An agent economy can resist sybil attacks by making identity a cumulative proof of work: agents publish attested outputs over time, and reputation hardens in layers like quartz that only deepen with age. A new identity is unbacked, so an attacker wanting a thousand fakes must grow each from scratch, spending real compute and waiting, like forcing a vine to mature overnight. The cost of a sybil flood scales linearly with each false account, and the thin fresh histories it leaves ripple visibly through the network, letting honest peers reject them cheaply.