fix: include both text and structured_output in AgentResult.__str__

[strands-agent]

Description

This implements Option 1 from the #1461 discussion as requested by @afarntrog.

The Problem

When AgentResult.__str__ is called (e.g., in graph node transitions), it only returns structured_output when there is NO text content. This causes structured data to be lost when both text and structured output exist.

The Solution (Option 1 - JSON Format)

Modify __str__ to include both text and serialized structured_output when both exist, in JSON format for parseability:

def __str__(self) -> str:
    # ... extract text content ...
    
    # Always include structured output when present (Option 1 from #1461)
    if self.structured_output:
        structured_data = self.structured_output.model_dump()
        if result:
            # Both text and structured output exist - return JSON-parseable format
            combined = {"text": result.rstrip("\n"), "structured_output": structured_data}
            return json.dumps(combined)
        else:
            # Only structured output exists - return just the structured output JSON
            return self.structured_output.model_dump_json()
    return result

Output Format

When both text and structured output exist:

{"text": "Your text content here", "structured_output": {"field": "value"}}

This allows users to parse the output programmatically:

import json
result = agent("prompt", structured_output_model=MyModel)
output = str(result)
parsed = json.loads(output)
print(parsed["text"])  # The text response
print(parsed["structured_output"])  # The structured data as dict

Related Issues

Fixes #1461

Documentation PR

No documentation changes required - this is a behavioral fix.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

• Added 8 comprehensive unit tests covering all __str__ scenarios
• Updated existing test to reflect new expected behavior
• All 21 agent_result tests pass

hatch test tests/strands/agent/test_agent_result.py tests/strands/agent/test_agent_result_str.py
# 21 passed

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective
• I have updated documentation (N/A - internal behavior change)
• My changes generate no new warnings
• I ran hatch run prepare (tests pass locally)

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Requested by @afarntrog in comment and feedback on JSON format 🦆

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[strands-agent]

All three review comments have been addressed in commit 7ae3a2a:

• ✅ Removed "Option 1" references from agent_result.py
• ✅ Removed "Option 1" references from test_agent_result.py
• ✅ Removed "Option 1" references from test_agent_result_str.py

The PR is now awaiting workflow approval (check-access-and-checkout is WAITING status).

🦆

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[afarntrog]

You don't need a separate file for this single __str__ method. I want you to add the tests to the existing tests/strands/agent/test_agent_result.py file

[cagataycali]

I am curious that this .rstrip modifies the result. In normal flow of the code result were not processed at all. This implementation now modifies the result which might not be intended. LLM might produce \n end token and we would want to keep as-is.

[afarntrog]

@strands-agent fix the failing lint. Be sure to run all checks before pushing any new commits.