Open a tab for your agent.

import { createPayFetch } from 'tab'

const payFetch = createPayFetch({
  agentKey: process.env.AGENT_KEY,
  owner: '0xYourWallet',
  maxAmount: '250000',   // refuse anything over 0.25 USDG
})

const res = await payFetch('https://example.com/research', {
  method: 'POST',
  body: JSON.stringify({ q: 'NVDA guidance' }),
})
console.log(res.paid, res.payment.txHash)

A 402 is answered by signing with the key and retrying once. Anything else passes through untouched. The key is a plain private key, so the same wrapper works from a cron job, a Claude tool, or an MCP server.

import { createPaywall } from 'tab'

const wall = createPaywall({
  facilitator: 'https://tab.example/x402',
  payTo: '0xYourWallet',
  asset: USDG, vault: VAULT, chainId: 4663,
  price: '10000',        // 0.01 USDG per call
})

app.post('/research', wall.middleware(async (req, res) => {
  res.json(await research(req.body.q))
}))

The route settles before it runs by default, so the work is paid for before it starts. Set settle to after if you would rather serve first.

const wall = createPaywall({
  ...common,
  // price is computed per request: 0.002 USDG per
  // 100 characters of input, floor 0.005
  price: req => {
    const chars = Number(req.headers['content-length'] || 0)
    return String(Math.max(5000, Math.ceil(chars / 100) * 2000))
  },
})

Price can be a function of the request. The 402 states the exact amount, the client checks it against its own limit, and the signature covers that amount and nothing else.

const res = await fetch(url)
if (res.status === 402) {
  const { accepts } = await res.json()
  const offer = accepts.find(a => a.scheme === 'tab-vault')
  const payload = await signPayment({ agentKey, owner, ...offer })
  return fetch(url, { headers: {
    'X-PAYMENT': btoa(JSON.stringify({
      x402Version: 1, scheme: 'tab-vault',
      network: offer.network, payload,
    })),
  }})
}

The wire format is two JSON documents and one EIP-712 signature. A client in any language can produce it. The protocol page has every field.