Context
AI agents are starting to call paid APIs, but API billing assumes a person: an account, a card, a monthly plan. An agent has none of those. It has a wallet and a single request to make.
x402 brings back HTTP’s long-unused 402 Payment Required status, so a client can pay inside the request itself. The server answers with a price, the client pays in USDC and retries, and the server serves the response. MonkePay is the layer that makes this one line for an API owner:
app.get("/v1/forecast", mp.gate({ price: "0.01" }), handler);What I built
- The SDK. TypeScript, shipped as ESM and CJS. A framework-agnostic core sits behind a nine-method adapter contract, with adapters for Express, Hono, Fastify and the Next.js App Router. The parity between them is documented, so a route behaves the same whichever framework it runs on. It supports per-request pricing, or one payment that unlocks permanent access with a signed token.
- Settlement. The agent pays into a per-developer Coinbase CDP wallet on Base. The SDK reports the payment to the backend over a signed channel. The backend confirms the actual USDC transfer on-chain before it records anything, then splits gross, fee and net.
- Money movement. Scheduled jobs sweep the 1% platform fee and pay out the rest to each developer’s address. Gas is topped up automatically before a sweep runs.
- The dashboard. Live payments, filters by agent and endpoint, and CSV export.
What was hard
Trusting nothing the SDK says about money. Every event from the SDK is HMAC-signed over a timestamp, a nonce and the payload, compared in constant time, and replay-blocked with a per-nonce key in Redis. A valid signature still isn’t enough. The claimed amount has to match the decoded transfer log, and a unique transaction hash stops the same payment being recorded twice. If the receipt isn’t on-chain yet, the backend retries until it is. It never records on a timer.
Moving money on a schedule, safely. Sweeps and payouts run under distributed Redis locks, with release and renewal written as Lua scripts that check the lock’s token, so one worker can never release another’s lock. Each transaction is claimed before it moves, keyed for idempotency, and released if the sweep fails, so a crash halfway through can be retried safely.
A bug worth keeping. Early on, a payout drew funds across networks. Balances are now strictly per network, and a monetary-safety regression suite guards that rule.
A payment failure is not an API failure. If recording a payment or a developer’s callback fails, the paid request still succeeds. The agent paid, so it gets its response.