X402 Payments

x402: Crypto Payments

The x402 protocol implements HTTP 402 (Payment Required) for pay-per-request APIs. Clients pay with signed EIP-3009 USDC authorizations — the payer signs off-chain (no gas), and the server or facilitator settles on-chain. Nethereum's Nethereum.X402 package provides X402HttpClient for automatic payments and X402Middleware for ASP.NET Core endpoint protection.

NuGet: Nethereum.X402

dotnet add package Nethereum.X402

Client: Pay Automatically

X402HttpClient handles the full 402 flow — detect payment requirement, sign EIP-3009 authorization, retry with payment:

using Nethereum.X402.Client;

var options = new X402HttpClientOptions
{
    MaxPaymentAmount = 0.1m,       // Safety limit per request
    PreferredNetwork = "base",
    TokenName = "USD Coin",
    TokenVersion = "2",
    ChainId = 8453,
    TokenAddress = "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"
};

var x402Client = new X402HttpClient(httpClient, privateKey, options);
var response = await x402Client.GetAsync("https://api.example.com/premium/content");

// Check payment result from response headers
if (response.HasPaymentResponse())
{
    var txHash = response.GetTransactionHash();
    var payer = response.GetPayerAddress();
    var success = response.IsPaymentSuccessful();
}

Supports GetAsync, PostAsync, PutAsync, DeleteAsync, SendAsync. Throws X402PaymentExceedsMaximumException if requested amount exceeds MaxPaymentAmount.

Manual Payment Flow

Pass explicit PaymentRequirements for full control:

var x402Client = new X402HttpClient(httpClient, privateKey, "USD Coin", "2", 8453, usdcAddress);

var requirements = new PaymentRequirements
{
    Scheme = "exact",
    Network = "base",
    MaxAmountRequired = "1000000",  // $1.00 USDC (6 decimals)
    PayTo = receiverAddress,
    Resource = "/api/premium",
    Description = "Premium content",
    MaxTimeoutSeconds = 60
};

var response = await x402Client.GetAsync("https://api.example.com/premium", requirements);

Server: Protect Endpoints

Use route-based middleware to gate endpoints:

using Nethereum.X402.AspNetCore;
using Nethereum.X402.Server;
using Nethereum.X402.Models;

builder.Services.AddX402Services("https://facilitator.x402.org");

app.UseX402(options =>
{
    options.Routes.Add(new RoutePaymentConfig("/api/premium/*", new PaymentRequirements
    {
        Scheme = "exact",
        Network = "base",
        MaxAmountRequired = "1000000",
        Asset = "USDC",
        PayTo = "0xYourAddress",
        Resource = "/api/premium",
        Description = "Premium API access",
        MaxTimeoutSeconds = 60
    }));
});

Self-Facilitated (No External Facilitator)

Process payments on-chain directly:

using Nethereum.X402.Extensions;

// Transfer model: facilitator account pays gas
builder.Services.AddX402TransferProcessor(
    facilitatorPrivateKey: Environment.GetEnvironmentVariable("KEY"),
    rpcEndpoints: new Dictionary<string, string> { ["base"] = "https://mainnet.base.org" },
    tokenAddresses: new Dictionary<string, string> { ["base"] = usdcAddress },
    chainIds: new Dictionary<string, int> { ["base"] = 8453 },
    tokenNames: new Dictionary<string, string> { ["base"] = "USD Coin" },
    tokenVersions: new Dictionary<string, string> { ["base"] = "2" });

// Or Receive model: receiver pays gas
builder.Services.AddX402ReceiveProcessor(
    receiverPrivateKey: Environment.GetEnvironmentVariable("KEY"),
    ...same config dictionaries...);

Two Payment Models

Both implement IX402PaymentProcessor with VerifyPaymentAsync, SettlePaymentAsync, GetSupportedAsync.

Build Authorizations Directly

For custom payment flows:

using Nethereum.X402.Signers;

var builder = new TransferWithAuthorisationBuilder();
var signer = new TransferWithAuthorisationSigner();

var authorization = builder.BuildFromPaymentRequirements(requirements, payerAddress);
// Default: validAfter = 10 min ago, validBefore = 1 hour from now

var signature = await signer.SignWithPrivateKeyAsync(
    authorization, "USD Coin", "2", chainId, usdcAddress, payerPrivateKey);

// Or sign with Web3 account (hardware wallets, KMS)
var signature = await signer.SignWithWeb3Async(
    authorization, "USD Coin", "2", chainId, usdcAddress, web3, signerAddress);

Error Codes

Supported Tokens

For full documentation, see: https://docs.nethereum.com/docs/defi/guide-x402-payments