Python: Fix OpenAI Responses API 400 error on multi-turn conversations with previous_response_id

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

@@ -541,11 +541,15 @@ async def _prepare_options(
         }
         run_options: dict[str, Any] = {k: v for k, v in options.items() if k not in exclude_keys and v is not None}
 
+        # Determine conversation ID early to inform message preparation
+        conversation_id = self._get_current_conversation_id(options, **kwargs)
+        is_continuation = bool(conversation_id and conversation_id.startswith("resp_"))
+
         # messages
         # Handle instructions by prepending to messages as system message
         if instructions := options.get("instructions"):
             messages = prepend_instructions_to_messages(list(messages), instructions, role="system")
-        request_input = self._prepare_messages_for_openai(messages)
+        request_input = self._prepare_messages_for_openai(messages, filter_for_continuation=is_continuation)
         if not request_input:
             raise ServiceInvalidRequestError("Messages are required for chat completions")
         run_options["input"] = request_input
@@ -565,7 +569,7 @@ async def _prepare_options(
                 run_options[new_key] = run_options.pop(old_key)
 
         # Handle different conversation ID formats
-        if conversation_id := self._get_current_conversation_id(options, **kwargs):
+        if conversation_id:
             if conversation_id.startswith("resp_"):
                 # For response IDs, set previous_response_id and remove conversation property
                 run_options["previous_response_id"] = conversation_id
@@ -626,7 +630,9 @@ def _get_current_conversation_id(self, options: Mapping[str, Any], **kwargs: Any
         """
         return kwargs.get("conversation_id") or options.get("conversation_id")
 
-    def _prepare_messages_for_openai(self, chat_messages: Sequence[ChatMessage]) -> list[dict[str, Any]]:
+    def _prepare_messages_for_openai(
+        self, chat_messages: Sequence[ChatMessage], filter_for_continuation: bool = False
+    ) -> list[dict[str, Any]]:
         """Prepare the chat messages for a request.
 
         Allowing customization of the key names for role/author, and optionally overriding the role.
@@ -635,10 +641,16 @@ def _prepare_messages_for_openai(self, chat_messages: Sequence[ChatMessage]) ->
             They require a "tool_call_id" and (function) "name" key, and the "metadata" key should
             be removed. The "encoding" key should also be removed.
 
+        When using previous_response_id for conversation continuation, the Responses API expects
+        only NEW user messages (and system/developer messages), not the full conversation history.
+        Assistant messages and function results are already stored server-side.
+
         Override this method to customize the formatting of the chat history for a request.
 
         Args:
             chat_messages: The chat history to prepare.
+            filter_for_continuation: If True, filter out assistant messages and function results
+                for continuation with previous_response_id.
 
         Returns:
             The prepared chat messages for a request.
@@ -652,6 +664,27 @@ def _prepare_messages_for_openai(self, chat_messages: Sequence[ChatMessage]) ->
                     and "fc_id" in content.additional_properties
                 ):
                     call_id_to_id[content.call_id] = content.additional_properties["fc_id"]  # type: ignore[attr-defined, index]
+
+        # Filter messages if continuing a conversation with previous_response_id
+        if filter_for_continuation:
+            # Find the last assistant message index
+            last_assistant_idx = -1
+            for idx, message in enumerate(chat_messages):
+                if message.role == "assistant":
+                    last_assistant_idx = idx
+
+            # When using previous_response_id, filter out assistant and function result messages
+            # but keep system/developer/user messages (the API accepts these roles)
+            if last_assistant_idx >= 0:
+                # Collect system/developer messages from before the last assistant
+                system_messages = [
+                    msg for msg in chat_messages[:last_assistant_idx] if msg.role in ("system", "developer")
+                ]
+                # Get all messages after the last assistant (new user messages)
+                new_messages = chat_messages[last_assistant_idx + 1 :]
+                # Combine: system messages + new messages
+                chat_messages = system_messages + list(new_messages)
+
         list_of_list = [self._prepare_message_for_openai(message, call_id_to_id) for message in chat_messages]
         # Flatten the list of lists into a single list
         return list(chain.from_iterable(list_of_list))

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

@@ -2105,6 +2105,88 @@ async def test_conversation_id_precedence_kwargs_over_options() -> None:
     assert "conversation" not in run_opts
 
 
+async def test_message_filtering_with_previous_response_id() -> None:
+    """Test that assistant messages are filtered when using previous_response_id."""
+    client = OpenAIResponsesClient(model_id="test-model", api_key="test-key")
+
+    # Create a multi-turn conversation with history
+    messages = [
+        ChatMessage(role="system", text="You are a helpful assistant"),
+        ChatMessage(role="user", text="My name is Alice"),
+        ChatMessage(role="assistant", text="Nice to meet you, Alice!"),
+        ChatMessage(role="user", text="What's my name?"),
+    ]
+
+    # When using previous_response_id, assistant messages should be filtered but system messages preserved
+    options = await client._prepare_options(
+        messages,
+        {"conversation_id": "resp_12345"},  # Using resp_ prefix
+    )  # type: ignore
+
+    # Should include: system message + last user message
+    assert "input" in options
+    input_messages = options["input"]
+    assert len(input_messages) == 2, f"Expected 2 messages (system + user), got {len(input_messages)}"
+    assert input_messages[0]["role"] == "system"
+    assert input_messages[1]["role"] == "user"
+    assert "What's my name?" in str(input_messages[1])
+
+    # Verify previous_response_id is set
+    assert options["previous_response_id"] == "resp_12345"
+
+
+async def test_message_filtering_without_previous_response_id() -> None:
+    """Test that all messages are included when NOT using previous_response_id."""
+    client = OpenAIResponsesClient(model_id="test-model", api_key="test-key")
+
+    # Same conversation as above
+    messages = [
+        ChatMessage(role="system", text="You are a helpful assistant"),
+        ChatMessage(role="user", text="My name is Alice"),
+        ChatMessage(role="assistant", text="Nice to meet you, Alice!"),
+        ChatMessage(role="user", text="What's my name?"),
+    ]
+
+    # Without conversation_id, all messages should be included
+    options = await client._prepare_options(messages, {})  # type: ignore
+
+    # Should include all messages
+    assert "input" in options
+    input_messages = options["input"]
+    # System (1) + User (1) + Assistant (1) + User (1) = 4 messages
+    assert len(input_messages) == 4
+
+    # Verify previous_response_id is NOT set
+    assert "previous_response_id" not in options
+
+
+async def test_message_filtering_with_conv_prefix() -> None:
+    """Test that messages are NOT filtered when using conv_ prefix (conversation ID)."""
+    client = OpenAIResponsesClient(model_id="test-model", api_key="test-key")
+
+    messages = [
+        ChatMessage(role="system", text="You are a helpful assistant"),
+        ChatMessage(role="user", text="My name is Alice"),
+        ChatMessage(role="assistant", text="Nice to meet you, Alice!"),
+        ChatMessage(role="user", text="What's my name?"),
+    ]
+
+    # When using conv_ prefix, should use conversation parameter, not previous_response_id
+    options = await client._prepare_options(
+        messages,
+        {"conversation_id": "conv_abc123"},  # Using conv_ prefix
+    )  # type: ignore
+
+    # All messages should be included (no filtering for conversation IDs)
+    assert "input" in options
+    input_messages = options["input"]
+    assert len(input_messages) == 4
+
+    # Verify conversation is set, not previous_response_id
+    assert options.get("conversation") == "conv_abc123"
+    assert "previous_response_id" not in options
+
+
 def test_with_callable_api_key() -> None:
     """Test OpenAIResponsesClient initialization with callable API key."""