fix validation: Only set tool_choice `auto` if at least one tool is provided (#8568)

tests/tool_use/test_chat_completion_request_validations.py

+import pytest
+
+from vllm.entrypoints.openai.protocol import ChatCompletionRequest
+
+
+def test_chat_completion_request_with_no_tools():
+    # tools key is not present
+    request = ChatCompletionRequest.model_validate({
+        'messages': [{
+            'role': 'user',
+            'content': 'Hello'
+        }],
+        'model':
+        'facebook/opt-125m',
+    })
+    assert request.tool_choice == 'none'
+
+    # tools key is None
+    request = ChatCompletionRequest.model_validate({
+        'messages': [{
+            'role': 'user',
+            'content': 'Hello'
+        }],
+        'model':
+        'facebook/opt-125m',
+        'tools':
+        None
+    })
+    assert request.tool_choice == 'none'
+
+    # tools key present but empty
+    request = ChatCompletionRequest.model_validate({
+        'messages': [{
+            'role': 'user',
+            'content': 'Hello'
+        }],
+        'model':
+        'facebook/opt-125m',
+        'tools': []
+    })
+    assert request.tool_choice == 'none'
+
+
+def test_chat_completion_request_with_tool_choice_but_no_tools():
+    with pytest.raises(ValueError,
+                       match="When using `tool_choice`, `tools` must be set."):
+        ChatCompletionRequest.model_validate({
+            'messages': [{
+                'role': 'user',
+                'content': 'Hello'
+            }],
+            'model':
+            'facebook/opt-125m',
+            'tool_choice':
+            'auto'
+        })
+
+    with pytest.raises(ValueError,
+                       match="When using `tool_choice`, `tools` must be set."):
+        ChatCompletionRequest.model_validate({
+            'messages': [{
+                'role': 'user',
+                'content': 'Hello'
+            }],
+            'model':
+            'facebook/opt-125m',
+            'tool_choice':
+            'auto',
+            'tools':
+            None
+        })

vllm/entrypoints/openai/protocol.py

@@ -386,7 +386,7 @@ class ChatCompletionRequest(OpenAIBaseModel):
 
         # if "tool_choice" is not specified but tools are provided,
         # default to "auto" tool_choice
-        if "tool_choice" not in data and "tools" in data:
+        if "tool_choice" not in data and data.get("tools"):
             data["tool_choice"] = "auto"
 
         # if "tool_choice" is specified -- validation