[Bugfix] Add validation for tool requests when tool_parser is unavailable

When tool_choice is "required" or a named tool, but the server was not
started with --enable-auto-tool-choice and --tool-call-parser, vLLM
would silently degrade instead of returning an error.

This fix adds validation to return a clear error message indicating that
these flags are required for tool parsing.

Fixes #29432

Author: majiayu000

tests/entrypoints/openai/test_serving_chat.py

@@ -1372,3 +1372,71 @@ async def test_non_tool_reasoning_empty_content_list(self, serving_chat):
                 },
             ],
         )
+
+
+@pytest.mark.asyncio
+async def test_tool_choice_validation_without_parser():
+    """Test that tool_choice='required' or named tool without tool_parser
+    returns an appropriate error message."""
+    mock_engine = MagicMock(spec=AsyncLLM)
+    mock_engine.get_tokenizer.return_value = get_tokenizer(MODEL_NAME)
+    mock_engine.errored = False
+    mock_engine.model_config = MockModelConfig()
+    mock_engine.input_processor = MagicMock()
+    mock_engine.io_processor = MagicMock()
+
+    models = OpenAIServingModels(
+        engine_client=mock_engine,
+        base_model_paths=BASE_MODEL_PATHS,
+    )
+    # Create serving_chat without tool_parser (enable_auto_tools=False)
+    serving_chat = OpenAIServingChat(
+        mock_engine,
+        models,
+        response_role="assistant",
+        chat_template=CHAT_TEMPLATE,
+        chat_template_content_format="auto",
+        request_logger=None,
+        enable_auto_tools=False,  # No tool parser
+    )
+
+    tools = [
+        {
+            "type": "function",
+            "function": {
+                "name": "get_weather",
+                "description": "Get the weather in a given location",
+                "parameters": {
+                    "type": "object",
+                    "properties": {"location": {"type": "string"}},
+                    "required": ["location"],
+                },
+            },
+        }
+    ]
+
+    # Test tool_choice="required" without tool_parser
+    req_required = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[{"role": "user", "content": "What's the weather?"}],
+        tools=tools,
+        tool_choice="required",
+    )
+    response_required = await serving_chat.create_chat_completion(req_required)
+    assert hasattr(response_required, "body")
+    error_body = response_required.body.decode()
+    assert "tool_choice" in error_body
+    assert "--enable-auto-tool-choice" in error_body
+
+    # Test named tool_choice without tool_parser
+    req_named = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[{"role": "user", "content": "What's the weather?"}],
+        tools=tools,
+        tool_choice={"type": "function", "function": {"name": "get_weather"}},
+    )
+    response_named = await serving_chat.create_chat_completion(req_named)
+    assert hasattr(response_named, "body")
+    error_body = response_named.body.decode()
+    assert "tool_choice" in error_body
+    assert "--enable-auto-tool-choice" in error_body

vllm/entrypoints/openai/serving_chat.py

@@ -217,6 +217,20 @@ async def create_chat_completion(
                     "--enable-auto-tool-choice and --tool-call-parser to be set"
                 )
 
+            # Validate that tool_parser is available when tool_choice requires
+            # tool parsing (i.e., "required" or a named tool)
+            if (
+                request.tool_choice is not None
+                and request.tool_choice not in ("none", "auto")
+                and tool_parser is None
+                and not isinstance(tokenizer, MistralTokenizer)
+                and not self.use_harmony
+            ):
+                return self.create_error_response(
+                    f'tool_choice="{request.tool_choice}" requires '
+                    "--enable-auto-tool-choice and --tool-call-parser to be set"
+                )
+
             if request.tools is None or (
                 request.tool_choice == "none"
                 and self.exclude_tools_when_tool_choice_none