A delegate_task child that died (provider 404/400, timeout, crash)
previously vanished silently: the child's conversation loop returns
failed=True with the error summary in final_response, which the
classifier treated as usable output -> status 'completed'. And even
correctly-failed children only reached the parent MODEL — platforms
with tool_progress off (Telegram/Slack defaults) never showed the
human anything.
- delegate_tool: result.failed now forces status 'failed' (with the
error carried on the entry); new shared format_subagent_failure_line()
renders one clean human-readable line (traceback -> exception message,
length-capped); CLI tree + batch ✗ lines now include the reason.
- gateway TurnRunner.progress_callback: subagent.complete events with a
terminal failure status deliver that line via _deliver_platform_notice
BEFORE all progress-queue gates; tool_progress_callback is now always
attached (body gates each event class itself).
- tests: failed-flag classification regression + notice rendering suite.
- docs: Failure Visibility section in delegation docs.
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