Summary

• Queues conversational gateway follow-ups instead of interrupting when the busy parent has active delegated child agents.
• Refs [Bug]: Sending a message while delegate_task is running kills the subagent — interrupt propagates unconditionally to children #30170.

Why

• In busy_input_mode: interrupt, a normal follow-up calls AIAgent.interrupt() on the parent, which recursively interrupts _active_children and kills in-flight delegate_task work.
• While delegate_task blocks the parent synchronously, the safest minimal behavior is to preserve the child work and replay the follow-up after delegation completes.

Changes

• gateway/run.py: detects active delegated children on the running parent agent and treats plain busy follow-ups as queue-mode instead of interrupt-mode.
• gateway/run.py: sends a delegation-specific busy acknowledgement so users know the subagent is still working.
• tests/gateway/test_busy_session_auth_bypass.py: covers the active-delegation path and preserves the existing interrupt behavior when no child is running.

Validation

• .venv/bin/python -m pytest tests/gateway/test_busy_session_auth_bypass.py tests/gateway/test_restart_drain.py tests/gateway/test_status_command.py -o 'addopts=' -q
• .venv/bin/python -m ruff check gateway/run.py tests/gateway/test_busy_session_auth_bypass.py

Scope

• In scope: normal conversational messages arriving through the gateway while a parent agent has active delegated child agents.
• Out of scope: steer-through to children, async delegation, and explicit cancel commands such as /stop or /new.