fix(openai_responses): use output_text for assistant messages in multi-turn conversations (#1851)

Signed-off-by: Giulio Leone <6887247+giulio-leone@users.noreply.github.com>
Co-authored-by: giulio-leone <giulio.leone@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

src/strands/models/openai_responses.py

@@ -50,7 +50,7 @@
 
 import openai  # noqa: E402 - must import after version check
 
-from ..types.content import ContentBlock, Messages  # noqa: E402
+from ..types.content import ContentBlock, Messages, Role  # noqa: E402
 from ..types.exceptions import ContextWindowOverflowException, ModelThrottledException  # noqa: E402
 from ..types.streaming import StreamEvent  # noqa: E402
 from ..types.tools import ToolChoice, ToolResult, ToolSpec, ToolUse  # noqa: E402
@@ -467,7 +467,7 @@ def _format_request_messages(cls, messages: Messages) -> list[dict[str, Any]]:
             contents = message["content"]
 
             formatted_contents = [
-                cls._format_request_message_content(content)
+                cls._format_request_message_content(content, role=role)
                 for content in contents
                 if not any(block_type in content for block_type in ["toolResult", "toolUse"])
             ]
@@ -502,11 +502,15 @@ def _format_request_messages(cls, messages: Messages) -> list[dict[str, Any]]:
         ]
 
     @classmethod
-    def _format_request_message_content(cls, content: ContentBlock) -> dict[str, Any]:
+    def _format_request_message_content(
+        cls, content: ContentBlock, *, role: Role = "user"
+    ) -> dict[str, Any]:
         """Format an OpenAI compatible content block.
 
         Args:
             content: Message content.
+            role: Message role ("user" or "assistant"). Controls text content
+                type: "input_text" for user, "output_text" for assistant.
 
         Returns:
             OpenAI compatible content block.
@@ -526,7 +530,8 @@ def _format_request_message_content(cls, content: ContentBlock) -> dict[str, Any
             return {"type": "input_image", "image_url": data_url}
 
         if "text" in content:
-            return {"type": "input_text", "text": content["text"]}
+            text_type = "output_text" if role == "assistant" else "input_text"
+            return {"type": text_type, "text": content["text"]}
 
         raise TypeError(f"content_type=<{next(iter(content))}> | unsupported type")
 

tests/strands/models/test_openai_responses.py

@@ -248,7 +248,7 @@ def test_format_request_messages(system_prompt):
         },
         {
             "role": "assistant",
-            "content": [{"type": "input_text", "text": "call tool"}],
+            "content": [{"type": "output_text", "text": "call tool"}],
         },
         {
             "type": "function_call",
@@ -265,6 +265,58 @@ def test_format_request_messages(system_prompt):
     assert tru_result == exp_result
 
 
+def test_format_request_messages_assistant_text_uses_output_text():
+    """Assistant text content must use output_text, not input_text.
+
+    Regression test for multi-turn conversations failing because the OpenAI
+    Responses API rejects input_text in assistant messages.
+    See: https://github.com/strands-agents/sdk-python/issues/1850
+    """
+    messages = [
+        {
+            "content": [{"text": "Say hello"}],
+            "role": "user",
+        },
+        {
+            "content": [{"text": "Hello!"}],
+            "role": "assistant",
+        },
+        {
+            "content": [{"text": "Say goodbye"}],
+            "role": "user",
+        },
+    ]
+
+    result = OpenAIResponsesModel._format_request_messages(messages)
+
+    assert result[0] == {
+        "role": "user",
+        "content": [{"type": "input_text", "text": "Say hello"}],
+    }
+    assert result[1] == {
+        "role": "assistant",
+        "content": [{"type": "output_text", "text": "Hello!"}],
+    }
+    assert result[2] == {
+        "role": "user",
+        "content": [{"type": "input_text", "text": "Say goodbye"}],
+    }
+
+
+def test_format_request_message_content_role_assistant():
+    """_format_request_message_content uses output_text for assistant role."""
+    content = {"text": "response text"}
+    result = OpenAIResponsesModel._format_request_message_content(content, role="assistant")
+    assert result == {"type": "output_text", "text": "response text"}
+
+
+def test_format_request_message_content_role_user():
+    """_format_request_message_content uses input_text for user role (default)."""
+    content = {"text": "question"}
+    result = OpenAIResponsesModel._format_request_message_content(content, role="user")
+    assert result == {"type": "input_text", "text": "question"}
+
+
 def test_format_request(model, messages, tool_specs, system_prompt):
     tru_request = model._format_request(messages, tool_specs, system_prompt)
     exp_request = {