fix: support assistant role in message conversion

- The _Converter.items_to_messages method was incorrectly rejecting 'assistant'
as a valid role in conversation messages, causing runtime errors when processing
standard chat completion message formats.
- This fix enables proper handling of
complete conversation contexts that include both user and assistant messages.

src/agents/models/openai_chatcompletions.py

@@ -808,6 +808,13 @@ class _Converter:
                         "content": cls.extract_text_content(content),
                     }
                     result.append(msg_developer)
+                elif role == "assistant":
+                    flush_assistant_message()
+                    msg_assistant: ChatCompletionAssistantMessageParam = {
+                        "role": "assistant",
+                        "content": cls.extract_text_content(content),
+                    }
+                    result.append(msg_assistant)
                 else:
                     raise UserError(f"Unexpected role in easy_input_message: {role}")
 

tests/test_openai_chatcompletions_converter.py

@@ -393,3 +393,35 @@ def test_unknown_object_errors():
     with pytest.raises(UserError, match="Unhandled item type or structure"):
         # Purposely ignore the type error
         _Converter.items_to_messages([TestObject()])  # type: ignore
+
+
+def test_assistant_messages_in_history():
+    """
+    Test that assistant messages are added to the history.
+    """
+    messages = _Converter.items_to_messages(
+            [
+                {
+                    "role": "user",
+                    "content": "Hello",
+                },
+                {
+                    "role": "assistant",
+                    "content": "Hello?",
+                },
+                {
+                    "role": "user",
+                    "content": "What was my Name?",
+                },
+            ]
+        )
+    
+    # OUTPUT is [{'role': 'user', 'content': 'Hello'}, {'role': 'assistant', 'content': 'Hello?'}, {'role': 'user', 'content': 'What was my Name?'}]
+    assert messages == [{'role': 'user', 'content': 'Hello'}, {'role': 'assistant', 'content': 'Hello?'}, {'role': 'user', 'content': 'What was my Name?'}]
+    assert len(messages) == 3
+    assert messages[0]["role"] == "user"
+    assert messages[0]["content"] == "Hello"
+    assert messages[1]["role"] == "assistant"
+    assert messages[1]["content"] == "Hello?"
+    assert messages[2]["role"] == "user"
+    assert messages[2]["content"] == "What was my Name?"