fix: handle OpenAI model responses with tool calls and no other assistant content

src/strands/models/litellm.py

@@ -194,7 +194,7 @@ def format_request_messages(
         formatted_messages = cls._format_system_messages(system_prompt, system_prompt_content=system_prompt_content)
         formatted_messages.extend(cls._format_regular_messages(messages))
 
-        return [message for message in formatted_messages if message["content"] or "tool_calls" in message]
+        return [message for message in formatted_messages if "content" in message or "tool_calls" in message]
 
     @override
     def format_chunk(self, event: dict[str, Any], **kwargs: Any) -> StreamEvent:

src/strands/models/openai.py

@@ -204,10 +204,18 @@ def format_request_tool_message(cls, tool_result: ToolResult, **kwargs: Any) ->
             ],
         )
 
+        formatted_contents = [cls.format_request_message_content(content) for content in contents]
+
+        # If single text content, use string format for better model compatibility
+        if len(formatted_contents) == 1 and formatted_contents[0].get("type") == "text":
+            content: str | list[dict[str, Any]] = formatted_contents[0]["text"]
+        else:
+            content = formatted_contents
+
         return {
             "role": "tool",
             "tool_call_id": tool_result["toolUseId"],
-            "content": [cls.format_request_message_content(content) for content in contents],
+            "content": content,
         }
 
     @classmethod
@@ -369,18 +377,21 @@ def _format_regular_messages(cls, messages: Messages, **kwargs: Any) -> list[dic
 
             formatted_message = {
                 "role": message["role"],
-                "content": formatted_contents,
+                **({"content": formatted_contents} if formatted_contents else {}),
                 **({"tool_calls": formatted_tool_calls} if formatted_tool_calls else {}),
             }
             formatted_messages.append(formatted_message)
 
             # Process tool messages to extract images into separate user messages
             # OpenAI API requires images to be in user role messages only
+            # All tool messages must be grouped together before any user messages with images
+            user_messages_with_images = []
             for tool_msg in formatted_tool_messages:
                 tool_msg_clean, user_msg_with_images = cls._split_tool_message_images(tool_msg)
                 formatted_messages.append(tool_msg_clean)
                 if user_msg_with_images:
-                    formatted_messages.append(user_msg_with_images)
+                    user_messages_with_images.append(user_msg_with_images)
+            formatted_messages.extend(user_messages_with_images)
 
         return formatted_messages
 
@@ -407,7 +418,7 @@ def format_request_messages(
         formatted_messages = cls._format_system_messages(system_prompt, system_prompt_content=system_prompt_content)
         formatted_messages.extend(cls._format_regular_messages(messages))
 
-        return [message for message in formatted_messages if message["content"] or "tool_calls" in message]
+        return [message for message in formatted_messages if "content" in message or "tool_calls" in message]
 
     def format_request(
         self,

tests/strands/models/test_litellm.py

@@ -812,3 +812,39 @@ def __init__(self, usage):
     assert metadata_events[0]["metadata"]["usage"]["inputTokens"] == 10
     assert metadata_events[0]["metadata"]["usage"]["outputTokens"] == 5
     assert metadata_events[0]["metadata"]["usage"]["totalTokens"] == 15
+
+
+def test_format_request_messages_with_tool_calls_no_content():
+    """Test that assistant messages with only tool calls are included and have no content field."""
+    messages = [
+        {"role": "user", "content": [{"text": "Use the calculator"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {"expression": "2+2"},
+                        "name": "calculator",
+                        "toolUseId": "c1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    tru_result = LiteLLMModel.format_request_messages(messages)
+
+    exp_result = [
+        {"role": "user", "content": [{"text": "Use the calculator", "type": "text"}]},
+        {
+            "role": "assistant",
+            "tool_calls": [
+                {
+                    "function": {"arguments": '{"expression": "2+2"}', "name": "calculator"},
+                    "id": "c1",
+                    "type": "function",
+                }
+            ],
+        },
+    ]
+    assert tru_result == exp_result

tests/strands/models/test_openai.py

@@ -180,6 +180,23 @@ def test_format_request_tool_message():
     assert tru_result == exp_result
 
