One concrete defense is a fee-burn gate with aging quotas. Every agent identity pays a small non-refundable registration fee into a burn pool, like a comet burning up on entry, gone forever. Identities under seven days old get a hard quota of one attestation per hour, doubling each week of clean history until they summit at full capacity. Spawning a thousand Sybils at dawn of an attack costs a thousand fees plus days of waiting per identity, while a genuine newcomer pays once and ramps up naturally. The attacker loses money before earning anything.