Add KimiK2Detector with tool interruption support (#19696)

Signed-off-by: Xinyuan Tong <xinyuantong.cs@gmail.com>

python/sglang/srt/parser/reasoning_parser.py

@@ -268,6 +268,34 @@ class KimiDetector(BaseReasoningFormatDetector):
         )
 
 
+class KimiK2Detector(BaseReasoningFormatDetector):
+    """
+    Detector for Kimi K2 models.
+    Assumes reasoning format:
+      (<think>)*(.*)</think>
+
+    Kimi K2 can switch from reasoning to tool-call section with
+    `<|tool_calls_section_begin|>` before emitting `</think>`.
+    """
+
+    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,
+            tool_start_token="<|tool_calls_section_begin|>",
+            continue_final_message=continue_final_message,
+            previous_content=previous_content,
+        )
+
+
 class Glm45Detector(BaseReasoningFormatDetector):
     """
     Detector for GLM-4.5 models.
@@ -431,7 +459,7 @@ class ReasoningParser:
         "glm45": Glm45Detector,
         "gpt-oss": GptOssDetector,
         "kimi": KimiDetector,
-        "kimi_k2": Qwen3Detector,
+        "kimi_k2": KimiK2Detector,
         "qwen3": Qwen3Detector,
         "qwen3-thinking": Qwen3Detector,
         "minimax": Qwen3Detector,

test/registered/parser/test_reasoning_parser.py

@@ -5,6 +5,7 @@ from sglang.srt.parser.reasoning_parser import (
     DeepSeekR1Detector,
     Glm45Detector,
     KimiDetector,
+    KimiK2Detector,
     Qwen3Detector,
     ReasoningParser,
     StreamingParseResult,
@@ -314,6 +315,51 @@ class TestKimiDetector(CustomTestCase):
         self.assertEqual(result.normal_text, "answer")
 
 
+class TestKimiK2Detector(CustomTestCase):
+    """Test cases for KimiK2 detector with tool interruption support."""
+
+    def setUp(self):
+        self.detector = KimiK2Detector()
+
+    def test_init(self):
+        """Test KimiK2Detector initialization."""
+        self.assertEqual(self.detector.think_start_token, "<think>")
+        self.assertEqual(self.detector.think_end_token, "</think>")
+        self.assertEqual(self.detector.tool_start_token, "<|tool_calls_section_begin|>")
+        self.assertFalse(self.detector._in_reasoning)
+        self.assertTrue(self.detector.stream_reasoning)
+
+    def test_detect_and_parse_tool_interrupt(self):
+        """Test parsing with Kimi-K2 tool-section interruption."""
+        text = "<think>thinking<|tool_calls_section_begin|><|tool_call_begin|>"
+        result = self.detector.detect_and_parse(text)
+        self.assertEqual(result.reasoning_text, "thinking")
+        self.assertEqual(
+            result.normal_text, "<|tool_calls_section_begin|><|tool_call_begin|>"
+        )
+
+    def test_streaming_tool_interrupt(self):
+        """Test streaming parse interrupted by tool section."""
+        self.detector.parse_streaming_increment("<think>")
+        result1 = self.detector.parse_streaming_increment("reasoning")
+        self.assertEqual(result1.reasoning_text, "reasoning")
+        self.assertEqual(result1.normal_text, "")
+
+        result2 = self.detector.parse_streaming_increment(
+            "<|tool_calls_section_begin|>"
+        )
+        self.assertEqual(result2.reasoning_text, "")
+        self.assertEqual(result2.normal_text, "<|tool_calls_section_begin|>")
+
+    def test_streaming_after_interrupt_is_normal(self):
+        """After interruption, subsequent chunks should be normal text."""
+        self.detector.parse_streaming_increment("<think>")
+        self.detector.parse_streaming_increment("reasoning<|tool_calls_section_begin|>")
+        result = self.detector.parse_streaming_increment("<|tool_call_begin|>")
+        self.assertEqual(result.reasoning_text, "")
+        self.assertEqual(result.normal_text, "<|tool_call_begin|>")
+
+
 class TestGlm45Detector(CustomTestCase):
     """Test cases for GLM45 detector with tool interruption support."""
 
@@ -478,6 +524,9 @@ class TestReasoningParser(CustomTestCase):
         parser = ReasoningParser("kimi")
         self.assertIsInstance(parser.detector, KimiDetector)
 
+        parser = ReasoningParser("kimi_k2")
+        self.assertIsInstance(parser.detector, KimiK2Detector)
+
         parser = ReasoningParser("glm45")
         self.assertIsInstance(parser.detector, Glm45Detector)
 
@@ -565,6 +614,37 @@ class TestReasoningParser(CustomTestCase):
         self.assertEqual(all_reasoning, "reasoning")
         self.assertEqual(all_normal, "<tool_call>tool args")
 
+    def test_kimik2_tool_interruption(self):
+        """Test Kimi-K2 tool interruption through ReasoningParser API."""
+        parser = ReasoningParser("kimi_k2")
+
+        # Non-streaming: tool interrupt
+        reasoning, normal = parser.parse_non_stream(
+            "<think>thinking<|tool_calls_section_begin|><|tool_call_begin|>"
+        )
+        self.assertEqual(reasoning, "thinking")
+        self.assertEqual(normal, "<|tool_calls_section_begin|><|tool_call_begin|>")
+
+        # Streaming: tool interrupt
+        parser = ReasoningParser("kimi_k2")
+        chunks = [
+            "<think>",
+            "reasoning",
+            "<|tool_calls_section_begin|>",
+            "<|tool_call_begin|>",
+        ]
+        all_reasoning = ""
+        all_normal = ""
+        for chunk in chunks:
+            reasoning, normal = parser.parse_stream_chunk(chunk)
+            if reasoning:
+                all_reasoning += reasoning
+            if normal:
+                all_normal += normal
+
+        self.assertEqual(all_reasoning, "reasoning")
+        self.assertEqual(all_normal, "<|tool_calls_section_begin|><|tool_call_begin|>")
+
 
 class TestIntegrationScenarios(CustomTestCase):
     """Integration tests for realistic usage scenarios."""