Autonomous Agent Payments

AI agents need to call APIs autonomously — without human-managed API keys, billing accounts, or pre-authorized subscriptions. Two open standards, x402 and MPP (Machine Payments Protocol), solve this by embedding micropayments directly into HTTP requests.

0x Swap API endpoints are available through both protocols. Agents pay $0.01 per request in USDC. No 0x API key required — payment is the authentication.

Using an AI coding agent?

Install the 0x-agentic-gateway skill. Your agent will handle protocol selection, package installation, wallet setup, and code generation automatically.

$
npx skills add 0xProject/0x-ai

Then activate it in your agent session:

/0x-agentic-gateway

Manual integration

The sections below cover endpoints, wallet requirements, and working code examples for both protocols.

How it works

Both protocols follow the same HTTP-native pattern:

Every payment includes the agent’s wallet address and an on-chain transaction hash in the PAYMENT-RESPONSE header — giving cryptographically verifiable per-caller attribution without API key management.

x402

x402 is an open standard that repurposes the HTTP 402 Payment Required status code for real-time stablecoin payments. Agents sign a USDC EIP-3009 transferWithAuthorization payload (EVM) or a Solana USDC transfer transaction (SVM), then retry the request with the encoded signature.

Endpoints

The same URLs are used regardless of payment network. The payment scheme your client registers (EVM or Solana) determines which chain the payment runs on. The swap execution chain is controlled by the chainId query parameter.

Cost

Wallet requirement: EVM wallet funded with USDC on Base (0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913), or a Solana wallet funded with USDC on Solana mainnet (EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v). When paying via Solana, an EVM wallet is also required — swap execution runs on EVM, so the taker in the quote must be an EVM address.

Get a swap price and quote

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import { createPublicClient, http } from "viem";
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import { base } from "viem/chains";
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import { privateKeyToAccount } from "viem/accounts";
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import { wrapFetchWithPayment, x402Client } from "@x402/fetch";
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import { ExactEvmScheme, toClientEvmSigner } from "@x402/evm";
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const account = privateKeyToAccount(process.env.WALLET_PRIVATE_KEY);
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const publicClient = createPublicClient({ chain: base, transport: http() });
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const signer = toClientEvmSigner(account, publicClient);
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const core = new x402Client();
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core.register("eip155:8453", new ExactEvmScheme(signer));
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// Refuse to sign any payment over $0.02
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core.registerPolicy((_version, requirements) =>
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  requirements.filter((r) => BigInt(r.amount) <= 20_000n),
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);
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const x402Fetch = wrapFetchWithPayment(fetch, core);
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const BASE = "https://agent.api.0x.org/v1/x402";
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const params =
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  "?chainId=8453" + // Base — any EVM chain supported by 0x Swap API works here
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  "&sellToken=0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913" + // USDC on Base
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  "&buyToken=0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE" + // ETH
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  "&sellAmount=100000"; // 0.1 USDC
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// Price — indicative, no taker required
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const priceRes = await x402Fetch(
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  `${BASE}/swap-allowance-holder-price/${params}`,
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);
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const price = await priceRes.json();
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// Quote — firm quote with transaction calldata
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const quoteRes = await x402Fetch(
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  `${BASE}/swap-allowance-holder-quote/${params}&taker=${account.address}`,
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);
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const quote = await quoteRes.json();

Reading the payment response

After a successful request, the PAYMENT-RESPONSE header contains base64-encoded settlement data:

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const settlement = JSON.parse(
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  Buffer.from(priceRes.headers.get("PAYMENT-RESPONSE"), "base64").toString(
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    "utf8",
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  ),
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);
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console.log(settlement.success); // true
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console.log(settlement.transaction); // on-chain tx hash
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console.log(settlement.network); // e.g. "eip155:8453" or "solana:5eykt4..."
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console.log(settlement.payer); // agent's wallet address

MPP

MPP (Machine Payments Protocol) is an open standard for machine-to-machine HTTP payments. It uses an extensible challenge–credential–receipt pattern: the server issues a WWW-Authenticate challenge, the client returns a credential (payment proof), and the server confirms with a receipt.

0x MPP endpoints use Tempo Mainnet (chainId 4217) as the payment rail, settled in USDC.e.

Endpoints

Both endpoints proxy the 0x Swap API v2 (AllowanceHolder flow). The chainId parameter controls which EVM chain the swap executes on — check valid chains in the Supported Chains list.

Cost

Wallet requirement: EVM wallet funded with USDC.e on Tempo Mainnet (0x20C000000000000000000000b9537d11c60E8b50).

Get a swap price and quote

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import "dotenv/config";
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import { Mppx, tempo } from "mppx/client";
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import { privateKeyToAccount } from "viem/accounts";
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const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
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const mppx = Mppx.create({
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  methods: [
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    tempo.charge({
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      account,
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      mode: "push", // client broadcasts the tx and sends the hash to the proxy
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    }),
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  ],
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});
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const BASE_URL = "https://agent.api.0x.org/v1/mpp-tempo";
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const params = new URLSearchParams({
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  chainId: "8453", // Base — any EVM chain supported by 0x Swap API works here
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  sellToken: "0x4200000000000000000000000000000000000006", // WETH on Base
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  buyToken: "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", // USDC on Base
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  sellAmount: "1000000000000000", // 0.001 WETH
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});
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// Price — indicative, no taker required
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const priceRes = await mppx.fetch(
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  `${BASE_URL}/swap-allowance-holder-price/?${params}`,
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);
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const price = await priceRes.json();
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// Quote — add taker address for firm quote with calldata
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params.set("taker", account.address);
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const quoteRes = await mppx.fetch(
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  `${BASE_URL}/swap-allowance-holder-quote/?${params}`,
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);
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const quote = await quoteRes.json();
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console.log(quote.transaction.to); // AllowanceHolder contract
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console.log(quote.transaction.data); // calldata to submit

Next steps

• 0x Agentic Gateway skill — install with npx skills add 0xProject/0x-ai
• x402 documentation — full protocol spec
• MPP documentation — full protocol spec and SDK reference
• 0x Swap API reference — query parameters, response fields, and error codes
• Supported Chains — chains available for swap execution