Python: fix(ag-ui): properly handle json serialize with handoff workflows as agent

[moonbox3]

Motivation and Context

When using AG-UI with a Handoff workflow (as an agent), the server crashes with TypeError: Object of type HandoffAgentUserRequest is not JSON serializable the first time a handoff agent requests user input. This occurs because HandoffAgentUserRequest is a dataclass containing an AgentResponse (which uses SerializationMixin), and the AG-UI event bridge calls json.dumps() directly on these objects without converting them to serializable dictionaries first.

The fix updates make _json_safe() in _utils.py to handle to_dict() methods (used by SerializationMixin) and to recursively process results from as_dict(), model_dump(), to_dict(), and dict() calls to make sure nested non-serializable objects are also converted. The three locations in _events.py that serialize function call arguments now use make_json_safe() to properly handle dataclasses and SerializationMixin objects before JSON encoding.

Description

• Fixes Python: [Bug]: agent-framework-ag-ui crashes when Handoff user-input requests emit non‑JSON‑serializable arguments #3239
• Add unit test coverage

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/ag-ui/agent_framework_ag_ui
_events.py	264	13	95%	105–106, 200, 219, 248, 262, 285, 365–366, 395, 406, 429, 485
_message_adapters.py	452	44	90%	66, 76–77, 86–89, 155, 255, 259, 262, 265, 269, 285, 302, 324, 360, 372, 425, 442–443, 506, 510, 516, 524–525, 527, 531–534, 547, 636–639, 641, 706, 763, 767, 770, 773, 781–782
_orchestrators.py	410	48	88%	111, 120, 227, 477, 532–534, 541–542, 550, 552–554, 558, 567, 600, 602–604, 606–609, 611, 613–614, 616, 618–625, 725–727, 731–736, 785–787, 790
_utils.py	101	7	93%	70, 73, 86, 109, 158, 257, 262
packages/ag-ui/agent_framework_ag_ui/_orchestration
_helpers.py	184	53	71%	57, 106, 109, 146, 149, 153, 171, 174, 178, 195, 198, 201, 221, 233, 237, 252, 257, 288–305, 307–310, 314–315, 329, 348–350, 357, 377, 379, 382–386
_state_manager.py	51	1	98%	97
TOTAL	17429	2706	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3180	213 💤	0 ❌	0 🔥	1m 3s ⏱️

[Copilot]

Pull request overview

This PR fixes a JSON serialization crash in AG-UI when using handoff workflows as agents. The server previously crashed with TypeError: Object of type HandoffAgentUserRequest is not JSON serializable when attempting to serialize dataclass objects containing nested SerializationMixin objects (which use to_dict() methods).

Changes:

• Enhanced make_json_safe() to support to_dict() method and recursively process all serialization method outputs
• Updated three locations in _events.py to use make_json_safe() before JSON encoding function call arguments
• Added comprehensive test coverage for dataclasses with nested objects and SerializationMixin patterns

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
python/packages/ag-ui/agent_framework_ag_ui/_utils.py	Added support for to_dict() method in make_json_safe() and made all serialization methods recursive
python/packages/ag-ui/agent_framework_ag_ui/_events.py	Applied make_json_safe() to function call arguments in three locations before JSON encoding
python/packages/ag-ui/tests/test_utils.py	Added unit tests for to_dict() method handling and nested dataclass serialization
python/packages/ag-ui/tests/test_events_comprehensive.py	Added integration tests for dataclass and nested dataclass arguments in function calls