Build an x402 Payment Server Create an Express server that charges USDC for API access using the x402 payment protocol. Callers pay per-request in USDC on Base — no accounts, API keys, or subscriptions needed. Your service is automatically discoverable by other agents via the x402 Bazaar. How It Works x402 is an HTTP-native payment protocol. When a client hits a protected endpoint without paying, the server returns HTTP 402 with payment requirements. The client signs a USDC payment and retries with a payment header. The facilitator verifies and settles the payment, and the server returns the response. Services register with the x402 Bazaar so other agents can discover and pay for them automatically. Confirm wallet is initialized and authed npx awal@2.0.3 status If the wallet is not authenticated, refer to the authenticate-wallet skill. Step 1: Get the Payment Address Run this to get the wallet address that will receive payments: npx awal@2.0.3 address Use this address as the payTo value. Step 2: Set Up the Project mkdir x402-server && cd x402-server npm init -y npm install express @x402/express @x402/core @x402/evm @x402/extensions Create index.js : const express = require ( "express" ) ; const { paymentMiddleware } = require ( "@x402/express" ) ; const { x402ResourceServer , HTTPFacilitatorClient } = require ( "@x402/core/server" ) ; const { ExactEvmScheme } = require ( "@x402/evm/exact/server" ) ; const app = express ( ) ; app . use ( express . json ( ) ) ; const PAY_TO = "
" ; // Create facilitator client and x402 resource server const facilitator = new HTTPFacilitatorClient ( { url : "https://x402.org/facilitator" } ) ; const server = new x402ResourceServer ( facilitator ) ; server . register ( "eip155:8453" , new ExactEvmScheme ( ) ) ; // x402 payment middleware — protects routes below app . use ( paymentMiddleware ( { "GET /api/example" : { accepts : { scheme : "exact" , price : "$0.01" , network : "eip155:8453" , payTo : PAY_TO , } , description : "Description of what this endpoint returns" , mimeType : "application/json" , } , } , server , ) , ) ; // Protected endpoint app . get ( "/api/example" , ( req , res ) => { res . json ( { data : "This costs $0.01 per request" } ) ; } ) ; app . listen ( 3000 , ( ) => console . log ( "Server running on port 3000" ) ) ; Step 3: Run It node index.js Test with curl — you should get a 402 response with payment requirements: curl -i http://localhost:3000/api/example API Reference paymentMiddleware(routes, server) Creates Express middleware that enforces x402 payments. Parameter Type Description routes object Route config mapping route patterns to payment config server x402ResourceServer Pre-configured x402 resource server instance x402ResourceServer Created with a facilitator client. Register payment schemes and extensions before passing to middleware. const { x402ResourceServer , HTTPFacilitatorClient } = require ( "@x402/core/server" ) ; const { ExactEvmScheme } = require ( "@x402/evm/exact/server" ) ; const facilitator = new HTTPFacilitatorClient ( { url : "https://x402.org" } ) ; const server = new x402ResourceServer ( facilitator ) ; server . register ( "eip155:8453" , new ExactEvmScheme ( ) ) ; Method Description register(network, scheme) Register a payment scheme for a CAIP-2 network identifier Route Config Each key in the routes object is "METHOD /path" . The value is a config object: { "GET /api/data" : { accepts : { scheme : "exact" , price : "$0.05" , network : "eip155:8453" , payTo : "0x..." , } , description : "Human-readable description of the endpoint" , mimeType : "application/json" , extensions : { ... declareDiscoveryExtension ( { output : { example : { result : "example response" } , schema : { properties : { result : { type : "string" } , } , } , } , } ) , } , } , } Accepts Config Fields The accepts field can be a single object or an array (for multiple payment options): Field Type Description scheme string Payment scheme: "exact" price string USDC price (e.g. "$0.01" , "$1.00" ) network string CAIP-2 network identifier (e.g. "eip155:8453" ) payTo string Ethereum address (0x...) to receive USDC payments Route-Level Fields Field Type Description accepts object or array Payment requirements (single or multiple) description string? What this endpoint does (shown to clients) mimeType string? MIME type of the response extensions object? Extensions config (e.g. Bazaar discovery) Discovery Extension The declareDiscoveryExtension function registers your endpoint with the x402 Bazaar so other agents can discover it: const { declareDiscoveryExtension } = require ( "@x402/extensions/bazaar" ) ; extensions : { ... declareDiscoveryExtension ( { output : { example : { / example response body / } , schema : { properties : { / JSON schema of the response / } , } , } , } ) , } Field Type Description output.example object Example response body for the endpoint output.schema object JSON schema describing the response format Supported Networks Network Description eip155:8453 Base mainnet (real USDC) eip155:84532 Base Sepolia testnet (test USDC) Patterns Multiple endpoints with different prices app . use ( paymentMiddleware ( { "GET /api/cheap" : { accepts : { scheme : "exact" , price : "$0.001" , network : "eip155:8453" , payTo : PAY_TO , } , description : "Inexpensive data lookup" , } , "GET /api/expensive" : { accepts : { scheme : "exact" , price : "$1.00" , network : "eip155:8453" , payTo : PAY_TO , } , description : "Premium data access" , } , "POST /api/query" : { accepts : { scheme : "exact" , price : "$0.25" , network : "eip155:8453" , payTo : PAY_TO , } , description : "Run a custom query" , } , } , server , ) , ) ; app . get ( "/api/cheap" , ( req , res ) => { / ... / } ) ; app . get ( "/api/expensive" , ( req , res ) => { / ... / } ) ; app . post ( "/api/query" , ( req , res ) => { / ... / } ) ; Wildcard routes app . use ( paymentMiddleware ( { "GET /api/" : { accepts : { scheme : "exact" , price : "$0.05" , network : "eip155:8453" , payTo : PAY_TO , } , description : "API access" , } , } , server , ) , ) ; app . get ( "/api/users" , ( req , res ) => { / ... / } ) ; app . get ( "/api/posts" , ( req , res ) => { / ... / } ) ; Health check (no payment) Register free endpoints before the payment middleware: app . get ( "/health" , ( req , res ) => res . json ( { status : "ok" } ) ) ; // Payment middleware only applies to routes registered after it app . use ( paymentMiddleware ( { / ... / } , server ) ) ; app . get ( "/api/data" , ( req , res ) => { / ... */ } ) ; POST with body and discovery extension app . use ( paymentMiddleware ( { "POST /api/analyze" : { accepts : { scheme : "exact" , price : "$0.10" , network : "eip155:8453" , payTo : PAY_TO , } , description : "Analyze text sentiment" , mimeType : "application/json" , extensions : { ... declareDiscoveryExtension ( { output : { example : { sentiment : "positive" , score : 0.95 } , schema : { properties : { sentiment : { type : "string" } , score : { type : "number" } , } , } , } , } ) , } , } , } , server , ) , ) ; app . post ( "/api/analyze" , ( req , res ) => { const { text } = req . body ; // ... your logic res . json ( { sentiment : "positive" , score : 0.95 } ) ; } ) ; Multiple payment options per endpoint Accept payments on multiple networks for the same endpoint: "GET /api/data" : { accepts : [ { scheme : "exact" , price : "$0.01" , network : "eip155:8453" , payTo : EVM_ADDRESS , } , { scheme : "exact" , price : "$0.01" , network : "eip155:84532" , payTo : EVM_ADDRESS , } , ] , description : "Data endpoint accepting Base mainnet or testnet" , } Using the CDP facilitator (authenticated) For production use with the Coinbase facilitator (supports mainnet): npm install @coinbase/x402 const { facilitator } = require ( "@coinbase/x402" ) ; const { HTTPFacilitatorClient } = require ( "@x402/core/server" ) ; const facilitatorClient = new HTTPFacilitatorClient ( facilitator ) ; const server = new x402ResourceServer ( facilitatorClient ) ; server . register ( "eip155:8453" , new ExactEvmScheme ( ) ) ; This requires CDP_API_KEY_ID and CDP_API_KEY_SECRET environment variables. Get these from https://portal.cdp.coinbase.com . Testing with the pay-for-service Skill Once the server is running, use the pay-for-service skill to test payments:Check the endpoint's payment requirements
npx awal@2.0.3 x402 details http://localhost:3000/api/example
Make a paid request
npx awal@2.0.3 x402 pay http://localhost:3000/api/example Pricing Guidelines Use Case Suggested Price Simple data lookup $0.001 - $0.01 API proxy / enrichment $0.01 - $0.10 Compute-heavy query $0.10 - $0.50 AI inference $0.05 - $1.00 Checklist Get wallet address with npx awal@2.0.3 address Install express , @x402/express , @x402/core , @x402/evm , and @x402/extensions Create x402ResourceServer with facilitator client and register ExactEvmScheme for eip155:8453 Define routes with prices, descriptions, and discovery extensions (Bazaar auto-registers when routes declare it) Register payment middleware before protected routes Keep health/status endpoints before payment middleware Test with curl (should get 402) and npx awal@2.0.3 x402 pay (should get 200) Announce your service so other agents can find and use it