x402-trinity

A drop-in fetch replacement that lets an AI agent pay for things by itself — with hard spending limits, no hosted wallet service, and no changes to the agent's own code.

Zero runtime dependencies. 7 KB gzipped. Buyer and seller in TypeScript; buyer in Python.

npm install x402-trinity

Why

When an agent hits a paid resource it gets 402 Payment Required. Without something to handle that, the request simply fails.

x402-trinity handles it: reads the challenge, checks it against limits you set, signs, retries. All of it locally — no hosted wallet service, no third-party API, and the key never leaves your process.

Gas exists but the payer doesn't pay it — under EIP-3009 the facilitator submits the transfer, so an agent's wallet only ever needs USDC.

Related MCP server: remit.md MCP Server

Buy (agent pays for things)

import { createX402Fetch } from 'x402-trinity';

const x402Fetch = createX402Fetch({
  privateKey: process.env.X402_PRIVATE_KEY,   // never hardcode
  policy: {
    maxAmountPerRequest: '5000',              // 0.005 USDC max per call  (REQUIRED)
    totalBudget: '1000000',                   // 1.00 USDC lifetime       (REQUIRED)
    allowHosts: ['api.example.com'],
    allowPayTo: ['0x...'],
  },
});

const r = await x402Fetch('https://api.example.com/data');   // 402 handled, returns 200

Or patch the global so unmodified code pays automatically:

import { installX402 } from 'x402-trinity';
const uninstall = installX402(cfg);   // globalThis.fetch now pays 402s

Confirm which wallet will pay before funding anything:

X402_PRIVATE_KEY=0x... npx x402-trinity-whoami

It prints the address and its balance on every chain, and never prints the key. If the address is not the wallet you meant, stop before sending anything.

Sell (charge for a resource, get paid)

import { createX402Seller } from 'x402-trinity/seller';
import { createFileNonceStore } from 'x402-trinity/budget-file';

const seller = createX402Seller({
  payTo: '0xYourWallet',      // 100% of every payment lands here
  price: '10000',             // 0.01 USDC, atomic units
  network: 'base',
  facilitator: 'https://your-facilitator.example',   // REQUIRED — no default exists
  nonceStore: createFileNonceStore('./.x402-nonces.json'),  // REQUIRED — see below
});

// in any fetch-style handler:
const gate = await seller.guard(request);
if (gate.response) return gate.response;          // unpaid or refused — hand back the 402
return new Response(yourData, { headers: seller.receiptHeader(gate.settlement) });

Both required fields are deliberate. There is no default facilitator because settling is someone's real money and guessing an endpoint is not a default. And the replay guard has to outlive the process: an in-memory one forgets every settled payment on restart, so a buyer could re-present a spent authorization and get the resource again for free. The constructor throws rather than let either be implicit.

The seller never holds a private key and never touches funds. It quotes a price and asks a facilitator to verify and settle; money moves buyer → you directly on-chain.

This release ships Base + USDC only. Any other EVM chain works through customChains; your address is the same on all of them.

What it does

Another chain

Base is the default. The signing is chain-agnostic, so add whatever you need — including a testnet to rehearse on:

const fetch2 = createX402Fetch({
  privateKey: process.env.X402_PRIVATE_KEY,
  maxAmountPerRequest: '10000',
  totalBudget: '100000',
  customChains: {
    'base-sepolia': { id: 84532, asset: '0x036cbd53842c5426634e7929541ec2318f3dcf7e', name: 'USDC', version: '2' },
  },
});

Verify the entry against the deployed contract first: call DOMAIN_SEPARATOR() and check it equals what this library computes. A wrong name or version produces a signature that looks valid and the contract rejects.

It deliberately will not

Broadcast to a chain · hold funds · need gas or an RPC · take a cut of a payment (structurally impossible — EIP-3009 has one recipient) · require a hosted signer · pay without limits · guess at a protocol it doesn't speak.

Safety — read this before real money

Caps are mandatory. maxAmountPerRequest and totalBudget have no defaults; the wrapper refuses to construct without them. An auto-payer without limits is a money leak controlled by whoever runs the server.

totalBudget alone is per-instance. It is an in-memory counter that resets on restart, on a new client, and on every Cloudflare Worker isolate — which is per request. On mainnet that turns a lifetime cap into a per-request cap. Mainnet therefore requires a durable budgetStore, or an explicit acknowledgeEphemeralBudget: true:

import { createFileBudgetStore } from 'x402-trinity/budget-file';

createX402Fetch({
  ...,
  budgetStore: createFileBudgetStore('./.x402-budget.json'),   // survives restarts
});

The key is in the process. That is the trade for having no hosted signer. Bound it: use a dedicated wallet holding only what you would accept losing, set allowPayTo so an exfiltrated key still cannot pay a stranger through this wrapper, and use remoteSign if you need enclave-grade custody.

Timing hardening is hardening, not a proof. Secret scalars use blinding plus always-add-and-double, which cut the timing spread from 99.6% to 12% (Python: 99.9% → 1.1%). BigInt arithmetic is itself variable-time and no pure-JS implementation removes that.

MCP server — give an agent a wallet

MCP is how an assistant gets tools. This exposes three:

{
  "mcpServers": {
    "x402-trinity": {
      "command": "npx",
      "args": ["-y", "x402-trinity-mcp"],
      "env": {
        "X402_PRIVATE_KEY": "0x...",
        "X402_MAX_PER_REQUEST": "50000",
        "X402_TOTAL_BUDGET": "1000000",
        "X402_BUDGET_FILE": "./.x402-budget.json",
        "X402_ALLOW_HOSTS": "api.example.com",
        "X402_NETWORKS": "base"
      }
    }
  }
}

A model decides when these run. It cannot be reasoned with about budgets, and a paywalled page can claim any price it likes. So the limits are not parameters the model can set — they come from the environment, and the server refuses to start without X402_PRIVATE_KEY, X402_MAX_PER_REQUEST and X402_TOTAL_BUDGET.

Set X402_BUDGET_FILE too, or the lifetime ceiling resets every restart. Set X402_ALLOW_HOSTS and nothing else can be paid, however convincing the challenge looks. Use a wallet holding only what you would accept losing.

Python

Same protocol, standard library only. Buyer only — to charge for a resource, use the TypeScript seller. Aimed at long-lived processes: on-body agents, robotics controllers, harvesting scripts.

from x402_trinity import X402Client, Policy

client = X402Client(
    private_key=os.environ["X402_PRIVATE_KEY"],
    policy=Policy(max_amount_per_request=5000, total_budget=1_000_000,
                  allow_hosts=["api.example.com"]),
)
body = client.urlopen("https://api.example.com/data").read()

Or as a decorator, so payment-unaware code just works:

from x402_trinity import x402_telemetry

@x402_telemetry(private_key=KEY, policy=Policy(...))
def harvest():
    return urllib.request.urlopen("https://sensor.local/v1/lidar").read()

The warm path is 1.1 µs — a long-lived controller is warm after its first payment.

Development

npm install
npm run build        # dist/*.js + *.min.js + *.d.ts

esbuild, typescript and wrangler are dev dependencies only, used for building. The shipped package has zero runtime dependencies.

License

Business Source License 1.1. The source is open to read, modify and use non-commercially; production use is granted except as a hosted or managed service offering its functionality to third parties. It converts to MIT on 2029-08-25. See LICENSE.

This software moves money — read the additional notice there, and set your spending caps.