[Model] Support SeedOss Reason Parser (#24263)

Signed-off-by: Yan Lu <luyan@nvidia.com>
Co-authored-by: Michael Goin <mgoin64@gmail.com>

vllm/reasoning/mistral_reasoning_parser.py

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+from functools import cached_property
+
 from vllm.logger import init_logger
 from vllm.reasoning import ReasoningParser, ReasoningParserManager
 from vllm.reasoning.deepseek_r1_reasoning_parser import (
@@ -31,11 +33,6 @@ class MistralReasoningParser(DeepSeekR1ReasoningParser):
                 "The model tokenizer must be passed to the ReasoningParser "
                 "constructor during construction.")
 
-        from mistral_common.tokens.tokenizers.base import SpecialTokens
-
-        self.start_token = SpecialTokens.begin_think
-        self.end_token = SpecialTokens.end_think
-
         self.start_token_id = tokenizer.tokenizer.get_control_token(
             self.start_token)
         self.end_token_id = tokenizer.tokenizer.get_control_token(
@@ -45,3 +42,15 @@ class MistralReasoningParser(DeepSeekR1ReasoningParser):
             raise RuntimeError(
                 "Mistral reasoning parser could not locate think start/end "
                 "tokens in the tokenizer!")
+
+    @cached_property
+    def start_token(self) -> str:
+        """The token that starts reasoning content."""
+        from mistral_common.tokens.tokenizers.base import SpecialTokens
+        return SpecialTokens.begin_think
+
+    @cached_property
+    def end_token(self) -> str:
+        """The token that ends reasoning content."""
+        from mistral_common.tokens.tokenizers.base import SpecialTokens
+        return SpecialTokens.end_think