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飛錢 Flying Money · open source · test network
Fund a budget for a seller you choose. Use it yourself, or let a person or an AI assistant use it. See what was spent and what’s left.
Set a budget in digital dollars (USDC) for one API or service. Your agent pays per request, the service checks each payment on the spot, and collects the total later in one transaction. The agent can’t raise its budget or spend past it, even if its key is stolen. After the end date, you can take back what’s left.
Live on Arbitrum Sepolia with test money · Open source (MIT) · Not yet audited · See networks →
Pays only Silk Road Oracle. 5.00 of 5.00 left.
Chapter 1 · The problem
Ways to let an agent pay
Give it a budget instead: one service, one amount, one end date.
Agents buy API calls, data and compute thousands of times a day, for fractions of a cent. The limit has to live outside the prompt, because prompts can be hijacked.
Ways to hand over money
Money for one place, with a limit that holds and leftovers you take back.
A parent, an employer or a friend wants to hand over money for one thing, without handing over a card. The shop wants to be sure the money is really there.
Chapter 2 · How it works
Chapter 3 · Trust
Why there’s no cancel button: a shop can accept a payment on the spot, even offline, only because the money can’t be pulled back halfway through. You control where, how much and how long, and whether to renew. Read the full guarantees →
Chapter 4 · Use cases
5 USDC for one data API, this week. It stops at 5.00.
Let Claude call paid tools inside a limit it can’t raise.
50 USDC at the school canteen, this term.
20 USDC at your usual café. No fee for you.
Fuel money for one station, with no company card to lose.
Sent as a link. After the end date, the sender can take back whatever isn’t spent.
Chapter 5 · For developers
Your agent uses a drop-in fetch that answers HTTP 402 “Payment Required” by itself. Your API adds one middleware, checks every payment on its own server, and collects in batches of up to 20.
import { createFlyingMoneyClient, fileStore } from '@flying-money/client'
import { privateKeyToAccount } from 'viem/accounts'
const fm = createFlyingMoneyClient({
chains: ['arbitrum-sepolia'],
spender: privateKeyToAccount(process.env.AGENT_KEY),
store: fileStore('.flying-money.json'), // crash-safe outbox
certificates: [process.env.AGENT_CERTIFICATES], // the budget your owner funded
maxPricePerRequest: 50_000n, // 0.05 USDC
})
const res = await fm.fetch('https://oracle.example/v1/tea-price?city=Luoyang')import { flyingMoney } from '@flying-money/server/hono'
import { upstashStore } from '@flying-money/server'
app.use('/v1/*', flyingMoney({
accepts: ['arbitrum-sepolia'],
payee: process.env.PAYEE_ADDRESS,
price: () => 10_000n, // 0.01 USDC per request
store: upstashStore({ url, token }), // durable, shared state
}))
app.get('/v1/tea-price', (c) => c.json({ price: 42 })){
"mcpServers": {
"flying-money": {
"command": "node",
"args": ["flying-money/packages/mcp/dist/bin.js"],
"env": {
"AGENT_KEY": "0x…",
"AGENT_CERTIFICATES": "0x…",
"FM_MAX_PRICE": "0.05"
}
}
}
}Agent quickstart →Seller quickstart →llms.txt for your agent →
Chapter 6 · Why “flying money”
In Tang-dynasty China, tea merchants were tired of hauling heavy strings of coin. They deposited the coin at an official office and travelled with a certificate that paid out when its tallies matched. People called it 飛錢, flying money. We use the same idea: put the money aside first, carry a proof instead, settle later.
It runs live on a test network in about 30 seconds. Cut its connection. Steal its key. Watch what gets refused.