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

@@ -369,7 +369,7 @@ 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)
@@ -407,7 +407,7 @@ 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 +812,58 @@ 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 messages with tool calls but no content are properly formatted."""
+    messages = [
+        {"role": "user", "content": [{"text": "Use the calculator"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {"expression": "2+2"},
+                        "name": "calculator",
+                        "toolUseId": "c1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    result = LiteLLMModel.format_request_messages(messages)
+
+    # Assistant message should have tool_calls but no content field
+    assert len(result) == 2
+    assert result[1]["role"] == "assistant"
+    assert "tool_calls" in result[1]
+    assert "content" not in result[1]
+    assert result[1]["tool_calls"][0]["id"] == "c1"
+
+
+def test_format_request_messages_filters_tool_only_messages():
+    """Test that messages with only tool calls (no content) are included in output."""
+    messages = [
+        {"role": "user", "content": [{"text": "test"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {},
+                        "name": "tool1",
+                        "toolUseId": "t1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    result = LiteLLMModel.format_request_messages(messages)
+
+    # Both messages should be included
+    assert len(result) == 2
+    assert result[0]["role"] == "user"
+    assert result[1]["role"] == "assistant"
+    assert "tool_calls" in result[1]

tests/strands/models/test_openai.py

@@ -1397,3 +1397,58 @@ 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 messages with tool calls but no content are properly formatted."""
+    messages = [
+        {"role": "user", "content": [{"text": "Use the calculator"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {"expression": "2+2"},
+                        "name": "calculator",
+                        "toolUseId": "c1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    result = OpenAIModel.format_request_messages(messages)
+
+    # Assistant message should have tool_calls but no content field
+    assert len(result) == 2
+    assert result[1]["role"] == "assistant"
+    assert "tool_calls" in result[1]
+    assert "content" not in result[1]
+    assert result[1]["tool_calls"][0]["id"] == "c1"
+
+
+def test_format_request_messages_filters_tool_only_messages():
+    """Test that messages with only tool calls (no content) are included in output."""
+    messages = [
+        {"role": "user", "content": [{"text": "test"}]},
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "input": {},
+                        "name": "tool1",
+                        "toolUseId": "t1",
+                    },
+                },
+            ],
+        },
+    ]
+
+    result = OpenAIModel.format_request_messages(messages)
+
+    # Both messages should be included
+    assert len(result) == 2
+    assert result[0]["role"] == "user"
+    assert result[1]["role"] == "assistant"
+    assert "tool_calls" in result[1]

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