[Bugfix] Fix the issue with interleaved thinking when using streaming (#30033)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>
Signed-off-by: Chauncey <chaunceyjiang@gmail.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>

tests/reasoning/test_base_thinking_reasoning_parser.py

@@ -112,7 +112,7 @@ class TestBaseThinkingReasoningParserMethods:
         """Test the is_reasoning_end method."""
         parser = TestThinkingReasoningParser(test_tokenizer)
         end_token_id = parser.end_token_id
-
+        start_token_id = parser.start_token_id
         # Test with end token present
         assert parser.is_reasoning_end([1, 2, end_token_id, 4]) is True
 
@@ -122,6 +122,16 @@ class TestBaseThinkingReasoningParserMethods:
         # Test with empty list
         assert parser.is_reasoning_end([]) is False
 
+        # Test with interleaved thinking
+        assert parser.is_reasoning_end([1, start_token_id, 2, end_token_id]) is True
+        assert parser.is_reasoning_end([1, start_token_id, 2, 3]) is False
+        assert (
+            parser.is_reasoning_end(
+                [1, start_token_id, 2, end_token_id, 2, 2, start_token_id]
+            )
+            is False
+        )
+
     def test_extract_content_ids(self, test_tokenizer):
         """Test the extract_content_ids method."""
         parser = TestThinkingReasoningParser(test_tokenizer)

vllm/reasoning/basic_parsers.py

@@ -64,8 +64,15 @@ class BaseThinkingReasoningParser(ReasoningParser):
             )
 
     def is_reasoning_end(self, input_ids: list[int]) -> bool:
+        start_token_id = self.start_token_id
         end_token_id = self.end_token_id
-        return any(input_id == end_token_id for input_id in reversed(input_ids))
+
+        for i in range(len(input_ids) - 1, -1, -1):
+            if input_ids[i] == start_token_id:
+                return False
+            if input_ids[i] == end_token_id:
+                return True
+        return False
 
     def extract_content_ids(self, input_ids: list[int]) -> list[int]:
         """