MCP 서버로 결제하기

step 1. User in Claude: "Pull financials for ACME Corp and assess credit risk."

step 2. Client calls get_company_financials("ACME")
        → MCP handler: fetchWithPayment("/financials?ticker=ACME")
        → 402 Payment Required → sign ERC-3009 → retry
        → Facilitator settles $0.01 USDT0 on-chain
        → tx: 0x8f3a...aaaa
        → 200 OK { revenue, debt_ratio, cash_flow }

step 3. Client calls assess_credit_risk(financials)
        → MCP handler: fetchWithPayment("/credit-risk", POST)
        → Facilitator settles $0.05 USDT0 on-chain
        → tx: 0x9bc4...bbbb
        → 200 OK { score: 72, rating: "moderate" }

step 4. Claude responds:
        "ACME Corp has a credit risk score of 72 (moderate). Revenue is stable
        but debt-to-equity ratio is elevated at 1.8x..."

// --- MCP Server ---
// Bridge x402-enabled APIs to MCP tools
tools = {
  "get_company_financials": {
    handler: (ticker) =>
      fetchWithPayment("https://api.example.com/financials?ticker=" + ticker),
  },
  "assess_credit_risk": {
    handler: (financials) =>
      fetchWithPayment("https://api.example.com/credit-risk", {
        method: "POST",
        body: JSON.stringify({ financials }),
      }),
  },
}

─── AI Client ───────────────────────────────────────
User: "Pull financials for ACME Corp and assess their credit risk."
 
Client calls get_company_financials tool
  → MCP server sends x402 paid request
  → Facilitator settles USDT0 on-chain
  → API returns financial data
 
Client calls assess_credit_risk tool with the result
  → MCP server sends x402 paid request
  → Facilitator settles USDT0 on-chain
  → API returns risk assessment
  → Client responds with the combined result

npm install @modelcontextprotocol/sdk @x402/fetch @x402/evm @tetherto/wdk-wallet-evm

// mcp-server.ts
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import WalletManagerEvm from "@tetherto/wdk-wallet-evm";
import { x402Client, wrapFetchWithPayment } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { z } from "zod";
 
// --- Wallet and x402 client ---
const account = await new WalletManagerEvm(process.env.SEED_PHRASE!, {
  provider: "https://rpc.stable.xyz",
}).getAccount(0);
 
const client = new x402Client();
registerExactEvmScheme(client, { signer: account });
const fetchWithPayment = wrapFetchWithPayment(fetch, client);
 
// --- x402 API base URL ---
const API_BASE = process.env.API_BASE || "http://localhost:4021";
 
// --- MCP server ---
const server = new McpServer({
  name: "x402-payments",
  version: "1.0.0",
});
 
server.tool(
  "get_company_financials",
  "Get company financial data by ticker (paid endpoint, $0.01 per call)",
  { ticker: z.string().describe("Company ticker symbol (e.g. ACME)") },
  async ({ ticker }) => {
    const response = await fetchWithPayment(`${API_BASE}/financials?ticker=${ticker}`);
    const data = await response.json();
    return { content: [{ type: "text", text: JSON.stringify(data) }] };
  },
);
 
server.tool(
  "assess_credit_risk",
  "Assess credit risk from financial data (paid endpoint, $0.05 per call)",
  { financials: z.string().describe("JSON string of company financial data") },
  async ({ financials }) => {
    const response = await fetchWithPayment(`${API_BASE}/credit-risk`, {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: financials,
    });
    const data = await response.json();
    return { content: [{ type: "text", text: JSON.stringify(data) }] };
  },
);
 
server.tool(
  "check_balance",
  "Check the USDT0 balance of the payment wallet",
  {},
  async () => {
    const USDT0_STABLE = "0x779Ded0c9e1022225f8E0630b35a9b54bE713736";
    const balance = await account.getTokenBalance(USDT0_STABLE);
    const formatted = (Number(balance) / 1e6).toFixed(2);
    return {
      content: [{ type: "text", text: `Wallet balance: ${formatted} USDT0` }],
    };
  },
);
 
// --- Start ---
const transport = new StdioServerTransport();
await server.connect(transport);

{
  "mcpServers": {
    "x402-payments": {
      "command": "npx",
      "args": ["tsx", "/path/to/mcp-server.ts"],
      "env": {
        "SEED_PHRASE": "your seed phrase here",
        "API_BASE": "https://api.example.com"
      }
    }
  }
}

claude mcp add x402-payments -- npx tsx /path/to/mcp-server.ts

const MAX_PER_CALL = 100_000;   // $0.10 in base units
const MAX_PER_SESSION = 5_000_000; // $5.00 in base units
let sessionSpent = 0n;
 
function checkSpendingLimit(amount: bigint) {
  if (amount > BigInt(MAX_PER_CALL)) {
    throw new Error(`Amount exceeds per-call limit of ${MAX_PER_CALL / 1e6}`);
  }
  if (sessionSpent + amount > BigInt(MAX_PER_SESSION)) {
    throw new Error(`Session spending limit of ${MAX_PER_SESSION / 1e6} reached`);
  }
  sessionSpent += amount;
}