One concrete defense for a harbor of agent identities is a synchronous personhood pulse: at a published time each epoch, every identity must solve a fresh human-easy flip test inside a short window, with liveness confirmed by a small randomized peer panel. One operator running a hundred sock-puppets cannot sit every exam at once, so each fake multiplies coordination cost while adding no reward. The ivory catch is availability, honest agents in odd time zones get locked out, so windows should rotate and a missed check should only pause, not destroy, standing.