A2A inbound converters do not restore custom_metadata and drop metadata-only events

[YizukiAme]

Bug Description

The public inbound converters convert_a2a_message_to_event() and convert_a2a_task_to_event() in to_adk_event.py only restore actions from A2A metadata. Other ADK event metadata fields (custom_metadata, usage_metadata, error_code, long_running_tool_ids, etc.) are silently dropped.

Additionally, _create_event() returns None when there are no parts and no actions, even if event_metadata is present. This means metadata-only A2A messages are discarded entirely.

Steps to Reproduce

from a2a.types import Message, Part, TextPart, Role

msg = Message(
    role=Role.agent,
    parts=[],
    metadata={"adk_custom_metadata": {"flag": True}}
)
event = convert_a2a_message_to_event(msg, author="agent")
assert event is not None  # FAILS — metadata-only message silently dropped

Expected Behavior

• custom_metadata, usage_metadata, and other semantic metadata fields should be restored on the resulting Event
• Metadata-only messages (no content parts, but valid metadata) should produce a valid Event with content=None

Affected Files

• src/google/adk/a2a/converters/to_adk_event.py

[llalitkumarrr]

Hello @YizukiAme ,

Thank you for raising this issue. We were able to reproduce it and observed the same behavior. There is a PR linked to this issue currently under review. It will be merged once all tests pass and the changes are approved.

Hello @wyf7107 ,
Could you please look at this? We also observed that some fields like usage_metadata return None when the event is returned by a remote agent. We use google-adk==1.28.0 for this reproduction.