[Bugfix] [Reasoning] Add reasoning_start_str/reasoning_end_str properties to...

[Bugfix] [Reasoning] Add reasoning_start_str/reasoning_end_str properties to reasoning parsers (#40566)
Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

vllm/reasoning/deepseek_v3_reasoning_parser.py

@@ -40,6 +40,14 @@ class DeepSeekV3ReasoningParser(ReasoningParser):
         else:
             self._parser = IdentityReasoningParser(tokenizer, *args, **kwargs)
 
+    @property
+    def reasoning_start_str(self) -> str | None:
+        return self._parser.reasoning_start_str
+
+    @property
+    def reasoning_end_str(self) -> str | None:
+        return self._parser.reasoning_end_str
+
     def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
         return self._parser.is_reasoning_end(input_ids)
 

vllm/reasoning/identity_reasoning_parser.py

@@ -33,6 +33,14 @@ class IdentityReasoningParser(ReasoningParser):
                 "constructor during construction."
             )
 
+    @property
+    def reasoning_start_str(self) -> str | None:
+        return None
+
+    @property
+    def reasoning_end_str(self) -> str | None:
+        return None
+
     def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
         # Always return True, since we never treat reasoning specially
         return True

vllm/reasoning/kimi_k2_reasoning_parser.py

@@ -65,6 +65,14 @@ class KimiK2ReasoningParser(ReasoningParser):
                 "tokens in the tokenizer!"
             )
 
+    @property
+    def reasoning_start_str(self) -> str | None:
+        return self._start_token
+
+    @property
+    def reasoning_end_str(self) -> str | None:
+        return self._end_token
+
     def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
         """
         Check if the reasoning content ends in the input_ids.

vllm/reasoning/olmo3_reasoning_parser.py

@@ -237,6 +237,14 @@ class Olmo3ReasoningParser(ReasoningParser):
             think_start=self.think_start, think_end=self.think_end
         )
 
+    @property
+    def reasoning_start_str(self) -> str:
+        return self.think_start
+
+    @property
+    def reasoning_end_str(self) -> str:
+        return self.think_end
+
     def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
         text = self.model_tokenizer.decode(input_ids)
         return self.think_end in text

vllm/reasoning/step3_reasoning_parser.py

@@ -29,6 +29,7 @@ class Step3ReasoningParser(ReasoningParser):
 
     def __init__(self, tokenizer: PreTrainedTokenizerBase, *args, **kwargs):
         super().__init__(tokenizer, *args, **kwargs)
+        self.think_start_token = "<think>"
         self.think_end_token = "</think>"
 
         self.reasoning_regex = re.compile(rf"(.*?){self.think_end_token}", re.DOTALL)
@@ -47,6 +48,14 @@ class Step3ReasoningParser(ReasoningParser):
             )
         self.think_end_token_id: int = think_end_token_id
 
+    @property
+    def reasoning_start_str(self) -> str:
+        return self.think_start_token
+
+    @property
+    def reasoning_end_str(self) -> str:
+        return self.think_end_token
+
     def extract_reasoning_streaming(
         self,
         previous_text: str,