JWT decode

Decode a JWT without verification: header (algorithm), payload claims (subject, issuer, expiration), expiry status, and time remaining. Send POST /api/jwt-decode with the required field token and pay $0.001 per call over x402 or MPP, or call it free by solving a proof-of-work challenge. It returns a JSON object with header, payload and expired.

(Decoding only - signatures are NOT verified.)

Category: Encoding & crypto · Tags: jwt token auth decode

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/jwt-decode \
  -H "Content-Type: application/json" \
  -d '{"token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"}'

Without payment this returns HTTP 402 Payment Required with the exact price for jwt-decode; any x402 v2 or MPP client pays it and retries.

Example response

{
  "header": {
    "alg": "HS256"
  },
  "payload": {
    "sub": "agent402",
    "exp": 9999999999
  },
  "expired": false
}

From an MCP client

catalog.call {
  "slug": "jwt-decode",
  "params": {
    "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"
  }
}

On the hosted connector at https://agent402.tools/mcp, catalog.call runs jwt-decode free (rate-limited, no wallet). Local install: npx -y agent402-mcp.

Errors and behavior

• token is required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Free tier: no outbound network call leaves the server for this tool, so proof-of-work (16 leading zero bits of sha256) pays for it.
• A GET or HEAD to /api/jwt-decode returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

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();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/jwt-decode", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"
  }),
});

No wallet? Pay with compute

Fetch a challenge, solve the sha256 puzzle (16 leading zero bits, a fraction of a second of CPU), 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-decode")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/jwt-decode", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJhZ2VudDQwMiIsIm5hbWUiOiJkZW1vIGFnZW50IiwiaWF0IjoxNzAwMDAwMDAwLCJleHAiOjk5OTk5OTk5OTl9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"}) });

Part of these workflows

JWT decode is one step in these 2 skill packs, each sold as a single call:

• Decode this blob - Hand the agent an opaque string - a JWT, a base64'd JSON payload, a gzip-encoded API response, a hex-encoded hash - and walk it through identifying what it is and unwrapping it layer by layer until it's human-readable.
• JWT toolkit - Decode and verify a JWT in one pass - see the payload and check the signature.

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