Add think chunk (#21333)

Signed-off-by: Julien Denize <julien.denize@mistral.ai>

vllm/reasoning/mistral_reasoning_parser.py

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from vllm.logger import init_logger
+from vllm.reasoning import ReasoningParser, ReasoningParserManager
+from vllm.reasoning.deepseek_r1_reasoning_parser import (
+    DeepSeekR1ReasoningParser)
+from vllm.transformers_utils.tokenizers.mistral import MistralTokenizer
+
+logger = init_logger(__name__)
+
+
+@ReasoningParserManager.register_module("mistral")
+class MistralReasoningParser(DeepSeekR1ReasoningParser):
+    """
+    Reasoning parser for Mistral models.
+
+    The Mistral models uses [THINK]...[/THINK] tokens to denote reasoning
+    text. This parser extracts the reasoning content from the model output.
+    """
+
+    def __init__(self, tokenizer: MistralTokenizer):
+        if not isinstance(tokenizer, MistralTokenizer):
+            raise ValueError(
+                "The tokenizer must be an instance of MistralTokenizer.")
+
+        ReasoningParser.__init__(self, tokenizer)
+
+        if not self.model_tokenizer:
+            raise ValueError(
+                "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(
+            self.end_token)
+
+        if self.start_token_id is None or self.end_token_id is None:
+            raise RuntimeError(
+                "Mistral reasoning parser could not locate think start/end "
+                "tokens in the tokenizer!")