probe402-check MCP server

probe402-check

Ask probe402's public record about an x402 endpoint before your agent pays it, and re-check a published archive head from your own terminal.

Three small tools, one repository, Apache-2.0:

every 402 is checked against probe402's record before the payment is signed, against a policy you write. It never touches your signer and never changes the payment.

• wrapFetchWithCheck — an x402 client hook. Wrap the fetch your x402 client pays through and

It reads that route's free grade on probe402.com and answers: was it quoting at the newest reading, quoting what, has probe402 itself paid it and how was the result graded, how old is that reading against the route's own cadence, and the address to cite. It compares the payment request you are holding against the recorded one if you pass it. The decision stays with your agent's policy.

• checkbeforepaying — an MCP server and a CLI. Give it the URL your agent is about to pay.

manifest from probe402.com, recomputes the head, and prints LINKS or DOES NOT LINK with both digests.

• verify-head — fetches one sealed day's head from probe402's public head table and that day's

The tool reads public surfaces of probe402.com and one public git repository, and nothing else. There is no key, no account and no payment anywhere in it.

Use it in 60 seconds

Claude Desktop — add this to claudedesktopconfig.json and restart:

{
  "mcpServers": {
    "probe402-check": {
      "command": "npx",
      "args": ["-y", "probe402-check", "--mcp"]
    }
  }
}

Claude Code — one line:

claude mcp add probe402-check -- npx -y probe402-check --mcp

OpenAI Agents SDK — copy examples/openai-agents-tool.ts into your project (it is forty lines, and it builds the tool with tool() from @openai/agents, which you install yourself):

import { Agent } from "@openai/agents";
import { checkTool } from "./openai-agents-tool.ts";

const agent = new Agent({
  name: "buyer",
  instructions: "Before paying any x402 endpoint, call check_before_paying and reason from its verdict.",
  tools: [checkTool],
});

Then ask for a route by URL: check https://datastand.dev/api/data/dev-signals before I pay it.

The npx forms above work once the package is on npm; until then, clone (see Install) and point the command at node /absolute/path/to/probe402-check/dist/cli/mcp.js.

At the payment: the x402 client hook

The MCP tool answers when an agent remembers to ask. An agent has to remember. The hook does not depend on remembering: it sits under the fetch your x402 client pays through, so every 402 is checked before the payment is signed.

One import, three lines:

import { wrapFetchWithPayment } from "@x402/fetch";
import { wrapFetchWithCheck } from "probe402-check";

const fetchWithPay = wrapFetchWithPayment(wrapFetchWithCheck(fetch), client);

It goes underneath the payment wrapper, because that is where the 402 arrives. x402-fetch (the v1 package) composes the same way. Now any request your agent makes is checked at the moment it would cost money:

const response = await fetchWithPay("https://datastand.dev/api/data/dev-signals");

What the hook does on a 402: reads the challenge (through a clone, so your client still reads it itself), asks probe402 about that exact URL — one request, to the free grade — compares every way to pay the 402 offers against the quote probe402 recorded, and applies your policy. Then it hands the 402 back untouched and your client signs, or it throws PaymentBlocked and your client never sees it.

It never touches your signer, your key or your wallet; it adds no header, removes none, and changes no payment. A request that already carries a payment header is your client's own retry and is passed straight through, so one payment costs one question.

The policy is yours

Shortened. The full README is on GitHub.