[Bugfix] Treat <tool_call> as implicit reasoning end in Qwen3 parser (#35687)

Signed-off-by: Doug Campos <qmx@qmx.me>

tests/reasoning/test_qwen3_reasoning_parser.py

@@ -78,6 +78,25 @@ WITHOUT_THINK_STREAM = {
     "content": None,
 }
 
+# --- <tool_call> without </think> (implicit reasoning end) ---
+
+TOOL_CALL_BODY = (
+    "<tool_call>\n<function=bash>\n<parameter=command>"
+    "\ncat /etc/hosts\n</parameter>\n</function>\n</tool_call>"
+)
+
+TOOL_CALL_NO_THINK_END = {
+    "output": "I need to read the file.\n\n" + TOOL_CALL_BODY,
+    "reasoning": "I need to read the file.\n\n",
+    "content": TOOL_CALL_BODY,
+}
+
+TOOL_CALL_WITH_THINK_NO_END = {
+    "output": "<think>I need to read the file.\n\n" + TOOL_CALL_BODY,
+    "reasoning": "I need to read the file.\n\n",
+    "content": TOOL_CALL_BODY,
+}
+
 # --- Edge cases ---
 
 COMPLETE_REASONING = {
@@ -199,6 +218,26 @@ TEST_CASES = [
         TRUNCATED_NO_START_TOKEN_STREAM,
         id="truncated_no_start_token_stream",
     ),
+    pytest.param(
+        False,
+        TOOL_CALL_NO_THINK_END,
+        id="tool_call_no_think_end",
+    ),
+    pytest.param(
+        True,
+        TOOL_CALL_NO_THINK_END,
+        id="tool_call_no_think_end_stream",
+    ),
+    pytest.param(
+        False,
+        TOOL_CALL_WITH_THINK_NO_END,
+        id="tool_call_with_think_no_end",
+    ),
+    pytest.param(
+        True,
+        TOOL_CALL_WITH_THINK_NO_END,
+        id="tool_call_with_think_no_end_stream",
+    ),
 ]
 
 
@@ -255,6 +294,13 @@ MULTI_TOKEN_DELTA_CASES = [
         "content",
         id="no_start_end_grouped_with_content",
     ),
+    pytest.param(
+        # <tool_call> arrives in a separate delta after reasoning text
+        ["I need to read the file.\n\n", "<tool_call>\n<function=bash>"],
+        "I need to read the file.\n\n",
+        "<tool_call>\n<function=bash>",
+        id="tool_call_implicit_reasoning_end",
+    ),
 ]
 
 
@@ -296,6 +342,12 @@ THINKING_DISABLED_CASES = [
         "Some output without think tokens",
         id="thinking_disabled_no_think_tokens",
     ),
+    pytest.param(
+        "I need to read the file.\n\n" + TOOL_CALL_BODY,
+        None,
+        "I need to read the file.\n\n" + TOOL_CALL_BODY,
+        id="thinking_disabled_with_tool_call",
+    ),
 ]
 
 

vllm/reasoning/qwen3_reasoning_parser.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from collections.abc import Sequence
+from collections.abc import Iterable, Sequence
 from typing import TYPE_CHECKING
 
 from vllm.entrypoints.openai.engine.protocol import DeltaMessage
@@ -31,6 +31,10 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
     use an older chat template where the model generates <think> itself.
     This parser handles both styles: if <think> appears in the generated output
     it is stripped before extraction (non-streaming) or skipped (streaming).
+
+    NOTE: Qwen3.5 models may emit <tool_call> inside the thinking block
+    without closing </think> first. <tool_call> is treated as an implicit
+    end of reasoning, matching the approach in KimiK2ReasoningParser.
     """
 
     def __init__(self, tokenizer: "TokenizerLike", *args, **kwargs):
@@ -41,6 +45,11 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
         # pure content when the user explicitly disables it.
         self.thinking_enabled = chat_kwargs.get("enable_thinking", True)
 
+        self._tool_call_tag = "<tool_call>"
+        self._tool_call_token_id = self.vocab.get(self._tool_call_tag)
+        self._tool_call_end_tag = "</tool_call>"
+        self._tool_call_end_token_id = self.vocab.get(self._tool_call_end_tag)
+
     @property
     def start_token(self) -> str:
         """The token that starts reasoning content."""
@@ -51,6 +60,58 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
         """The token that ends reasoning content."""
         return "</think>"
 
