fix: stop-string check misses early matches during speculative decoding (#23802)

Co-authored-by: xythink <xythink@users.noreply.github.com>
Co-authored-by: hnyls2002 <lsyincs@gmail.com>
Co-authored-by: Liangsheng Yin <hnyls2002@gmail.com>

python/sglang/srt/managers/schedule_batch.py

@@ -1192,7 +1192,7 @@ class Req(ReqDllmMixin):
 
         return self.surr_and_decode_ids, self.read_offset - self.surr_offset
 
-    def tail_str(self) -> str:
+    def tail_str(self, new_accepted_len: int = 1) -> str:
         # Check stop strings and stop regex patterns together
         if (
             len(self.sampling_params.stop_strs) == 0
@@ -1205,7 +1205,12 @@ class Req(ReqDllmMixin):
             self.sampling_params.stop_regex_max_len + 1,
         )
 
-        tail_len = min(max_len_tail_str, len(self.output_ids))
+        # Spec decode accepts multiple tokens per step; widen the window to cover
+        # the whole accepted chunk so a stop string landing mid-chunk (with more
+        # tokens accepted after it) is not pushed out of view.
+        tail_len = min(
+            max_len_tail_str + max(new_accepted_len - 1, 0), len(self.output_ids)
+        )
         return self.tokenizer.decode(self.output_ids[-tail_len:])
 
     def check_match_stop_str_prefix(self) -> bool:
@@ -1261,12 +1266,12 @@ class Req(ReqDllmMixin):
 
         return False
 
-    def _check_str_based_finish(self):
+    def _check_str_based_finish(self, new_accepted_len: int = 1):
         if (
             len(self.sampling_params.stop_strs) > 0
             or len(self.sampling_params.stop_regex_strs) > 0
         ):
-            tail_str = self.tail_str()
+            tail_str = self.tail_str(new_accepted_len)
 
             # Check stop strings
             if len(self.sampling_params.stop_strs) > 0:
@@ -1331,7 +1336,7 @@ class Req(ReqDllmMixin):
         if self._check_vocab_boundary_finish(new_accepted_tokens):
             return
 
-        if self._check_str_based_finish():
+        if self._check_str_based_finish(new_accepted_len):
             return
 
     def reset_for_retract(self):

test/registered/unit/managers/test_stop_str_speculative.py

@@ -0,0 +1,60 @@
+"""Regression: under speculative decoding (multi-token commits) a stop string
+committed mid-chunk must still trigger the finish check, else the request
+over-generates. Drives the real `Req.update_finish_state`; pure CPU."""
+
+import unittest
+from array import array
+
+from sglang.srt.managers.schedule_batch import Req
+from sglang.srt.sampling.sampling_params import SamplingParams
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+STOP_ID = 1
+ID_TO_TEXT = {STOP_ID: "STOP", **{i: chr(ord("a") + i % 26) for i in range(10, 40)}}
+
+# "STOP" (index 3) sits 6 tokens back: outside the old (stop_str_max_len + 1)
+# window, inside the one widened by new_accepted_len.
+MIDCHUNK = [10, 11, 12, STOP_ID, 20, 21, 22, 23, 24]
+
+
+class _FakeTokenizer:
+    eos_token_id = -1
+    additional_stop_token_ids = None
+
+    def decode(self, ids):
+        return "".join(ID_TO_TEXT[int(i)] for i in ids)
+
+
+def _make_req(output_ids, stop):
+    sp = SamplingParams(max_new_tokens=1000, stop=stop)
+    sp.normalize(tokenizer=None)  # char-based stop_str_max_len
+    req = Req(
+        rid="t",
+        origin_input_text="",
+        origin_input_ids=array("q", [0]),
+        sampling_params=sp,
+        eos_token_ids=set(),
+        vocab_size=10_000,
+    )
+    req.tokenizer = _FakeTokenizer()
+    req.output_ids = array("q", output_ids)
+    return req
+
+
+class TestStopStrSpeculative(unittest.TestCase):
+    def test_stop_str_midchunk_finishes(self):
+        req = _make_req(MIDCHUNK, stop=["STOP"])
+        req.update_finish_state(new_accepted_len=6)
+        self.assertTrue(req.finished())
+        self.assertEqual(req.finished_reason.matched, "STOP")
+
+    def test_no_stop_str_does_not_finish(self):
+        req = _make_req([10, 11, 12, 20, 21, 22, 23, 24], stop=["STOP"])
+        req.update_finish_state(new_accepted_len=6)
+        self.assertFalse(req.finished())
+
+
+if __name__ == "__main__":
+    unittest.main()