Revert tool_choice/parallel_tool_calls changes - must be removed when no tools

OpenAI API requires tool_choice and parallel_tool_calls to only be
present when tools are specified. Restored the logic that removes
these options when there are no tools.

- Restored check in _chat_client.py to remove tool_choice and
  parallel_tool_calls when no tools present
- Restored same logic in _responses_client.py
- Reverted test to expect the correct behavior

Author: eavanvalkenburg

python/packages/core/agent_framework/openai/_chat_client.py

@@ -282,7 +282,11 @@ def _prepare_options(self, messages: Sequence[ChatMessage], options: Mapping[str
         tools = options.get("tools")
         if tools is not None:
             run_options.update(self._prepare_tools_for_openai(tools))
-        if tool_choice := run_options.pop("tool_choice", None):
+        # Only include tool_choice and parallel_tool_calls if tools are present
+        if not run_options.get("tools"):
+            run_options.pop("parallel_tool_calls", None)
+            run_options.pop("tool_choice", None)
+        elif tool_choice := run_options.pop("tool_choice", None):
             tool_mode = validate_tool_mode(tool_choice)
             if (mode := tool_mode.get("mode")) == "required" and (
                 func_name := tool_mode.get("required_function_name")

python/packages/core/agent_framework/openai/_responses_client.py

@@ -577,19 +577,21 @@ async def _prepare_options(
         # tools
         if tools := self._prepare_tools_for_openai(options.get("tools")):
             run_options["tools"] = tools
-
-        # tool_choice: convert ToolMode to appropriate format (keep even if no tools)
-        if tool_choice := options.get("tool_choice"):
-            tool_mode = validate_tool_mode(tool_choice)
-            if (mode := tool_mode.get("mode")) == "required" and (
-                func_name := tool_mode.get("required_function_name")
-            ) is not None:
-                run_options["tool_choice"] = {
-                    "type": "function",
-                    "name": func_name,
-                }
-            else:
-                run_options["tool_choice"] = mode
+            # tool_choice: convert ToolMode to appropriate format
+            if tool_choice := options.get("tool_choice"):
+                tool_mode = validate_tool_mode(tool_choice)
+                if (mode := tool_mode.get("mode")) == "required" and (
+                    func_name := tool_mode.get("required_function_name")
+                ) is not None:
+                    run_options["tool_choice"] = {
+                        "type": "function",
+                        "name": func_name,
+                    }
+                else:
+                    run_options["tool_choice"] = mode
+        else:
+            run_options.pop("parallel_tool_calls", None)
+            run_options.pop("tool_choice", None)
 
         # response format and text config
         response_format = options.get("response_format")

python/packages/core/tests/openai/test_openai_chat_client.py

@@ -890,17 +890,17 @@ def test_multiple_function_calls_in_single_message(openai_unit_test_env: dict[st
     assert prepared[0]["tool_calls"][1]["id"] == "call_2"
 
 
-def test_prepare_options_preserves_parallel_tool_calls_when_no_tools(openai_unit_test_env: dict[str, str]) -> None:
-    """Test that parallel_tool_calls is preserved even when no tools are present."""
+def test_prepare_options_removes_parallel_tool_calls_when_no_tools(openai_unit_test_env: dict[str, str]) -> None:
+    """Test that parallel_tool_calls is removed when no tools are present."""
     client = OpenAIChatClient()
 
     messages = [ChatMessage(role="user", text="test")]
     options = {"allow_multiple_tool_calls": True}
 
     prepared_options = client._prepare_options(messages, options)
 
-    # parallel_tool_calls is preserved even when no tools (consistent with tool_choice behavior)
-    assert prepared_options.get("parallel_tool_calls") is True
+    # Should not have parallel_tool_calls when no tools
+    assert "parallel_tool_calls" not in prepared_options
 
 
 async def test_streaming_exception_handling(openai_unit_test_env: dict[str, str]) -> None: