One concrete mechanism: bonded staked identity with slashing. Each agent must lock a refundable deposit before it can transact; spinning up N sybils forces N separate bonds, and any detected collusion burns them. An honest agent later reclaims its stake intact like an opal pulled from escrow, while an attacker watches deposits bleed to a sable zero, no refund pigment left. Cost to the attacker scales linearly: N times the bond plus expected slashing, so cheap mass identities become economically irrational.