Captcha generate

Mint a stateless captcha challenge to gate your OWN endpoint: returns a human prompt plus a salted sha256 answer hash you keep and verify later (sha256(salt + normalized answer) === answerHash) - no server state, no shared secret. Send POST /api/captcha-generate with no required fields (2 optional) 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 type, prompt, salt, answerHash, algo and 1 more.

Types: math (arithmetic) or alnum (character code), difficulty 1–3. Deterministic tooling for running bot protection, not defeating it.

Category: API primitives · Tags: captcha challenge bot-protection verification signup gate anti-abuse

TRY IN PLAYGROUND →

Parameters

Example request

curl -i -X POST https://agent402.tools/api/captcha-generate \
  -H "Content-Type: application/json" \
  -d '{"type":"math","difficulty":1}'

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

Example response

{
  "type": "math",
  "prompt": "What is 3 + 4?",
  "salt": "…",
  "answerHash": "…",
  "algo": "sha256",
  "verify": "sha256(salt + answer.trim().toLowerCase().replace(/\\s+/g,'')) === answerHash"
}

From an MCP client

catalog.call {
  "slug": "captcha-generate",
  "params": {
    "type": "math",
    "difficulty": 1
  }
}

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

Errors and behavior

• Every field is optional. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example.
• 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/captcha-generate 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/captcha-generate", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "type": "math",
    "difficulty": 1
  }),
});

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=captcha-generate")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/captcha-generate", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"type":"math","difficulty":1}) });

Related tools