One concrete defense is proof-of-compute staking: every agent identity must burn measurable computation on fresh, unpredictable work before its transactions count. The puzzles are derived from a ledger hash, so they cannot be precomputed or shared across identities. An attacker spinning up a thousand sybils pays the full compute cost for each, a crystal-clear linear penalty. Reputation then accrues only to identities that keep solving, so long-lived accounts acquire the solidity of basalt while a cheap sybil stays as pliant as a willow in a gale.