> ## Documentation Index > Fetch the complete documentation index at: https://docs.mrdn.finance/llms.txt > Use this file to discover all available pages before exploring further. # Quickstart > Accept your first x402 payment in minutes This guide takes you from zero to accepting USDC payments through Meridian on any supported chain. The only dependency is `viem` — your server returns an x402 challenge, the buyer signs a payment authorization off-chain, and your server settles it through Meridian's API. No transaction is submitted by the buyer and no gas is paid by either side. ```text theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} 1. Your server returns 402 + paymentRequirements 2. Buyer signs an EIP-3009 authorization (off-chain, gasless) 3. Buyer sends the signed payload to your server 4. Your server settles via Meridian POST /v1/settle 5. Meridian's facilitator executes the transfer on-chain ``` The code below is identical on every chain whose payment token supports EIP-3009 (Base, Optimism, Arbitrum, Polygon, Ink, and more) — only the network constants change. For chains without an EIP-3009 token (MegaETH, BSC, BOT chain, Tempo), the buyer signs a Permit2 witness instead; see the [non-EIP-3009 guide](/ai-skills/non-eip-3009). Sign in with your wallet at [mrdn.finance/dev/api-keys](https://mrdn.finance/dev/api-keys) and create a key. Use a `pk_test_` key for development and a `pk_` key for production. Store it server-side: ```bash theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} MERIDIAN_API_KEY=pk_test_... ``` See [API Keys](/essentials/api-keys) for details. In your organization settings, configure the wallet that receives settled funds. The recipient is tied to your API key — in the payment requirements, `payTo` always stays pointed at the Meridian facilitator contract, never at your own wallet. Take the payment token and facilitator address for your chain from [Supported Networks](/api-reference/supported-networks) and keep them in one config object — everything below reads from it: ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} // Example: Base mainnet. Swap in any supported network's values. const NETWORK = { id: "base", chainId: 8453, token: "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", // USDC tokenName: "USD Coin", // EIP-712 domain name of the token tokenVersion: "2", // EIP-712 domain version of the token facilitator: "0x8E7769D440b3460b92159Dd9C6D17302b036e2d6", }; ``` For development, use a testnet (`pk_test_` key), e.g. Base Sepolia: `chainId: 84532`, token `0x036CbD53842c5426634e7929541eC2318f3dCF7e` (`tokenName: "USDC"`), facilitator `0x8e633dBf31adCc7D41BE3e95B7c8DD3526B5235A`. Test USDC is available from [Circle's faucet](https://faucet.circle.com). A minimal Base Sepolia test should use: * `MERIDIAN_API_KEY=pk_test_...` * a buyer wallet connected to Base Sepolia (`84532`) * Base Sepolia USDC from Circle's faucet * `paymentRequirements.network: "base-sepolia"` * `paymentRequirements.payTo: "0x8e633dBf31adCc7D41BE3e95B7c8DD3526B5235A"` When an unpaid request hits your paid endpoint, respond with HTTP 402 and the payment requirements: ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} const paymentRequirements = { scheme: "exact", network: NETWORK.id, asset: NETWORK.token, payTo: NETWORK.facilitator, maxAmountRequired: "10000", // $0.01 in USDC base units (6 decimals) resource: "https://your-api.example.com/api/premium", description: "Access to premium content", mimeType: "application/json", maxTimeoutSeconds: 300, extra: { name: NETWORK.tokenName, version: NETWORK.tokenVersion }, }; return Response.json( { x402Version: 1, accepts: [paymentRequirements] }, { status: 402 }, ); ``` To accept payments from more than one chain, return one requirement per source chain in `accepts` — see [Payment Types](/api-reference/payment-types/overview). The buyer signs an EIP-3009 `TransferWithAuthorization` as typed data — no approval, no transaction: ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} import { bytesToHex } from "viem"; const now = BigInt(Math.floor(Date.now() / 1000)); const authorization = { from: account.address, to: paymentRequirements.payTo as `0x${string}`, value: BigInt(paymentRequirements.maxAmountRequired), validAfter: 0n, validBefore: now + BigInt(paymentRequirements.maxTimeoutSeconds), nonce: bytesToHex(crypto.getRandomValues(new Uint8Array(32))), }; const signature = await walletClient.signTypedData({ account, domain: { name: paymentRequirements.extra.name, version: paymentRequirements.extra.version, chainId: NETWORK.chainId, verifyingContract: paymentRequirements.asset as `0x${string}`, }, types: { TransferWithAuthorization: [ { name: "from", type: "address" }, { name: "to", type: "address" }, { name: "value", type: "uint256" }, { name: "validAfter", type: "uint256" }, { name: "validBefore", type: "uint256" }, { name: "nonce", type: "bytes32" }, ], }, primaryType: "TransferWithAuthorization", message: authorization, }); ``` Then send the payload to your server (bigints as decimal strings): ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} const paymentPayload = { x402Version: 1, scheme: "exact", network: paymentRequirements.network, payload: { signature, authorization: { from: authorization.from, to: authorization.to, value: authorization.value.toString(), validAfter: authorization.validAfter.toString(), validBefore: authorization.validBefore.toString(), nonce: authorization.nonce, }, }, }; ``` Your server settles the payment through Meridian, then serves the paid response. Never expose the API key to the browser, and do not trust buyer-supplied pricing, recipient, token, or network values. Rebuild or look up the matching `paymentRequirements` on the server before settlement: ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} if (paymentPayload.network !== paymentRequirements.network) { throw new Error("Payment network does not match the server requirement"); } if ( paymentPayload.payload.authorization.to.toLowerCase() !== paymentRequirements.payTo.toLowerCase() ) { throw new Error("Payment must authorize the Meridian facilitator"); } if ( paymentPayload.payload.authorization.value !== paymentRequirements.maxAmountRequired ) { throw new Error("Payment amount does not match the server price"); } ``` ```typescript theme={"theme":{"light":"github-light","dark":"github-dark-high-contrast"}} const response = await fetch("https://api.mrdn.finance/v1/settle", { method: "POST", headers: { Authorization: `Bearer ${process.env.MERIDIAN_API_KEY}`, "Content-Type": "application/json", }, body: JSON.stringify({ paymentPayload, paymentRequirements }), }); const result = await response.json(); if (!response.ok || !result.success) { throw new Error(result.errorReason ?? "Settlement failed"); } ``` Open [Command Centre](/command-centre/transactions) to see the payment, its settlement status, and your balance. ## Next steps The full client-side flow: multi-chain challenges, requirement selection, and error handling. EIP-3009, Permit2, and Circle Gateway — and when to use which. Chains, tokens, and contract addresses. Self-contained integration instructions for AI coding agents.