[Bugfix] Honor tool_choice="none" in Chat Completions streaming (#42752)

Signed-off-by: hoobnn <111053672+hoobnn@users.noreply.github.com>
Signed-off-by: sfeng33 <4florafeng@gmail.com>
Co-authored-by: sfeng33 <4florafeng@gmail.com>

tests/parser/test_streaming.py

@@ -36,11 +36,24 @@ def tokenizer():
     return get_tokenizer("Qwen/Qwen3-32B")
 
 
+TOOLS = [
+    {
+        "type": "function",
+        "function": {
+            "name": "get_weather",
+            "parameters": {"type": "object", "properties": {}},
+        },
+    }
+]
+
+
 @pytest.fixture
 def request_obj():
     return ChatCompletionRequest(
         model="test-model",
         messages=[{"role": "user", "content": "hi"}],
+        tools=TOOLS,
+        tool_choice="auto",
     )
 
 
@@ -328,3 +341,27 @@ def test_parse_delta_finished_appends_remaining_args(tokenizer, request_obj):
         tc.function.arguments for tc in tool_calls if tc.function.arguments
     )
     assert tool_args.endswith(remainder)
+
+
+def test_parse_delta_tool_choice_none(tokenizer, request_obj):
+    parser = make_parser(tokenizer, reasoning=False, tool=True)
+    request = request_obj.model_copy(update={"tool_choice": "none"})
+    results = stream_text(parser, tokenizer, MODEL_OUTPUT, request, prompt_token_ids=[])
+    reasoning, content, tool_calls = collect_fields(results)
+
+    assert reasoning == ""
+    assert len(tool_calls) == 0
+    assert "<tool_call>" in content
+    assert "get_weather" in content
+
+
+def test_parse_delta_tool_choice_none_with_reasoning(tokenizer, request_obj):
+    parser = make_parser(tokenizer, reasoning=True, tool=True)
+    request = request_obj.model_copy(update={"tool_choice": "none"})
+    results = stream_text(parser, tokenizer, MODEL_OUTPUT, request, prompt_token_ids=[])
+    reasoning, content, tool_calls = collect_fields(results)
+
+    assert "let me think about this" in reasoning
+    assert len(tool_calls) == 0
+    assert "<tool_call>" in content
+    assert "get_weather" in content

vllm/parser/abstract_parser.py

@@ -706,6 +706,9 @@ class DelegatingParser(Parser):
         tool_call_id_type: str = "random",
         function_name_returned: bool = False,
     ) -> tuple[DeltaMessage | None, bool]:
+        if request.tool_choice == "none":
+            return (DeltaMessage(content=delta_text) if delta_text else None), False
+
         assert self._tool_parser is not None
         supports_required_and_named = self._tool_parser.supports_required_and_named
         if (