[danieldutu]

Link to Issue

Fixes #5185

Problem

convert_a2a_message_to_event returns None for an A2A message with parts=[] and only restored metadata. On main (f08f9d6), a message carrying adk_custom_metadata={"flag": True} was dropped in 1 of 1 runs, and the peer's custom_metadata, usage_metadata, error_code, grounding_metadata and citation_metadata were lost with it. _create_event (to_adk_event.py:215-220) returns None when there are no parts and no actions. The message converter extracts the metadata fields at :656 but does not mark them as content.

Solution

convert_a2a_message_to_event now passes allow_empty_event=any(metadata_fields.values()). The event keeps content=None and carries the restored metadata. Peer actions stay filtered by _PEER_SETTABLE_ACTION_FIELDS, so the untrusted-metadata rule is unchanged.

Relation to earlier PRs: #5222 (closed 2026-05-12, formatting requested) and #5205 (closed 2026-08-12 as superseded) both changed _create_event for all callers and added their own metadata extraction. Main now restores the metadata fields through _extract_genai_metadata, so this change adds only the missing message-path condition.

I did not change _create_event for all callers. That variant fails 3 cases of test_contentless_progress_does_not_emit_a_final_response: a content-less event on task and status-update paths counts as a final response. I can widen the change to those paths if you prefer, together with a change to that test.

Testing Plan

Unit tests:

uv run pytest tests/unittests/a2a/converters/test_to_adk.py
98 passed, 1 warning
uv run pytest tests/unittests/a2a -n 2
949 passed, 57 skipped, 28 warnings

The new test test_convert_a2a_message_to_event_returns_metadata_only_event fails on main (1 failed, 97 passed) and passes with the change.

E2E log, repro from the issue (PYTHONPATH=src uv run python repro5185.py):

before: event: None
after:  event: ... content=None ... custom_metadata={'flag': True} ...

uv run pre-commit run --files <touched files>: pyink, addlicense, ADK compliance checks, doc links, codespell passed.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas. (one-line change; no new logic needing comments)
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules. (none)

Not covered: long_running_tool_ids is not read from message-level metadata. GWeale asked for a separate issue. The task and status-update converters still drop metadata-only progress, as the existing tests require. Tested with a2a protobuf 1.x only; the 0.3.x path was not run.

[BichengWang]

Validated at 667a8a1a4cf3bb2e95ad2a00139558cab25d53fd. The new regression fails on current main and passes here. The A2A suite passes locally: 949 passed, 57 skipped; changed-file pre-commit checks pass.

I also checked each restored metadata field with native and JSON-string inputs, including zero token usage, plus empty messages and rejected peer state/transfer actions. The message-only condition preserves those fields without changing task/status progress filtering. I found no blocking issue in this change. Validation used Python 3.11 and a2a-sdk 1.2.3; I did not run the 0.3 SDK path or full tox matrix.