diff --git a/python/sglang/srt/managers/schedule_batch.py b/python/sglang/srt/managers/schedule_batch.py
index e29d6413b..acd61e279 100755
--- a/python/sglang/srt/managers/schedule_batch.py
+++ b/python/sglang/srt/managers/schedule_batch.py
@@ -1330,7 +1330,7 @@ class Req(ReqDllmMixin):
 
     def _check_vocab_boundary_finish(self, new_accepted_tokens: List[int] = None):
         for i, token_id in enumerate(new_accepted_tokens):
-            if token_id > self.vocab_size or token_id < 0:
+            if token_id >= self.vocab_size or token_id < 0:
                 offset = len(self.output_ids) - len(new_accepted_tokens) + i
                 if self.sampling_params.stop_token_ids:
                     self.output_ids[offset] = next(
diff --git a/test/registered/unit/managers/test_vocab_boundary_finish.py b/test/registered/unit/managers/test_vocab_boundary_finish.py
new file mode 100644
index 000000000..beb59d076
--- /dev/null
+++ b/test/registered/unit/managers/test_vocab_boundary_finish.py
@@ -0,0 +1,82 @@
+"""Regression tests for Req._check_vocab_boundary_finish NaN guard boundary."""
+
+import unittest
+
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase, maybe_stub_sgl_kernel
+
+maybe_stub_sgl_kernel()
+
+from sglang.srt.managers.schedule_batch import FINISH_MATCHED_STR, Req
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+VOCAB_SIZE = 1000
+
+
+def _make_req(*, output_ids: list, eos_token_ids: set, stop_token_ids: set) -> Req:
+    # Build a bare Req without running __init__; only the fields touched by
+    # _check_vocab_boundary_finish are populated.
+    req = Req.__new__(Req)
+    req.output_ids = list(output_ids)
+    req.vocab_size = VOCAB_SIZE
+    req.eos_token_ids = eos_token_ids
+    req.finished_reason = None
+    req.finished_len = None
+
+    class _SamplingParams:
+        pass
+
+    req.sampling_params = _SamplingParams()
+    req.sampling_params.stop_token_ids = stop_token_ids
+    return req
+
+
+class TestVocabBoundaryFinish(CustomTestCase):
+    def test_token_equal_to_vocab_size_is_out_of_bounds(self):
+        # Valid ids are [0, vocab_size); id == vocab_size must trip the NaN guard.
+        req = _make_req(
+            output_ids=[5, VOCAB_SIZE], eos_token_ids={2}, stop_token_ids=set()
+        )
+        self.assertTrue(req._check_vocab_boundary_finish([5, VOCAB_SIZE]))
+        self.assertIsInstance(req.finished_reason, FINISH_MATCHED_STR)
+        self.assertEqual(req.finished_reason.matched, "NaN happened")
+        self.assertEqual(req.finished_len, 2)
+        # The offending slot is rewritten to the eos token.
+        self.assertEqual(req.output_ids[1], 2)
+
+    def test_token_above_vocab_size_is_out_of_bounds(self):
+        # A wildly large garbage id (typical of NaN sampling) is caught.
+        req = _make_req(
+            output_ids=[5, VOCAB_SIZE + 12345], eos_token_ids={2}, stop_token_ids=set()
+        )
+        self.assertTrue(req._check_vocab_boundary_finish([5, VOCAB_SIZE + 12345]))
+        self.assertEqual(req.finished_len, 2)
+
+    def test_negative_token_is_out_of_bounds(self):
+        # Negative ids also indicate corrupted sampling output.
+        req = _make_req(output_ids=[5, -1], eos_token_ids={2}, stop_token_ids=set())
+        self.assertTrue(req._check_vocab_boundary_finish([5, -1]))
+        self.assertEqual(req.output_ids[1], 2)
+
+    def test_max_valid_token_is_in_bounds(self):
+        # id == vocab_size - 1 is the largest valid token and must not trip.
+        req = _make_req(
+            output_ids=[5, VOCAB_SIZE - 1], eos_token_ids={2}, stop_token_ids=set()
+        )
+        self.assertFalse(req._check_vocab_boundary_finish([5, VOCAB_SIZE - 1]))
+        self.assertIsNone(req.finished_reason)
+        self.assertIsNone(req.finished_len)
+        self.assertEqual(req.output_ids[1], VOCAB_SIZE - 1)
+
+    def test_stop_token_used_when_no_eos(self):
+        # Without eos tokens, the slot is rewritten to a stop token instead.
+        req = _make_req(
+            output_ids=[VOCAB_SIZE], eos_token_ids=set(), stop_token_ids={7}
+        )
+        self.assertTrue(req._check_vocab_boundary_finish([VOCAB_SIZE]))
+        self.assertEqual(req.output_ids[0], 7)
+
+
+if __name__ == "__main__":
+    unittest.main()