 
+def test_format_request_tool_message_single_text_returns_string():
+    """Test that single text content is returned as string for model compatibility."""
+    tool_result = {
+        "content": [{"text": '{"result": "success"}'}],
+        "status": "success",
+        "toolUseId": "c1",
+    }
+
+    tru_result = OpenAIModel.format_request_tool_message(tool_result)
+    exp_result = {
+        "content": '{"result": "success"}',
+        "role": "tool",
+        "tool_call_id": "c1",
+    }
+    assert tru_result == exp_result
+
+
 def test_split_tool_message_images_with_image():
     """Test that images are extracted from tool messages."""
     tool_message = {
@@ -441,7 +458,7 @@ def test_format_request_messages(system_prompt):
             ],
         },
         {
-            "content": [{"text": "4", "type": "text"}],
+            "content": "4",
             "role": "tool",
             "tool_call_id": "c1",
         },
@@ -1397,3 +1414,122 @@ def test_format_request_filters_location_source_document(model, caplog):
     assert len(formatted_content) == 1
     assert formatted_content[0]["type"] == "text"
     assert "Location sources are not supported by OpenAI" in caplog.text
+
+
+def test_format_request_messages_with_tool_calls_no_content():
+    """Test that assistant messages with only tool calls are included and have no content field."""
+    messages = [
+        {"role": "user", "content": [{"text": "Use the calculator"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {"expression": "2+2"},
+                        "name": "calculator",
+                        "toolUseId": "c1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    tru_result = OpenAIModel.format_request_messages(messages)
+
+    exp_result = [
+        {"role": "user", "content": [{"text": "Use the calculator", "type": "text"}]},
+        {
+            "role": "assistant",
+            "tool_calls": [
+                {
+                    "function": {"arguments": '{"expression": "2+2"}', "name": "calculator"},
+                    "id": "c1",
+                    "type": "function",
+                }
+            ],
+        },
+    ]
+    assert tru_result == exp_result
+
+
+def test_format_request_messages_multiple_tool_calls_with_images():
+    """Test that multiple tool calls with image results are formatted correctly.
+
+    OpenAI requires all tool response messages to immediately follow the assistant
+    message with tool_calls, before any other messages. When tools return images,
+    the images are moved to user messages, but these must come after ALL tool messages.
+    """
+    messages = [
+        {"role": "user", "content": [{"text": "Run the tools"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {"toolUse": {"input": {}, "name": "tool1", "toolUseId": "call_1"}},
+                {"toolUse": {"input": {}, "name": "tool2", "toolUseId": "call_2"}},
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {
+                    "toolResult": {
+                        "toolUseId": "call_1",
+                        "content": [{"image": {"format": "png", "source": {"bytes": b"img1"}}}],
+                        "status": "success",
+                    }
+                },
+                {
+                    "toolResult": {
+                        "toolUseId": "call_2",
+                        "content": [{"image": {"format": "png", "source": {"bytes": b"img2"}}}],
+                        "status": "success",
+                    }
+                },
+            ],
+        },
+    ]
+
+    tru_result = OpenAIModel.format_request_messages(messages)
+
+    image_placeholder = (
+        "Tool successfully returned an image. The image is being provided in the following user message."
+    )
+    exp_result = [
+        {"role": "user", "content": [{"text": "Run the tools", "type": "text"}]},
+        {
+            "role": "assistant",
+            "tool_calls": [
+                {"function": {"arguments": "{}", "name": "tool1"}, "id": "call_1", "type": "function"},
+                {"function": {"arguments": "{}", "name": "tool2"}, "id": "call_2", "type": "function"},
+            ],
+        },
+        {
+            "role": "tool",
+            "tool_call_id": "call_1",
+            "content": [{"type": "text", "text": image_placeholder}],
+        },
+        {
+            "role": "tool",
+            "tool_call_id": "call_2",
+            "content": [{"type": "text", "text": image_placeholder}],
+        },
+        {
+            "role": "user",
+            "content": [
+                {
+                    "image_url": {"detail": "auto", "format": "image/png", "url": "data:image/png;base64,aW1nMQ=="},
+                    "type": "image_url",
+                }
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {
+                    "image_url": {"detail": "auto", "format": "image/png", "url": "data:image/png;base64,aW1nMg=="},
+                    "type": "image_url",
+                }
+            ],
+        },
+    ]
+    assert tru_result == exp_result

tests_integ/test_multiagent_swarm.py

@@ -113,6 +113,7 @@ def capture_first_node(self, event):
     return VerifyHook()
 
 
+@pytest.mark.timeout(120)
 def test_swarm_execution_with_string(researcher_agent, analyst_agent, writer_agent, hook_provider):
     """Test swarm execution with string input."""
     # Create the swarm