Interop fixture: live Base x402 MCP that uses x402/payment-required and HTTP retry

[cursor]

withX402Client / mcpay connect --evm is a real wallet-bearing 402 loop. The published client currently:

1. Treats an MCP tool error as payable only when _meta["x402/error"].accepts is present.
2. Signs with createPaymentHeader (default x402 version 1) using req.maxAmountRequired.
3. Retries the same MCP callTool, attaching _meta["x402/payment"].
4. Defaults the CLI EVM network to base-sepolia (--evm-network base is documented for mainnet).

I work on zFinia (https://zfinia.com). This is not a request to list us on mcpay.tech/register, wrap us in the MCPay proxy, endorse the product, or buy anything.

If you want an independent Base mainnet seller to regression-test that client against a host that is not an MCPay-proxied MCP, this origin is live:

• MCP (challenge-handoff only, never a wallet): https://api.zfinia.com/mcp
• Cheapest paid HTTP resource: POST https://api.zfinia.com/x402/v1/accessibility-contrast-calculator
• Exact USDC on eip155:8453, amount 1000 atomic ($0.001)
• Catalog: https://api.zfinia.com/.well-known/x402.json

Unpaid MCP tools/call for accessibility_contrast_calculator with

{"foreground":"#000000","background":"#ffffff","large_text":false}

returns isError: true plus:

• structuredContent.code = PAYMENT_REQUIRED
• retry_unchanged: false
• resourceIdentity = the HTTP POST above
• _meta["x402/payment-required"] (not _meta["x402/error"]) with x402Version: 2 and accepts[].amount (not maxAmountRequired)
• Instructions: pay from an external buyer wallet, then retry the paid HTTP resource outside MCP

CLI shape that matches the live network (wallet still required on the caller):

npx mcpay connect -u "https://api.zfinia.com/mcp" --evm "$EVM_PRIVATE_KEY" --evm-network base --max-atomic 1000

After independent HTTP settlement and retry, black-on-white paid JSON includes contrast_ratio: 21. An unpaid HTTP POST to the same URL returns HTTP 402 with a PAYMENT-REQUIRED header.

If the useful outcome is “MCPay client should ignore this host until v2 amount / x402/payment-required / HTTP-retry is supported,” that is a valid close. Reply here or to support@zfinia.com if you want the 402 terms in a different shape for withX402Client.