x402-payments

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Translated

Build applications using the x402 protocol — Coinbase's open standard for HTTP-native stablecoin payments using the HTTP 402 status code. Use this skill when: - Creating APIs that require USDC payments per request (seller/server side) - Building clients or AI agents that pay for x402-protected resources (buyer/client side) - Implementing MCP servers with paid tools for Claude Desktop - Adding payment middleware to Express, Hono, or Next.js applications - Working with Base (EVM) or Solana (SVM) payment flows - Building machine-to-machine or agent-to-agent payment systems - Integrating micropayments, pay-per-use billing, or paid API access Triggers: x402, HTTP 402, payment required, USDC payments, micropayments, pay-per-use API, agentic payments, stablecoin payments, paid API endpoint, paywall middleware

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Sourceaznatkoiny/zai-skills

Added on2026-02-11

NPX Install

npx skill4agent add aznatkoiny/zai-skills x402-payments

SKILL.md Content

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x402 Protocol Skill

Protocol Overview

x402 embeds stablecoin payments into HTTP by using the 402 "Payment Required" status code. A server responds with payment requirements; the client signs a payment authorization, resubmits the request, and gets the resource after verification and settlement.

Payment flow:

Client sends HTTP request → Server returns
```
402
```
+
```
PAYMENT-REQUIRED
```
header (base64 JSON)
Client reads requirements, creates signed payment payload
Client resubmits request with
```
PAYMENT-SIGNATURE
```
header (base64 JSON)
Server verifies payment via facilitator
```
POST /verify
```
Server performs work, settles via facilitator
```
POST /settle
```
Server returns
```
200
```
+ resource +
```
PAYMENT-RESPONSE
```
header (contains txHash)

Key concepts:

Facilitators verify and settle payments without holding funds. Use
```
https://x402.org/facilitator
```
for testnet, CDP facilitator for mainnet.
Schemes:
```
exact
```
(fixed price per request) is the production scheme.
```
upto
```
and
```
deferred
```
are proposed.
Networks: Identified by CAIP-2 format —
```
eip155:84532
```
(Base Sepolia),
```
eip155:8453
```
(Base Mainnet),
```
solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1
```
(Solana Devnet).
EVM uses EIP-3009 gasless
```
TransferWithAuthorization
```
. Solana uses SPL token transfers.

Quick-Start: Protect an API Endpoint (Seller)

bash

npm install @x402/express @x402/core @x402/evm

typescript

import express from "express";
import { paymentMiddleware } from "@x402/express";
import { x402ResourceServer, HTTPFacilitatorClient } from "@x402/core/server";
import { registerExactEvmScheme } from "@x402/evm/exact/server";

const app = express();
const payTo = process.env.PAY_TO!;

const facilitatorClient = new HTTPFacilitatorClient({
  url: "https://x402.org/facilitator",
});
const server = new x402ResourceServer(facilitatorClient);
registerExactEvmScheme(server);

app.use(
  paymentMiddleware(
    {
      "GET /weather": {
        accepts: [
          { scheme: "exact", price: "$0.001", network: "eip155:84532", payTo },
        ],
        description: "Get current weather data",
        mimeType: "application/json",
      },
    },
    server,
  ),
);

app.get("/weather", (req, res) => {
  res.json({ weather: "sunny", temperature: 70 });
});

app.listen(4021, () => console.log("Server on :4021"));

Quick-Start: Pay for x402 Resources (Buyer/Agent)

bash

npm install @x402/fetch @x402/core @x402/evm viem

typescript

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client, x402HTTPClient } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const signer = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as `0x${string}`);
const client = new x402Client();
registerExactEvmScheme(client, { signer });

const fetchWithPayment = wrapFetchWithPayment(fetch, client);

const response = await fetchWithPayment("http://localhost:4021/weather");
const data = await response.json();
console.log(data);

// Read payment receipt
const httpClient = new x402HTTPClient(client);
const receipt = httpClient.getPaymentSettleResponse(
  (name) => response.headers.get(name),
);
console.log("Tx:", receipt?.txHash);

Decision Tree

Decision	Choice	Packages
Server: Express	`paymentMiddleware` from `@x402/express`	`@x402/express @x402/core @x402/evm`
Server: Next.js	`paymentProxy` from `@x402/next`	`@x402/next @x402/core @x402/evm`
Server: Hono	`paymentMiddleware` from `@x402/hono`	`@x402/hono @x402/core @x402/evm`
Client: fetch	`wrapFetchWithPayment`	`@x402/fetch @x402/core @x402/evm viem`
Client: axios	`wrapAxiosWithPayment`	`@x402/axios @x402/core @x402/evm viem axios`
Client: manual	`x402Client` + `x402HTTPClient` from `@x402/core`	`@x402/core @x402/evm viem`
Chain: EVM	`registerExactEvmScheme`	`@x402/evm` + `viem`
Chain: Solana	`registerExactSvmScheme`	`@x402/svm` + `@solana/kit @scure/base`
Chain: both	Register both schemes on same client/server	All chain deps
Env: testing	Facilitator `https://x402.org/facilitator`	Base Sepolia / Solana Devnet
Env: production	CDP facilitator + API keys	Base Mainnet / Solana Mainnet
Agent: MCP	MCP server with `@x402/axios`	See `references/agentic-patterns.md`
Agent: Anthropic	Tool-use with `@x402/fetch`	See `references/agentic-patterns.md`

Reference File Navigation

Task	Read this file
Headers, payloads, CAIP-2 IDs, facilitator API, V1→V2 changes	`references/protocol-spec.md`
Express / Hono / Next.js middleware, multi-route, dynamic pricing	`references/server-patterns.md`
Fetch / axios client, wallet setup, lifecycle hooks, error handling	`references/client-patterns.md`
AI agent payments, MCP server, tool discovery, budget controls	`references/agentic-patterns.md`
Testnet→mainnet migration, CDP keys, faucets, security, sessions	`references/deployment.md`

Critical Implementation Notes

Register schemes before wrapping fetch/axios — order matters.

Two equivalent registration APIs:

Function:

registerExactEvmScheme(server)

/

registerExactEvmScheme(client, { signer })

Method:

server.register("eip155:84532", new ExactEvmScheme())

V2 headers (current):

PAYMENT-REQUIRED

,

PAYMENT-SIGNATURE

,

PAYMENT-RESPONSE

. V1 headers (legacy):

X-PAYMENT

,

X-PAYMENT-RESPONSE

. SDK is backward-compatible.

Price format:
```
"$0.001"
```
(dollar string) — SDK converts to atomic units (6 decimals for USDC).
Python SDK uses V1 patterns only. Use TypeScript or Go for V2.
Node.js v24+ required for the TypeScript SDK.

Repo:

https://github.com/coinbase/x402

— canonical examples in

examples/typescript/

.

Docs:

https://docs.cdp.coinbase.com/x402/welcome

and

https://x402.gitbook.io/x402

.