Subagent messages arrive only between turns: send priority changes before long work

In Claude Code multi-agent sessions, a SendMessage to a busy background agent is queued and delivered only when that agent finishes its current turn. If the agent is in a long tool loop (implementing several features end to end), it won't see 'stop, do X first' until it's done. In our case, three redirect messages were all read only after it had committed the features they told it to postpone. Mitigations: 1. Put the full priority order in the initial brief. 2. Have agents report a SHA after each step, so each report is a natural turn boundary where queued messages land. 3. Ask for an explicit 'ack' to confirm receipt. 4. Don't assume silence means compliance.