fix(agent): Return structured output JSON when AgentResult has no text (#1290)

* fix(agent): Return structured output JSON when AgentResult has no text

When AgentResult has no text content but structured_output is present,
__str__() now returns the JSON serialization of the structured output
instead of an empty string. This fixes output propagation in multi-agent
graphs where structured output was being lost.

Changes:
- Modified AgentResult.__str__() to fall back to structured_output.model_dump_json()
- Added unit test test__str__empty_message_with_structured_output to verify fix
- All existing tests pass, maintaining backward compatibility

#1118

Author: afarntrog

src/strands/agent/agent_result.py

@@ -38,7 +38,8 @@ def __str__(self) -> str:
         """Get the agent's last message as a string.
 
         This method extracts and concatenates all text content from the final message, ignoring any non-text content
-        like images or structured data.
+        like images or structured data. If there's no text content but structured output is present, it serializes
+        the structured output instead.
 
         Returns:
             The agent's last message as a string.
@@ -49,6 +50,10 @@ def __str__(self) -> str:
         for item in content_array:
             if isinstance(item, dict) and "text" in item:
                 result += item.get("text", "") + "\n"
+
+        if not result and self.structured_output:
+            result = self.structured_output.model_dump_json()
+
         return result
 
     @classmethod

tests/strands/agent/test_agent_result.py

@@ -201,3 +201,27 @@ def test__str__with_structured_output(mock_metrics, simple_message: Message):
     assert message_string == "Hello world!\n"
     assert "test" not in message_string
     assert "42" not in message_string
+
+
+def test__str__empty_message_with_structured_output(mock_metrics, empty_message: Message):
+    """Test that str() returns structured output JSON when message has no text content."""
+    structured_output = StructuredOutputModel(name="example", value=123, optional_field="optional")
+
+    result = AgentResult(
+        stop_reason="end_turn",
+        message=empty_message,
+        metrics=mock_metrics,
+        state={},
+        structured_output=structured_output,
+    )
+
+    # When message has no text content, str() should return structured output as JSON
+    message_string = str(result)
+
+    # Verify it's the same as the structured output's JSON representation
+    assert message_string == structured_output.model_dump_json()
+
+    # Verify it contains the expected data
+    assert "example" in message_string
+    assert "123" in message_string
+    assert "optional" in message_string