
+            penalty_factor=1.0,
+            max_response_length=100,
+            cache_length=0,
+        )
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(99)), 0.0)
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(100)), 0.0)
+        self.assertEqual(fn.compute_overlong_penalty(torch.zeros(101)), -1.0)
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/trinity/common/rewards/dapo_reward.py b/trinity/common/rewards/dapo_reward.py
index dcc194e683a..d715b014f38 100644
--- a/trinity/common/rewards/dapo_reward.py
+++ b/trinity/common/rewards/dapo_reward.py
@@ -89,7 +89,7 @@ def compute_overlong_penalty(self, response_token):
         response_len = len(response_token)
         expected_len = self.max_response_length - self.cache_length
 
-        if response_len < expected_len:
+        if response_len <= expected_len:
             return 0.0
         elif response_len > self.max_response_length:
             return -self.penalty_factor