How AI agents get a real inbox, pay per call instead of by the seat, and send and receive email without a human in the loop. Guides, comparisons, and deep dives from the team building DimeBox.
Replay protection, facilitator trust, and what happens when a call fails partway — a working threat model for pay-per-call APIs, not a yes/no answer.
Testing x402 payments against real USDC costs real money on every mistake. Base Sepolia runs the same protocol with test tokens that don't.
A signed payment isn't money moved yet. A facilitator verifies it and submits it on-chain — a job deliberately kept separate from the API server.
EIP-3009 lets an agent sign a USDC payment without holding a separate gas token at all — here's what the signature actually contains.
x402's default payment method pays the exact quoted amount for the exact request that triggered it — no running balance, no invoice, no tip jar.
Why DimeBox prices every call in USDC and settles on Base specifically, not any stablecoin on any chain.
What's actually inside a 402 response, what a client has to build in reply, and what a server checks before it runs the route — walked through stage by stage.
API keys answer who's calling. x402 answers whether this specific call got paid for — a narrower question that fits an autonomous caller better.
HTTP reserved a status code for payment in 1997. It took cheap stablecoin settlement and machine callers with no use for a checkout page to finally use it.
x402 turns the long-unused HTTP 402 status code into a working payment handshake: call an endpoint, get asked to pay, sign a payment, get your response.