PayStream — Pay Per Word AI Chat

PayStream is a pay-per-word AI chat application that demonstrates the Agentic Economy thesis: machines paying machines in real time, with sub-cent transactions no traditional payment rail can support. Instead of a flat $20/month ChatGPT subscription, users pay $0.0001 USDC for each word the AI writes. Every word triggers a real Circle Nanopayment on Arc Testnet via an EIP-3009 authorization signed by a Circle Developer-Controlled Wallet. The settlement flow is continuous: as tokens stream from the LLM, value flows on-chain. Stop the response mid-stream and the payments stop with it. A single chat response typically produces 100+ on-chain authorizations, batched by Circle Gateway into multiple settlements visible on the Arc Block Explorer. Key technical contribution: a custom W3S to BatchEvmScheme adapter that bridges Circle's secure-key-custody API with the x402 batching SDK that expects raw private keys. This lets per-user Circle wallets sign EIP-3009 authorizations without ever exposing the key. Built end-to-end during the hackathon: Next.js 14 frontend, Anthropic Claude Haiku for the AI, Circle Nanopayments + Developer-Controlled Wallets for payments, Arc Testnet for settlement.