zap-x402

Problem we solve: x402 “exact” waits on-chain per call; x402‑flash still does per‑request escrow checks (RPC + payload verify), adding tens of ms. Both are too slow for HFT/data-stream agents.

• Core idea (channel‑mini‑cover): combine a small live deposit channel D with a reusable bond B, and enforce D + B ≥ W (max unpaid window). This guarantees zero server loss while minimizing user lockup.

• Request flow:

  • Client opens once: post bond B (security deposit) and open channel with deposit D (live cover, TTL).

  • Each API call: client sends headers {Channel‑Id, cumulative Paid, Sig(id||paid)}.

  • Server verifies the ECDSA signature in memory, checks paid advanced by ≥ price, atomically updates lastPaid, and returns 200 immediately (no chain reads).

• Settlement and safety:

  • Background worker periodically calls claim on the channel using the latest signed cumulative paid; the contract releases the delta to the server.

  • If claim fails or deposit is short, the worker slashes the bond for the shortfall and pauses the channel. Next requests get 402 until the client refills.

  • Because D + B ≥ W by policy, recovery (claimed + slashed) ≥ all served usage; server loss is 0.

• Why faster than x402‑flash:

  • Hot path has zero JSON‑RPC and no escrow state reads—only one signature check and a RAM counter update—so added latency is ~0–2 ms versus 20–70 ms.

  • No 402→retry round‑trip once a channel is open; first request can be 200.

• Capital efficiency:

  • Users lock only D + B, not their full working float; the bond is reusable across sessions and the deposit is withdrawable after TTL.

• Failure handling:

  • Revocation/disconnect: server still holds the latest signed state; claim or slash ensures full recovery, then the channel is paused.

  • Replay/underpay: cumulative paid must increase; atomic CAS prevents races; invalid sigs or short increments trigger 402.

• Interop:

  • Expose this as an additional x402 scheme alongside “exact” and “flash”; fallback paths remain available for clients that don’t support channels.

We built ZAP x402-flash, a revolutionary micropayment protocol that enables instant sub-cent API monetization without blockchain settlement delays. Our full-stack solution features a modern React/Next.js frontend with Tailwind UI, Express.js backend with custom x402 middleware, and Solidity smart contracts deployed on Polygon Amoy testnet. We implemented two groundbreaking payment schemes: traditional ZAP flow and our innovative "Channel Mini Cover" with mathematical zero-loss guarantees (D + B ≥ W). The system processes payments in milliseconds using EIP-712 signatures, pre-funded escrow channels, and automatic bond slashing, making micropayments practical for real-world API usage.