Flush dropped reasoning at stream end when stream_reasoning=False (#32225)

python/sglang/srt/parser/reasoning_parser.py

@@ -227,26 +227,36 @@ class BaseReasoningFormatDetector:
 
         return StreamingParseResult()
 
-    def finish(self) -> StreamingParseResult:
+    def _strip_leading_think_start(self, text: str) -> str:
-        """
-        Called once when the stream ends. If force_nonempty_content is set
-        and the stream ended mid-reasoning, reclassifies the accumulated
-        reasoning (plus any partial token still buffered) as normal text.
-        """
-        if self._force_nonempty_content and self._in_reasoning:
-            # stream_reasoning=False never clears _buffer, so the opening think
-            # token (stripped only from the base class's local view) survives here.
-            buffer = self._buffer
         think_start_text = self.think_start_token + self.think_start_self_label
-            if buffer.startswith(think_start_text):
+        if text.startswith(think_start_text):
-                buffer = buffer[len(think_start_text) :]
+            return text[len(think_start_text) :]
+        return text
+
+    def finish(self) -> StreamingParseResult:
+        """Flush reasoning buffered under stream_reasoning=False when the stream ends
+        before the end token (e.g. max_tokens cut it short), instead of dropping it.
+        force_nonempty_content emits it as normal_text, else as reasoning_text."""
+        if not self._in_reasoning:
+            return StreamingParseResult()
+
+        # stream_reasoning=False never clears _buffer, so the opening think token
+        # (stripped only from the base class's local view) survives here.
+        buffer = self._strip_leading_think_start(self._buffer)
+        self._buffer = ""
+
+        if self._force_nonempty_content:
             normal_text = self._accumulated_reasoning + buffer
             self._accumulated_reasoning = ""
-            self._buffer = ""
             if normal_text:
                 return StreamingParseResult(normal_text=normal_text)
             return StreamingParseResult()
 
+        if not self.stream_reasoning and buffer:
+            return StreamingParseResult(reasoning_text=buffer)
+
+        return StreamingParseResult()
+
 
 class DeepSeekR1Detector(BaseReasoningFormatDetector):
     """
@@ -1401,6 +1411,21 @@ class CohereCommand4Detector(BaseReasoningFormatDetector):
 
         return StreamingParseResult()
 
+    def finish(self) -> StreamingParseResult:
+        # _in_reasoning stays pinned True here (phase tracked via _reasoning_done), so
+        # the base finish() would misfile a truncated answer tail as reasoning.
+        buffer = self._buffer
+        self._buffer = ""
+        if not self._reasoning_done:
+            ret = StreamingParseResult(
+                reasoning_text=self._strip_leading_think_start(buffer)
+            )
+        elif self._saw_text_start and not self._saw_text_end:
+            ret = StreamingParseResult(normal_text=buffer)
+        else:
+            return StreamingParseResult()
+        return self._maybe_apply_force_nonempty_content(ret)
+
 
 class ReasoningParser:
     """

test/registered/unit/parser/test_reasoning_parser.py

@@ -5,6 +5,7 @@ import unittest
 from sglang.srt.parser.reasoning_parser import (
     Apertus2509Detector,
     BaseReasoningFormatDetector,
+    CohereCommand4Detector,
     DeepSeekR1Detector,
     DeepSeekV4Detector,
     Gemma4Detector,
@@ -135,6 +136,43 @@ class TestBaseReasoningFormatDetector(CustomTestCase):
         self.assertEqual(result.reasoning_text, "reasoning")
         self.assertEqual(result.normal_text, "normal")
 
