Add Mistral Small 4 (Pixtral) support (#20708)

Signed-off-by: Xinyuan Tong <xinyuantong.cs@gmail.com>
Co-authored-by: Alex Nails <alexnails@radixark.ai>
Co-authored-by: Dimitrios Bariamis <12195802+dbari@users.noreply.github.com>
Co-authored-by: dbari <dbari@users.noreply.github.com>

python/sglang/srt/parser/reasoning_parser.py

@@ -450,6 +450,33 @@ class Nemotron3Detector(BaseReasoningFormatDetector):
         return ret
 
 
+class MistralDetector(BaseReasoningFormatDetector):
+    """
+    Detector for Mistral models with reasoning (e.g., Mistral-Small-4-119B-2603).
+    Assumes reasoning format:
+      [THINK]reasoning content[/THINK]answer
+
+    Reasoning is optional — it only appears when reasoning_effort="high" is set.
+    When reasoning_effort="none", the model outputs directly without thinking tokens.
+    """
+
+    def __init__(
+        self,
+        stream_reasoning: bool = True,
+        force_reasoning: bool = False,
+        continue_final_message: bool = False,
+        previous_content: str = "",
+    ):
+        super().__init__(
+            "[THINK]",
+            "[/THINK]",
+            force_reasoning=force_reasoning,
+            stream_reasoning=stream_reasoning,
+            continue_final_message=continue_final_message,
+            previous_content=previous_content,
+        )
+
+
 class ReasoningParser:
     """
     Parser that handles both streaming and non-streaming scenarios for extracting
@@ -474,6 +501,7 @@ class ReasoningParser:
         "minimax-append-think": MiniMaxAppendThinkDetector,
         "step3": DeepSeekR1Detector,
         "step3p5": DeepSeekR1Detector,
+        "mistral": MistralDetector,
         "nemotron_3": Nemotron3Detector,
         "interns1": Qwen3Detector,
     }