Tax middleware for MPP (Machine Payments Protocol). Makes any machine payment endpoint tax-compliant with one line of code.

MPP enables AI agents to pay for services programmatically. But every transaction is a taxable digital service in most US states — and nobody is calculating that tax. This middleware fills the gap.

npm install @agenttax/mppx

import { Mppx, tempo } from 'mppx/express'
import { agentTax } from '@agenttax/mppx'

const mppx = Mppx.create({
  secretKey: process.env.MPP_SECRET_KEY,
  methods: [tempo({ recipient: '0x...', currency: USDC })],
})

const tax = agentTax({
  apiKey: process.env.AGENTTAX_API_KEY,
  transactionType: 'compute',
  workType: 'compute',
})

// Replace mppx.charge() with tax.charge()
app.get('/api/gpu-hour',
  tax.charge(mppx, { amount: '1.00', description: 'GPU compute hour' }),
  (req, res) => { res.json({ result: '...' }) }
)

The middleware:

1. Detects buyer jurisdiction (IP geolocation + optional X-Buyer-State header)
2. Calls AgentTax API to calculate sales tax
3. Adjusts the 402 challenge to the tax-inclusive amount
4. Attaches an X-Tax-Receipt header to the response

const tax = agentTax({
  apiKey: process.env.AGENTTAX_API_KEY,
  transactionType: 'compute',
  taxReserveWallet: '0x...your-tax-reserve',
})

Tax portion automatically routes to a separate wallet via MPP splits. Both wallets belong to the merchant. AgentTax never touches the money.

const tax = agentTax({
  apiKey: process.env.AGENTTAX_API_KEY,
  transactionType: 'compute',
  asset: {
    symbol: 'GPU_HOUR',
    trackGains: true,
    accountingMethod: 'fifo',
    residentState: 'TX',
  },
})

Every payment logged as a trade. Sell-side responses include realized gain/loss.

Buyers can self-report jurisdiction for higher accuracy:

• X-Buyer-State: TX — 2-letter state code
• X-Buyer-Zip: 78701 — 5-digit zip for local rates

The middleware cross-verifies against IP and flags mismatches. Datacenter/VPN IPs are detected and flagged automatically.

By default, if the AgentTax API can't be reached, the middleware rejects the charge with HTTP 503. This is the conservative default — charging base-amount-only with no tax receipt is a compliance gap. Your caller sees the error and can retry.

You can opt into legacy fail-open behavior with onTaxUnavailable: 'allow'. Read the warning below before doing this.

Setting onTaxUnavailable: 'allow' causes the middleware to proceed with a $0-tax receipt when the AgentTax API is unreachable. The charge still completes; the buyer is undercharged; you have no calculation trail for that transaction.

Use this setting only if both are true:

1. Your flow is demonstrably non-taxable in every jurisdiction you reach (e.g. SKUs limited to no-sales-tax states, or an exempt-sale-only platform).
2. You have a separate compliance control outside this middleware — an independent tax engine, a manual review queue, or a documented legal opinion that $0 tax is correct for every possible buyer you can reach.

Every fail-open invocation now emits:

• A console.warn line prefixed [agenttax/mppx] FAIL-OPEN: containing a structured JSON payload with timestamp, buyer state/ZIP, base amount, transaction type, and counterparty ID.
• An optional onFailOpenAudit(entry) callback you provide in config — use it to ship the event to Sentry, Datadog, a DB audit table, or anywhere else your retention policy requires.

const tax = agentTax({
  apiKey: process.env.AGENTTAX_API_KEY,
  transactionType: 'compute',
  onTaxUnavailable: 'allow',            // opt-in; read warning above
  onFailOpenAudit: (entry) => {
    await db.query(
      'INSERT INTO fail_open_audit(event, ts, state, amount, counterparty, tx_type) VALUES ($1,$2,$3,$4,$5,$6)',
      [entry.event, entry.timestamp, entry.buyer_state, entry.base_amount, entry.counterparty_id, entry.transaction_type]
    );
  },
});

If you run with 'allow' and no onFailOpenAudit sink, you still get the stderr line — but shipping those to a proper audit store (not just Vercel function logs, which rotate) is on you.

curl -X POST https://agenttax.io/api/v1/auth/signup \
  -H "Content-Type: application/json" \
  -d '{"email": "you@example.com", "password": "securepass", "agent_name": "my-agent"}'

Free tier: 100 calls/month. Save the api_key.key from the response — it's only shown once.

• AgentTax
• MPP Protocol
• Agent Integration Guide
• MCP Server

MIT