Behind feature flag DELEGATION_SYNC_VIA_INBOX (default off). When set,
tool_delegate_task no longer holds an HTTP message/send connection
through the platform proxy waiting for the callee's reply. Instead:
1. POST /workspaces/<src>/delegate (returns 202 + delegation_id)
— platform's executeDelegation goroutine handles A2A dispatch
in the background. No client-side timeout dependency on the
platform holding a connection open.
2. Poll GET /workspaces/<src>/delegations every 3s for a row with
matching delegation_id reaching terminal status (completed/failed).
3. Return the response_preview text on completed; surface the
wrapped _A2A_ERROR_PREFIX error on failed (so caller error
detection stays unchanged).
This closes the bug class that broke Hongming's home hermes on
2026-05-05 ("message/send queued but result not available after 600s
timeout" while the callee was actively heartbeating "iteration 14/90").
## Compatibility
Default-off feature flag — flag-off path is byte-identical to the
legacy send_a2a_message behavior, pinned by
TestFlagOffLegacyPath::test_flag_off_uses_send_a2a_message_not_polling.
Idempotency-key derivation matches tool_delegate_task_async (SHA-256
of source:target:task) so a restart-mid-delegation gets the same key
and the platform returns the existing delegation_id.
## Recovery on timeout
If the polling budget (DELEGATION_TIMEOUT, default 300s) elapses
without a terminal status, the error message includes the
delegation_id + a "call check_task_status('<id>') to retrieve later"
hint. The platform's durable row is still live — work is NOT lost,
just the synchronous wait is over. Caller can poll for the result
later via the existing check_task_status tool.
## Stack with PR-2
PR-2 added the SERVER-SIDE result-push to the caller's a2a_receive
inbox row. PR-5 (this PR) adds the AGENT-SIDE cutover. Together they
remove the proxy-blocked sync path entirely. PR-2 default-off keeps
existing behavior; PR-5 default-off keeps existing behavior. Operators
flip both for full effect after staging burn-in.
## Coverage
9 unit tests:
- flag off → byte-identical to legacy (send_a2a_message called,
_delegate_sync_via_polling NOT called)
- dispatch HTTP exception → wrapped error
- dispatch non-2xx → wrapped error mentioning HTTP code
- dispatch missing delegation_id → wrapped error
- completed first poll → response_preview returned
- failed status → wrapped error with error_detail
- transient poll error → keeps polling, eventually succeeds
- deadline exceeded → wrapped timeout error mentions delegation_id +
check_task_status hint for recovery
- filters by delegation_id (other delegations' rows ignored)
All passing locally. CI will run the same suite on a clean env.
Refs RFC #2829.
5950d4cd81