JWT verify (HMAC)

Verify an HS256/384/512 JWT signature against a secret and check expiry. Returns valid + decoded payload. (HMAC algorithms only.)

Input

Field	Type	Description
token *	string
secret *	string

Example output

{
  "valid": true,
  "algorithm": "HS256",
  "expired": false,
  "payload": {
    "sub": "agent402"
  }
}

Try it - see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/jwt-verify \
  -H "Content-Type: application/json" \
  -d '{"token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.NqggPBGuLX1OA7YuSlQ4S0INJfCOWnwXWT0XUIUrt3s","secret":"my-secret"}'

The response is HTTP 402 Payment Required with exact payment requirements. Any x402 v2 client pays automatically and retries:

Paid call (JavaScript agent)

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

const client = new x402Client();
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/jwt-verify", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.NqggPBGuLX1OA7YuSlQ4S0INJfCOWnwXWT0XUIUrt3s",
    "secret": "my-secret"
  }),
});

No wallet? Pay with compute

This is a pure-CPU tool, so an agent without a wallet can pay with proof-of-work instead of USDC: fetch a challenge, solve the sha256 puzzle (16 leading zero bits - a fraction of a second of CPU, no money, no AI tokens), and resend with the X-Pow-Solution header.

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://agent402.tools/api/pow/challenge?slug=jwt-verify")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/jwt-verify", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.NqggPBGuLX1OA7YuSlQ4S0INJfCOWnwXWT0XUIUrt3s","secret":"my-secret"}) });

Part of these workflows

This tool is one step in 2 curated multi-tool workflows - agents can fetch the whole sequence as an MCP prompt or call https://agent402.tools/api/skill-packs/{slug}/prompt.

• JWT forensics - Someone hands you a JWT and asks 'is this valid?' Decode without verification first to see the shape, render the time claims (iat/nbf/exp) in human time, compute exactly how long until expiry, then HMAC-verify against the secret. Optional follow-ups: decode any base64-looking custom claims, verify embedded SHA fingerprints.
• JWT toolkit - Decode and verify a JWT in one pass - see the payload and check the signature.

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