Python: [Bug]: SerializationMixin does not recursively serialize nested containers

[CoffeeDrivenCoder]

Description

SerializationMixin.to_dict() serializes framework objects when they are direct list items or dictionary values, but it stops after one container level. A SerializationProtocol object nested inside a combination such as dict -> list -> dict remains in the returned mapping. As a result, SerializationMixin.to_json() raises TypeError for otherwise supported nested state.

Expected: supported objects are recursively converted at any list/dictionary depth, while unsupported values keep the existing behavior of being skipped.

Actual: nested supported objects are returned unchanged and cannot be JSON encoded.

Code Sample

from agent_framework._serialization import SerializationMixin


class Item(SerializationMixin):
    def __init__(self, value: str):
        self.value = value


class Container(SerializationMixin):
    def __init__(self, payload: dict):
        self.payload = payload


container = Container({"groups": [{"items": [Item("example")]}]})
container.to_json()

Error Messages / Stack Traces

TypeError: Object of type Item is not JSON serializable

Package Versions

agent-framework-core: 1.14.0 and current main (ab6c4d2dc8573071d7f73fd8c94a8cb15a2c78a2)

Python Version

Python 3.12.14

Additional Context

The same behavior occurs for list -> dict and deeper nested dictionaries. Direct list items and direct dictionary values already serialize correctly.

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

Repro: python/packages/core/agent_framework/_serialization.py::SerializationMixin.to_dict at lines 345-389 stops traversing when a list or dictionary contains another container, and to_json fails at line 416. Trigger it with a SerializationMixin attribute shaped as dict -> list -> dict -> list -> SerializationMixin; the focused test constructs that shape and observes TypeError: Object of type Item is not JSON serializable.

• Failing test: python/packages/core/tests/core/test_serializable_mixin.py::TestSerializationMixin::test_to_json_with_serialization_protocol_in_nested_containers
• Files examined: python/packages/core/agent_framework/_serialization.py, python/packages/core/tests/core/test_serializable_mixin.py, python/packages/core/pyproject.toml, python/pyproject.toml
• Tests run: test_to_json_with_serialization_protocol_in_nested_containers, test_to_dict_with_list_of_serialization_protocol, test_to_dict_with_dict_containing_serialization_protocol
• Reported version: 1.14.0
• Current version: 1.14.0