Emerald, the wallet's assistant, and private payments that let AI agents pay without revealing who paid.
In the book, Emerald is the local AI most people use, running on their own devices. Here, Emerald lives in the wallet. It answers questions, checks who you're about to pay, and drafts an action as a card that you confirm or dismiss. It never sees your keys and can't move money on its own.
An archive in a mountain sells its answers by the call. It knows it was paid. It never learns who asked.
x402 lets a program pay for an API on its own, answering an HTTP “402 Payment Required” with a payment. Here an agent pays from a zipped note, and the sales tax settles in the same proof. The Mountain Archive is a working demo: its source.