[Frontend] Align tool_choice="required" behavior with OpenAI when tools is empty (#21052)

Signed-off-by: Sungyoon Jeong <sungyoon.jeong@furiosa.ai>

vllm/entrypoints/openai/protocol.py

@@ -859,6 +859,15 @@ class ChatCompletionRequest(OpenAIBaseModel):
                     'are supported.'
                 )
 
+            # if tool_choice is "required" but the "tools" list is empty,
+            # override the data to behave like "none" to align with
+            # OpenAI’s behavior.
+            if data["tool_choice"] == "required" and isinstance(
+                    data["tools"], list) and len(data["tools"]) == 0:
+                data["tool_choice"] = "none"
+                del data["tools"]
+                return data
+
             # ensure that if "tool_choice" is specified as an object,
             # it matches a valid tool
             correct_usage_message = 'Correct usage: `{"type": "function",' \