JWT sign

Mint a signed JSON Web Token (HMAC: HS256 default, HS384, HS512) from a payload + secret. Pairs with jwt-decode/jwt-verify to complete the trio. Deterministic - same payload, secret, and alg always produce the same token.

Input

Field	Type	Description
payload *	object	claims to encode (object)
secret *	string	HMAC signing secret
alg	string	HS256 (default) | HS384 | HS512

Example output

{
  "token": "eyJhbGci...",
  "alg": "HS256"
}

Try it - see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/jwt-sign \
  -H "Content-Type: application/json" \
  -d '{"payload":{"sub":"123","role":"admin"},"secret":"s3cr3t","alg":"HS256"}'

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-sign", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "payload": {
      "sub": "123",
      "role": "admin"
    },
    "secret": "s3cr3t",
    "alg": "HS256"
  }),
});

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-sign")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/jwt-sign", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"payload":{"sub":"123","role":"admin"},"secret":"s3cr3t","alg":"HS256"}) });

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.

• Server-side identity mint - The server-side identity-issuance round-trip: mint a random user-id (UUIDv4), derive a deterministic cross-system correlation-id (UUIDv5), turn the display name into a URL-safe handle, generate a one-time recovery password, hash it for storage, sign a session JWT with HMAC, and base64-encode the recovery bundle for transport. Seven pure-CPU tools - the canonical 'create-user' write path done as one deterministic pipeline.
• JWT toolkit - Decode and verify a JWT in one pass - see the payload and check the signature.

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