Fix regression: validate tool_choice=auto when enable_auto_tools=True but tool_parser=None

The previous logic missed the case where:
- tool_choice="auto"
- enable_auto_tools=True
- tool_parser=None

In this scenario, no error was returned, but it should error since
tool_parser is still required.

Also added regression test to cover this case.

Signed-off-by: majiayu000 <[email protected]>

Author: majiayu000

tests/entrypoints/openai/test_serving_chat.py

@@ -1440,3 +1440,27 @@ async def test_tool_choice_validation_without_parser():
     error_body = response_named.body.decode()
     assert "tool_choice" in error_body
     assert "--tool-call-parser" in error_body
+
+    # Test tool_choice="auto" with enable_auto_tools=True but no tool_parser
+    # This is a regression test: enable_auto_tools is set but tool_parser is None
+    serving_chat_auto_enabled = OpenAIServingChat(
+        mock_engine,
+        models,
+        response_role="assistant",
+        chat_template=CHAT_TEMPLATE,
+        chat_template_content_format="auto",
+        request_logger=None,
+        enable_auto_tools=True,  # Enabled but no tool_parser configured
+    )
+
+    req_auto = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[{"role": "user", "content": "What's the weather?"}],
+        tools=tools,
+        tool_choice="auto",
+    )
+    response_auto = await serving_chat_auto_enabled.create_chat_completion(req_auto)
+    assert hasattr(response_auto, "body")
+    error_body = response_auto.body.decode()
+    assert "auto" in error_body
+    assert "--tool-call-parser" in error_body

vllm/entrypoints/openai/serving_chat.py

@@ -216,14 +216,21 @@ async def create_chat_completion(
                 None,
                 "none",
             ):
-                if request.tool_choice == "auto" and not self.enable_auto_tools:
-                    # for hf tokenizers, "auto" tools requires
-                    # --enable-auto-tool-choice and --tool-call-parser
+                if request.tool_choice == "auto":
+                    if not self.enable_auto_tools:
+                        # for hf tokenizers, "auto" tools requires
+                        # --enable-auto-tool-choice and --tool-call-parser
+                        return self.create_error_response(
+                            '"auto" tool choice requires '
+                            "--enable-auto-tool-choice and "
+                            "--tool-call-parser to be set"
+                        )
+                    # enable_auto_tools is set but tool_parser is None
                     return self.create_error_response(
                         '"auto" tool choice requires '
-                        "--enable-auto-tool-choice and --tool-call-parser to be set"
+                        "--tool-call-parser to be set"
                     )
-                elif request.tool_choice != "auto":
+                else:
                     # "required" or named tool requires tool parser
                     return self.create_error_response(
                         f'tool_choice="{request.tool_choice}" requires '