[Bugfix] Fix Qwen3/Qwen3.5 Reasoning Parser (#34779)

Signed-off-by: Roger Wang <hey@rogerw.io>

tests/reasoning/test_qwen3_reasoning_parser.py

@@ -4,46 +4,79 @@
 import pytest
 from transformers import AutoTokenizer
 
-from tests.reasoning.utils import run_reasoning_extraction
+from tests.reasoning.utils import (
+    StreamingReasoningReconstructor,
+    run_reasoning_extraction,
+    run_reasoning_extraction_streaming,
+)
 from vllm.reasoning import ReasoningParser, ReasoningParserManager
 
 parser_name = "qwen3"
 start_token = "<think>"
 end_token = "</think>"
 
-REASONING_MODEL_NAME = "Qwen/Qwen3-0.6B"
+REASONING_MODEL_NAMES = [
+    "Qwen/Qwen3-0.6B",
+    "Qwen/Qwen3.5-397B-A17B",
+    "Qwen/Qwen3-4B-Thinking-2507",
+]
 
 
-@pytest.fixture(scope="module")
-def qwen3_tokenizer():
-    return AutoTokenizer.from_pretrained(REASONING_MODEL_NAME)
+@pytest.fixture(scope="module", params=REASONING_MODEL_NAMES)
+def qwen3_tokenizer(request):
+    return AutoTokenizer.from_pretrained(request.param)
 
 
-# 带 <think></think>，非stream
+# --- <think> in prompt, only </think> in output (typical) ---
+
+WITHOUT_START_TOKEN = {
+    "output": "This is a reasoning section</think>This is the rest",
+    "reasoning": "This is a reasoning section",
+    "content": "This is the rest",
+}
+WITHOUT_START_TOKEN_STREAM = {
+    "output": "This is a reasoning section</think>This is the rest",
+    "reasoning": "This is a reasoning section",
+    "content": "This is the rest",
+}
+WITHOUT_START_TOKEN_COMPLETE_REASONING = {
+    "output": "This is a reasoning section</think>",
+    "reasoning": "This is a reasoning section",
+    "content": None,
+}
+
+# --- <think> present in output (old template / edge case) ---
+
 WITH_THINK = {
     "output": "<think>This is a reasoning section</think>This is the rest",
     "reasoning": "This is a reasoning section",
     "content": "This is the rest",
 }
-# 带 <think></think>，stream
 WITH_THINK_STREAM = {
     "output": "<think>This is a reasoning section</think>This is the rest",
     "reasoning": "This is a reasoning section",
     "content": "This is the rest",
 }
-# 不带 <think></think>，非stream
+
+# --- No think tokens at all (thinking disabled) ---
+
 WITHOUT_THINK = {
     "output": "This is the rest",
     "reasoning": None,
     "content": "This is the rest",
 }
-# 不带 <think></think>，stream
+# In streaming, the parser cannot distinguish "thinking disabled" from
+# "reasoning in progress" when no think tokens have appeared yet.
+# It assumes reasoning. The serving layer handles the "thinking disabled"
+# case by checking prompt_is_reasoning_end_arr before calling the parser.
 WITHOUT_THINK_STREAM = {
     "output": "This is the rest",
-    "reasoning": None,
-    "content": "This is the rest",
+    "reasoning": "This is the rest",
+    "content": None,
 }
 
+# --- Edge cases ---
+
 COMPLETE_REASONING = {
     "output": "<think>This is a reasoning section</think>",
     "reasoning": "This is a reasoning section",
@@ -57,7 +90,7 @@ MULTILINE_REASONING = {
 ONLY_OPEN_TAG = {
     "output": "<think>This is a reasoning section",
     "reasoning": None,
-    "content": "<think>This is a reasoning section",
+    "content": "This is a reasoning section",
 }
 
 ONLY_OPEN_TAG_STREAM = {
@@ -67,6 +100,26 @@ ONLY_OPEN_TAG_STREAM = {
 }
 
 TEST_CASES = [
+    pytest.param(
+        False,
+        WITHOUT_START_TOKEN,
+        id="without_start_token",
+    ),
+    pytest.param(
+        True,
+        WITHOUT_START_TOKEN_STREAM,
+        id="without_start_token_stream",
+    ),
+    pytest.param(
+        False,
+        WITHOUT_START_TOKEN_COMPLETE_REASONING,
+        id="without_start_token_complete_reasoning",
+    ),
+    pytest.param(
+        True,
+        WITHOUT_START_TOKEN_COMPLETE_REASONING,
+        id="without_start_token_complete_reasoning_stream",
+    ),
     pytest.param(
         False,
         WITH_THINK,
@@ -140,3 +193,59 @@ def test_reasoning(
 
     assert reasoning == param_dict["reasoning"]
     assert content == param_dict["content"]
+
+
+# Multi-token delta tests: simulate real-world streaming where a single
+# delta can contain multiple tokens (e.g., speculative decoding).
+MULTI_TOKEN_DELTA_CASES = [
+    pytest.param(
+        # <think> grouped with following text in one delta
+        ["<think>This is a reasoning section", "</think>", "This is the rest"],
+        "This is a reasoning section",
+        "This is the rest",
+        id="start_token_grouped_with_text",
+    ),
+    pytest.param(
+        # </think> grouped with following content in one delta
+        ["reasoning section", "</think>This is the rest"],
+        "reasoning section",
+        "This is the rest",
+        id="end_token_grouped_with_content",
+    ),
+    pytest.param(
+        # <think> and </think> in the same delta, no content after
+        ["<think>reasoning</think>"],
+        "reasoning",
+        None,
+        id="start_and_end_in_one_delta_no_content",
+    ),
+    pytest.param(
+        # No start token, end grouped with content (Qwen3.5 style)
+        ["reasoning section", "</think>content"],
+        "reasoning section",
+        "content",
+        id="no_start_end_grouped_with_content",
+    ),
+]
+
+
+@pytest.mark.parametrize(
+    "deltas, expected_reasoning, expected_content", MULTI_TOKEN_DELTA_CASES
+)
+def test_reasoning_streaming_multi_token_deltas(
+    deltas: list[str],
+    expected_reasoning: str | None,
+    expected_content: str | None,
+    qwen3_tokenizer,
+):
+    """Test that multi-token deltas don't leak <think> into reasoning."""
+    parser: ReasoningParser = ReasoningParserManager.get_reasoning_parser(parser_name)(
+        qwen3_tokenizer
+    )
+
+    reconstructor: StreamingReasoningReconstructor = run_reasoning_extraction_streaming(
+        parser, deltas
+    )
+
+    assert reconstructor.reasoning == expected_reasoning
+    assert (reconstructor.other_content or None) == expected_content