+    def test_finish_flushes_truncated_reasoning_no_stream_reasoning(self):
+        """Bug regression: with stream_reasoning=False the base detector buffers
+        the whole thinking block and only emits it on </think>. A stream cut
+        short (e.g. max_tokens) before </think> left the trace stuck in _buffer,
+        and finish() dropped it. finish() must now emit it as reasoning, with the
+        opening think token stripped, matching the non-streaming path."""
+        detector = BaseReasoningFormatDetector(
+            "<think>", "</think>", stream_reasoning=False
+        )
+        detector.parse_streaming_increment("<think>")
+        self.assertEqual(
+            detector.parse_streaming_increment("half a thought").reasoning_text, ""
+        )
+        end = detector.finish()
+        self.assertEqual(end.reasoning_text, "half a thought")
+        self.assertEqual(end.normal_text, "")
+        # State is cleared, so a second finish() is a no-op (no duplicate flush).
+        self.assertEqual(detector._buffer, "")
+        self.assertEqual(detector.finish().reasoning_text, "")
+
+    def test_finish_drops_partial_end_tag_when_streaming_reasoning(self):
+        """With stream_reasoning=True the reasoning is emitted chunk by chunk, so
+        finish() must not re-emit. Only a partial end-tag fragment can linger in
+        _buffer; that fragment is an incomplete token, not content, and must be
+        dropped rather than surfaced as reasoning."""
+        detector = BaseReasoningFormatDetector(
+            "<think>", "</think>", stream_reasoning=True
+        )
+        self.assertEqual(
+            detector.parse_streaming_increment("<think>thought").reasoning_text,
+            "thought",
+        )
+        self.assertEqual(detector.parse_streaming_increment("</thi").reasoning_text, "")
+        end = detector.finish()
+        self.assertEqual(end.reasoning_text, "")
+        self.assertEqual(end.normal_text, "")
+
 
 class TestDeepSeekR1Detector(CustomTestCase):
     def setUp(self):
@@ -155,6 +193,22 @@ class TestDeepSeekR1Detector(CustomTestCase):
         self.assertEqual(result.reasoning_text, "I need to think about this.")
         self.assertEqual(result.normal_text, "The answer is 42.")
 
+    def test_finish_flushes_truncated_forced_reasoning(self):
+        """Bug regression: DeepSeek-R1 forces reasoning without a <think> start
+        token, so the whole output is reasoning until </think>. With
+        stream_reasoning=False a stream cut before </think> buffered the trace;
+        finish() must flush it as reasoning instead of dropping it."""
+        detector = DeepSeekR1Detector(stream_reasoning=False)
+        self.assertEqual(
+            detector.parse_streaming_increment(
+                "reasoning with no end token"
+            ).reasoning_text,
+            "",
+        )
+        end = detector.finish()
+        self.assertEqual(end.reasoning_text, "reasoning with no end token")
+        self.assertEqual(end.normal_text, "")
+
 
 class TestQwen3Detector(CustomTestCase):
     def setUp(self):
@@ -1277,5 +1331,37 @@ class TestPoolsideV1Registered(CustomTestCase):
         self.assertTrue(rp.detector.thinks_internally)
 
 
+class TestCohereCommand4DetectorFinish(CustomTestCase):
+    """finish() flush for Cohere's custom streaming state machine.
+
+    This detector pins _in_reasoning True for its whole run and tracks phase via
+    _reasoning_done, so it overrides finish() rather than inheriting the base
+    one, which keys on _in_reasoning."""
+
+    def test_finish_flushes_truncated_reasoning(self):
+        """Stream cut mid-thinking (stream_reasoning=False, no <|END_THINKING|>)
+        must flush the buffered trace as reasoning instead of dropping it."""
+        detector = CohereCommand4Detector(stream_reasoning=False)
+        self.assertEqual(
+            detector.parse_streaming_increment("partial thinking").reasoning_text,
+            "",
+        )
+        end = detector.finish()
+        self.assertEqual(end.reasoning_text, "partial thinking")
+        self.assertEqual(end.normal_text, "")
+
+    def test_finish_flushes_answer_tail_as_normal_text(self):
+        """Regression guard for the base-class fix: once reasoning has closed, a
+        truncated answer tail (stream ended before <|END_TEXT|>) must be flushed
+        as normal_text. The base finish() keyed on _in_reasoning would misfile it
+        as reasoning because this detector never clears _in_reasoning."""
+        detector = CohereCommand4Detector(stream_reasoning=False)
+        detector.parse_streaming_increment("thinking<|END_THINKING|>")
+        detector.parse_streaming_increment("<|START_TEXT|>the answer")
+        end = detector.finish()
+        self.assertEqual(end.normal_text, "the answer")
+        self.assertEqual(end.reasoning_text, "")
+
+
 if __name__ == "__main__":
     unittest.main()