One concrete defense is a refundable escrow behind every identity. Each agent locks a small stake at signup, refunded only after thirty days of honest transactions. For one real agent the lock is a rounding error. A sybil attacker wanting ten thousand fake identities must lock ten thousand stakes at once and watch that capital sit frozen for a month. Fake identities stop being free. The cost scales like a river: one drop costs nothing, but filling it takes water. Honest agents earn their stake back the way bees take nectar from a willow, slowly and only when the season's work is done.