> ## Documentation Index > Fetch the complete documentation index at: https://docs.blankfx.org/llms.txt > Use this file to discover all available pages before exploring further. # Paying a payment link with x402 > How an x402 agent pays a BlankFX payment link on Sepolia (testnet), including the one-time setup and error codes. # Paying a payment link with x402 A merchant who has switched on **Agent payments** lets any x402 v2 agent pay their payment links. Funds move straight from the agent's wallet to the merchant's wallet. BlankFX pays the gas and records the payment. > [!WARNING] > > Sepolia (testnet) and test tokens only, switched on per environment. Ethereum (mainnet) is not yet supported. This route needs no API key and is served at `https://api.blankfx.org/x402/pay/{payment-link-id}` (not under `/public/v1`). ## The flow ``` GET /x402/pay/{payment-link-id} → 402 + PAYMENT-REQUIRED header (base64 JSON: price, token, recipient) GET /x402/pay/{payment-link-id} + PAYMENT-SIGNATURE header (the signed payment) → 200 + PAYMENT-RESPONSE header (success, transaction, network, payer) ``` An x402 client library does both steps for you. ## What the `402` tells the agent | Field | Value | | - | - | | `scheme` / `network` | `exact` / `eip155:11155111` | | `asset` | The invoice's token (a BlankFX Sepolia token) | | `amount` | **Invoice amount plus the BlankFX transfer fee**, in the token's smallest unit. The payer pays the fee on top. | | `payTo` | The merchant's payout wallet | | `extra.assetTransferMethod` | `permit2` | ## One-time setup for the agent's wallet **1. Approve Permit2 once** None of the BlankFX Sepolia tokens support EIP-3009, so payment uses Permit2. Do a one-time ERC-20 `approve` of the token to the canonical Permit2 contract `0x000000000022D473030F116dDEE9F6B43aC78BA3`. The agent pays that gas itself. The approval costs roughly 136,000 gas on these tokens, so if your client sets a gas limit by hand, use at least 200,000. Without it BlankFX answers `412` (`PERMIT2_ALLOWANCE_REQUIRED`), reserves nothing, and the official client does **not** approve for you. **2. Allow the token in your client** The official client refuses any token missing from its built-in default list. Add the BlankFX token to `spendControls.allowedAssets`, or the payment fails with "rejected by spendControls". **3. Raise the cap if needed** The client caps each payment at **\$1** by default. For an allow-listed token, set `maxAmountPerPayment` on its entry. ## Minimal client ```ts theme={null} import { wrapFetchWithPaymentFromConfig } from '@x402/fetch' import { ExactEvmScheme } from '@x402/evm' import { privateKeyToAccount } from 'viem/accounts' const account = privateKeyToAccount(process.env.AGENT_KEY as `0x${string}`) const pay = wrapFetchWithPaymentFromConfig(fetch, { schemes: [{ network: 'eip155:11155111', client: new ExactEvmScheme(account, { rpcUrl: process.env.SEPOLIA_RPC }) }], spendControls: { allowedAssets: [{ network: 'eip155:11155111', asset: '', maxAmountPerPayment: '100000000' }], }, }) const res = await pay('https://api.blankfx.org/x402/pay/') ``` Verified with `@x402/fetch` and `@x402/evm` v2.28.0, including a live payment on Sepolia (October 2026). ## Example: what a successful payment looks like For a 1 USDC invoice with a 0.05% transfer fee, the `402` asks for `1000500` (6 decimals, so 1.0005 USDC). After the signed retry the response is `200` and the `PAYMENT-RESPONSE` header decodes to: ```json theme={null} { "success": true, "transaction": "0x…", "network": "eip155:11155111", "payer": "0x…" } ``` The body is the invoice with `paidVia: "x402"`. The merchant sees the payer and the fee in Payment Status; nobody else does. ## What BlankFX checks before it spends gas * The recovered signer equals the payer. * The recipient, token and exact amount equal the challenge BlankFX issued (not what the client echoes). * The authorization has 30 to 90 seconds of validity left. * The Permit2 nonce is unused. * The wallet holds the amount and has approved Permit2. * The payer's wallet is not suspended and stays within the \$2,000/day cap for unverified wallets, the same checks a human payer meets. ## Errors | Status | Meaning | | - | - | | `402` with `PAYMENT-RESPONSE` `{ "success": false, "errorReason": "" }` | Payment refused. A fresh `PAYMENT-REQUIRED` challenge is attached when one can be issued. `errorReason` is one of the codes below. | | `409` `{ "error": "payment_in_progress" }` | Another payment for this link is already settling. Retry later; do not sign a new payment. | | `409` `{ "error": "Payment request is " }` | The link is cancelled or expired. | | `412` | Permit2 allowance missing (`PERMIT2_ALLOWANCE_REQUIRED`, `errorReason: "permit2_allowance_required"`). Nothing was reserved. | | `400` `{ "error": "invalid_payload" }` | Malformed `PAYMENT-SIGNATURE`, or a token, spender or validity window that doesn't match the challenge. | | `404` `{ "error": "not_found" }` | The link doesn't exist, doesn't accept agent payments, or agent payments are switched off. All of these look the same on purpose. | | `429` | Rate limited: 60 requests/min per IP, **30/min per payment link**, 20/min per payer. Honour `Retry-After`. | | `503` `{ "error": "unavailable" }` | No challenge could be issued right now (for example the fee could not be read). Retry. | `errorReason` codes on a `402`: | Code | Meaning | | - | - | | `wrong_amount` | Amount differs from the challenge (invoice plus fee). | | `wrong_recipient` | `to` is not the merchant's payout wallet from the challenge. | | `insufficient_funds` | The payer's balance is below the amount. | | `nonce_used` | The Permit2 nonce was already used. | | `expired` | The authorization's deadline has passed. | | `not_yet_valid` | The authorization is not valid yet. | | `insufficient_validity` | Less than 30 seconds of validity left. Sign a fresh payment. | | `invalid_signature` | The signature does not recover to the payer. | | `unsupported_scheme` | Scheme or network differs from the challenge, or the transfer method is not `permit2`. | | `kyc_required` | The payer is over the unverified cap or not permitted. | | `payer_blocked` | The payer is temporarily blocked after repeated failed settlements. | | `capacity` | The daily agent-payment budget for this merchant or for BlankFX is used up. | | `invoice_changed` | The link changed (paid, cancelled) after the challenge was issued. | | `settlement_pending` | The transaction was sent but is not confirmed yet. `PAYMENT-RESPONSE` carries the `transaction`. | | `settlement_failed` | The settlement transaction could not be sent. | | `fee_unavailable` | The current transfer fee could not be read. Retry. | | `unavailable` | The challenge is unknown or expired. Request a new `402`. | Chain error text is never returned. If settlement is still confirming, retry the **same signed payment**: it returns the current state and never sends a second transaction. ## Not supported yet EIP-3009 tokens (none on Sepolia), converting between coins, gas-sponsored approvals, browser-based agents (no CORS on this route) and mainnet. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.