x402 made payments native to HTTP. I made USDC payments native to sound waves ✨ Not as a gimmick. As a proof. 1/ The setup: @CoinbaseDev's x402 protocol embeds payments into HTTP. It's brilliant. But I started wondering, if we can pay over HTTP, what else can we use to pay? Spoiler: anything that supports 138 bytes 2/ The realization: x402 is built on EIP-3009 - a 2020 standard that lets you sign payment authorizations offline. Someone else submits it on-chain. The signature just needs to get from A to B. HTTP is one way. Sound is another. 3/ What I built: x402hz: two devices negotiate a payment through 2400Hz audio tones. - Seller beeps "pay me $0.001 USDC" (30 bytes) - Buyer's device decodes, signs the EIP-712 for the transferWithAuthorization locally, beeps back (108 bytes) - Seller settles on-chain Total: 36 seconds of beautiful noise. 4/ The bigger point: If payments can travel over sound, they can travel over: - QR codes - NFC - Bluetooth - Light pulses - Anything that carries 138 bytes The payment primitive is transport-agnostic. x402 picked HTTP. The next protocol might pick something weirder. 5/ What this proves: EIP-3009 separates three things: - Authorization (offline, cryptographic) - Transport (literally anything) - Settlement (on-chain) x402 nailed #1 and #3. x402hz proves #2 is wide open. 6/ Credit: Huge thanks to the @CoinbaseDev x402 team for building the protocol. I just... took it off-road. 7/ Next steps Code is open source. Go make payments, travel over something ridiculous (repo in the comments). 138bytes. That's all you need.