anthropic-mcp

An x402-gated MCP server that proxies Anthropic inference over Streamable HTTP. Pay in USDC on Base, get a Claude response.

Live endpoint: https://anthropic.yeetful.com/api/mcp/mcp

How it works

1. Client calls POST /api/mcp/mcp with an MCP JSON-RPC request.
2. The Next.js proxy (proxy.ts, Next.js 16's replacement for middleware.ts) returns HTTP 402 Payment Required with an x402 payment challenge describing the required USDC amount, network, and receiving wallet.
3. Client signs an EIP-3009 transferWithAuthorization for the challenge using an x402 client helper (e.g. x402-fetch / x402-axios) and retries the request with an X-PAYMENT header.
4. The x402 facilitator verifies + settles the payment on Base. On success, the request is forwarded to the MCP handler, which invokes the Anthropic SDK using the operator's ANTHROPIC_API_KEY.
5. The MCP tool result (Claude's text response) is returned.

Tools exposed

Tool	Purpose
ask_claude	Single-prompt text completion.
claude_chat	Multi-turn chat with a messages array.

Setup

cd anthropic-mcp
npm install
cp .env.example .env.local   # fill in ANTHROPIC_API_KEY
npm run dev

Defaults:

• Receiving wallet: 0x66268791B55e1F5fA585D990326519F101407257
• Network: base (mainnet). Set X402_NETWORK=base-sepolia for test USDC.
• Price: $0.005 per inference call. Override with X402_PRICE_USD.
• Default model: claude-sonnet-4-6. Override with ANTHROPIC_DEFAULT_MODEL.

Facilitator

• Mainnet (base): the middleware defaults to Coinbase CDP's hosted facilitator bundled with x402-next.
• Testnet (base-sepolia): set X402_FACILITATOR_URL=https://x402.org/facilitator.

Endpoints

Path	Gated	Description
/	no	Human-readable summary page.
/api/info	no	JSON describing pricing + tools.
/api/mcp/mcp	yes	MCP Streamable HTTP endpoint.

Testing

npm test

Three test suites:

• tests/config.test.ts — env parsing, price formatting, default wallet.
• tests/anthropic.test.ts — the Anthropic SDK wrapper, with a mocked client.
• tests/payment-gate.test.ts — the proxy returns a valid 402 x402 challenge pointing at the configured wallet, and /api/info publishes the right payment metadata.

Manually validating the live 402

curl -i -X POST https://anthropic.yeetful.com/api/mcp/mcp \
  -H 'content-type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

(Swap the host for http://localhost:3000 when running npm run dev locally.)

You should see HTTP/1.1 402 Payment Required and a JSON body containing accepts[].payTo equal to the configured wallet.

Paying with an x402 client

import { wrapFetchWithPayment } from "x402-fetch";
import { privateKeyToAccount } from "viem/accounts";

const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const fetchWithPayment = wrapFetchWithPayment(fetch, account);

const res = await fetchWithPayment("https://anthropic.yeetful.com/api/mcp/mcp", {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify({
    jsonrpc: "2.0",
    id: 1,
    method: "tools/call",
    params: {
      name: "ask_claude",
      arguments: { prompt: "What is the capital of France?" },
    },
  }),
});
console.log(await res.json());