Webhook signature verify

Verify a webhook's HMAC signature against the correct per-provider scheme: GitHub (X-Hub-Signature-256, sha256=hex), Stripe (Stripe-Signature t/v1 over "<t>.<body>" with replay tolerance), Shopify (X-Shopify-Hmac-Sha256, base64), Slack (X-Slack-Signature, v0:<ts>:<body> with replay tolerance). Send POST /api/webhook-verify with the required fields provider, payload, secret and signature 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 valid, provider, scheme and reason.

Constant-time comparison; the secret is never echoed. Pass the RAW request body string - signatures are over the raw bytes. Deterministic.

Category: Validation & parsing · Tags: webhook hmac signature security github stripe shopify slack

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/webhook-verify \
  -H "Content-Type: application/json" \
  -d '{"provider":"github","payload":"{\"hello\":\"world\"}","secret":"it'\''s a secret","signature":"sha256=8d4063f0a81aa1531d9891a028a68cf2bb537ecdf0e82557674d71e168d570f9"}'

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

Example response

{
  "valid": true,
  "provider": "github",
  "scheme": "X-Hub-Signature-256: sha256=hex(HMAC-SHA256(secret, rawBody))",
  "reason": "signature matches the recomputed HMAC for this payload and secret"
}

From an MCP client

catalog.call {
  "slug": "webhook-verify",
  "params": {
    "provider": "github",
    "payload": "{\"hello\":\"world\"}",
    "secret": "it's a secret",
    "signature": "sha256=8d4063f0a81aa1531d9891a028a68cf2bb537ecdf0e82557674d71e168d570f9"
  }
}

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

Errors and behavior

• provider, payload, secret and signature are 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/webhook-verify 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/webhook-verify", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "provider": "github",
    "payload": "{\"hello\":\"world\"}",
    "secret": "it's a secret",
    "signature": "sha256=8d4063f0a81aa1531d9891a028a68cf2bb537ecdf0e82557674d71e168d570f9"
  }),
});

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=webhook-verify")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/webhook-verify", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"provider":"github","payload":"{\"hello\":\"world\"}","secret":"it's a secret","signature":"sha256=8d4063f0a81aa1531d9891a028a68cf2bb537ecdf0e82557674d71e168d570f9"}) });

Part of these workflows

Webhook signature verify is one step in this skill pack, each sold as a single call:

• Webhook secure intake - The production ingest path for every incoming webhook: verify the provider signature (GitHub / Stripe / Shopify / Slack, constant-time, replay-window enforced), schema-validate the now-trusted body against the provider envelope, fingerprint the raw bytes for redelivery dedup, normalize the event timestamp to UTC + epoch, and redact PII before anything hits a log. Five pure-CPU tools - the accept-or-reject gate, run on every event.

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