+    def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
+        start_token_id = self.start_token_id
+        end_token_id = self.end_token_id
+        tool_call_token_id = self._tool_call_token_id
+        tool_call_end_token_id = self._tool_call_end_token_id
+
+        for i in range(len(input_ids) - 1, -1, -1):
+            token_id = input_ids[i]
+            if token_id == start_token_id:
+                # Found <think> before </think> or <tool_call>
+                return False
+            if token_id == end_token_id:
+                return True
+            if tool_call_token_id is not None and token_id == tool_call_token_id:
+                # Only treat as implicit reasoning end if this <tool_call>
+                # is NOT followed by </tool_call>.  Paired occurrences are
+                # template examples in the prompt, not model output.
+                if tool_call_end_token_id is not None and any(
+                    input_ids[j] == tool_call_end_token_id
+                    for j in range(i + 1, len(input_ids))
+                ):
+                    continue
+                return True
+        return False
+
+    def is_reasoning_end_streaming(
+        self, input_ids: Sequence[int], delta_ids: Iterable[int]
+    ) -> bool:
+        if super().is_reasoning_end_streaming(input_ids, delta_ids):
+            return True
+        if self._tool_call_token_id is not None:
+            return self._tool_call_token_id in delta_ids
+        return False
+
+    def extract_content_ids(self, input_ids: list[int]) -> list[int]:
+        """
+        Extract content token ids from the input_ids.
+        """
+        result = super().extract_content_ids(input_ids)
+        if result:
+            return result
+        # Fall back: content starts at <tool_call> (implicit reasoning end).
+        if (
+            self._tool_call_token_id is not None
+            and self._tool_call_token_id in input_ids
+        ):
+            tool_call_index = (
+                len(input_ids) - 1 - input_ids[::-1].index(self._tool_call_token_id)
+            )
+            return input_ids[tool_call_index:]
+        return []
+
     def extract_reasoning(
         self, model_output: str, request: "ChatCompletionRequest | ResponsesRequest"
     ) -> tuple[str | None, str | None]:
@@ -78,20 +139,24 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
             model_output_parts[2] if model_output_parts[1] else model_output_parts[0]
         )
 
-        if self.end_token not in model_output:
+        if self.end_token in model_output:
+            reasoning, _, content = model_output.partition(self.end_token)
+            return reasoning, content or None
+
         if not self.thinking_enabled:
             # Thinking explicitly disabled — treat everything as content.
             return None, model_output
+
+        # No </think> — check for implicit reasoning end via <tool_call>.
+        tool_call_index = model_output.find(self._tool_call_tag)
+        if tool_call_index != -1:
+            reasoning = model_output[:tool_call_index]
+            content = model_output[tool_call_index:]
+            return reasoning or None, content or None
         # Thinking enabled but no </think>: output was truncated.
         # Everything generated so far is reasoning.
         return model_output, None
 
-        # Extract reasoning content from the model output.
-        reasoning, _, content = model_output.partition(self.end_token)
-
-        final_content = content or None
-        return reasoning, final_content
-
     def extract_reasoning_streaming(
         self,
         previous_text: str,
@@ -135,6 +200,20 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
             # end_token_id in IDs but not in text (already stripped)
             return None
 
+        # Implicit reasoning end via <tool_call>.
+        if (
+            self._tool_call_token_id is not None
+            and self._tool_call_token_id in delta_token_ids
+        ):
+            tool_index = delta_text.find(self._tool_call_tag)
+            if tool_index >= 0:
+                reasoning = delta_text[:tool_index]
+                content = delta_text[tool_index:]
+                return DeltaMessage(
+                    reasoning=reasoning if reasoning else None,
+                    content=content if content else None,
+                )
+
         # No end token in this delta.
         if not delta_text:
             # Nothing left after stripping start token.
@@ -142,6 +221,11 @@ class Qwen3ReasoningParser(BaseThinkingReasoningParser):
         elif self.end_token_id in previous_token_ids:
             # End token already passed: everything is content now.
             return DeltaMessage(content=delta_text)
+        elif (
+            self._tool_call_token_id is not None
+            and self._tool_call_token_id in previous_token_ids
+        ):
+            return DeltaMessage(content=delta_text)
         else:
             # No end token yet: still in reasoning phase.
             return DeltaMessage(reasoning=delta_text)