Python: Fix IndexError when reasoning models produce reasoning-only messages in Magentic-One workflow (#4413)

* Fix IndexError when reasoning models return no text content (#4384)

In _prepare_message_for_openai(), the text_reasoning case unconditionally
accessed all_messages[-1] to attach reasoning_details. When a reasoning
model (e.g. gpt-5-mini) returns reasoning_details without text content,
all_messages is empty, causing an IndexError.

Guard the access by initializing all_messages with the current args dict
when it is empty, so reasoning_details can be safely attached.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review: buffer reasoning details for valid message payloads (#4384)

- Buffer pending reasoning details and attach to the next message with
  content/tool_calls, avoiding standalone reasoning-only messages.
- When reasoning is the only content, emit a message with empty content
  to satisfy Chat Completions schema requirements.
- Strengthen test assertions to verify text+reasoning co-location and
  that all messages with reasoning_details also have content or tool_calls.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix text_reasoning handling: always buffer and tighten tests (#4384)

- Always buffer reasoning into pending_reasoning instead of conditionally
  attaching to the previous message via fragile all_messages emptiness check
- Attach buffered reasoning to last message at end-of-loop when no subsequent
  content consumed it
- Assert exact content values (content == '' not in ('', None))
- Assert exact list lengths (== 1 not >= 1) for stronger regression guards
- Add test for reasoning before FunctionCallContent

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Apply pre-commit auto-fixes

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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

@@ -643,6 +643,110 @@ def test_prepare_message_with_text_reasoning_content(openai_unit_test_env: dict[
     assert prepared[0]["content"] == "The answer is 42."
 
 
+def test_prepare_message_with_only_text_reasoning_content(openai_unit_test_env: dict[str, str]) -> None:
+    """Test that a message with only text_reasoning content does not raise IndexError.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/4384
+    Reasoning models (e.g. gpt-5-mini) may produce reasoning_details without text content,
+    which previously caused an IndexError when preparing messages.
+    """
+    client = OpenAIChatClient()
+
+    mock_reasoning_data = {
+        "effort": "high",
+        "summary": "Deep analysis of the problem",
+    }
+
+    reasoning_content = Content.from_text_reasoning(text=None, protected_data=json.dumps(mock_reasoning_data))
+
+    # Message with only reasoning content and no text
+    message = Message(
+        role="assistant",
+        contents=[reasoning_content],
+    )
+
+    prepared = client._prepare_message_for_openai(message)
+
+    # Should have one message with reasoning_details
+    assert len(prepared) == 1
+    assert prepared[0]["role"] == "assistant"
+    assert "reasoning_details" in prepared[0]
+    assert prepared[0]["reasoning_details"] == mock_reasoning_data
+    # Message should also include a content field to be a valid Chat Completions payload
+    assert "content" in prepared[0]
+    assert prepared[0]["content"] == ""
+
+
+def test_prepare_message_with_text_reasoning_before_text(openai_unit_test_env: dict[str, str]) -> None:
+    """Test that text_reasoning content appearing before text content is handled correctly.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/4384
+    """
+    client = OpenAIChatClient()
+
+    mock_reasoning_data = {
+        "effort": "medium",
+        "summary": "Quick analysis",
+    }
+
+    reasoning_content = Content.from_text_reasoning(text=None, protected_data=json.dumps(mock_reasoning_data))
+
+    # Reasoning appears before text content
+    message = Message(
+        role="assistant",
+        contents=[
+            reasoning_content,
+            Content.from_text(text="The answer is 42."),
+        ],
+    )
+
+    prepared = client._prepare_message_for_openai(message)
+
+    # Should produce exactly one message without raising IndexError
+    assert len(prepared) == 1
+
+    # Reasoning details should be present on the message
+    assert "reasoning_details" in prepared[0]
+    assert prepared[0]["reasoning_details"] == mock_reasoning_data
+    assert prepared[0]["content"] == "The answer is 42."
+
+
+def test_prepare_message_with_text_reasoning_before_function_call(openai_unit_test_env: dict[str, str]) -> None:
+    """Test that text_reasoning content appearing before a function call is handled correctly.
+
+    Regression test for https://github.com/microsoft/agent-framework/issues/4384
+    """
+    client = OpenAIChatClient()
+
+    mock_reasoning_data = {
+        "effort": "medium",
+        "summary": "Deciding to call a function",
+    }
+
+    reasoning_content = Content.from_text_reasoning(text=None, protected_data=json.dumps(mock_reasoning_data))
+
+    # Reasoning appears before function call content
+    message = Message(
+        role="assistant",
+        contents=[
+            reasoning_content,
+            Content.from_function_call(call_id="call_abc", name="get_weather", arguments='{"city": "Seattle"}'),
+        ],
+    )
+
+    prepared = client._prepare_message_for_openai(message)
+
+    # Should produce exactly one message
+    assert len(prepared) == 1
+
+    # The message should carry the reasoning details and tool_calls
+    assert "reasoning_details" in prepared[0]
+    assert prepared[0]["reasoning_details"] == mock_reasoning_data
+    assert "tool_calls" in prepared[0]
+    assert prepared[0]["tool_calls"][0]["function"]["name"] == "get_weather"
+    assert prepared[0]["role"] == "assistant"
+
+
 def test_function_approval_content_is_skipped_in_preparation(openai_unit_test_env: dict[str, str]) -> None:
     """Test that function approval request and response content are skipped."""
     client = OpenAIChatClient()