Webhook Toolkit MCP server

webhook-toolkit

Receive, forward, sign and verify webhooks from your terminal, your test suite and your AI agent. One package: a CLI (webhook-toolkit, alias whtk), a typed library and an MCP server, backed by webhook-toolkit.com.

Quick start

npx webhook-toolkit listen --forward http://localhost:3000/webhooks

That's it: no account, no config. You get a public URL; paste it into Stripe, GitHub, Shopify or any other sender, and every webhook shows up in your terminal and is re-sent to your local handler:

  webhook-toolkit · listening

  Webhook URL   https://webhook-toolkit.com/r/l90c9MKIdo0o
  Inspector     https://webhook-toolkit.com/e/l90c9MKIdo0o
  Forwarding    → http://localhost:3000/webhooks
  Expires       in 7 days (2026-09-25 15:42)
                Anonymous URL. Run `webhook-toolkit login` to get a permanent one.

  Waiting for webhooks. Ctrl+C to stop.

  15:42:11  POST   /stripe  stripe checkout.session.completed  248 B
            ↳ 200 OK · 3 ms
  15:42:11  POST   /github  github push  269 B
            ↳ 500 Internal Server Error · 1 ms  {"error":"handler crashed: cannot read properties of undefined"}

Every request is also kept in a web inspector (headers, raw body, replay), so nothing is lost when your handler crashes.

Contents

• CLI
• Test webhooks in your test suite
• Use it from AI agents (MCP)
• Signing and verifying
• Relay: a real tunnel (paid)
• Free vs paid
• How it compares
• Library reference
• Configuration

CLI

npm i -g webhook-toolkit     # or run any command with npx webhook-toolkit …

Command	What it does
whtk listen [--forward <url>]	Public URL + live stream of incoming requests, optionally re-sent to localhost. Reuses your last URL while it is alive (--new for a fresh one).
whtk relay --to <url|port>	Real tunnel: callers get your local app's response. Paid.
whtk sign <provider> --secret <s>	Build a validly signed webhook: prints headers + a ready-to-run curl, or sends it with --send.
whtk verify <provider> --secret <s> …	Check a signature, and when it fails, say why.
whtk replay <token> <id> --to <url>	Re-send a captured request from your machine (localhost works).
whtk requests <token>	List captured requests (--json for scripts).
whtk endpoints	List your account's URLs.
whtk login / logout / whoami	Save, remove or inspect your API key.
whtk mcp	Start the MCP server on stdio.

A few things worth knowing:

• Sub-paths and query strings are kept. A request to …/r/<token>/stripe?x=1 is forwarded to http://localhost:3000/webhooks/stripe?x=1, so one URL can feed several handlers.
• Headers are forwarded as received (minus hop-by-hop headers, host and content-length), so signature checks in your handler pass exactly as in production.
• --json prints NDJSON (listening, request, forward events) for scripting: whtk listen -f 3000 --json | jq .request.event.
• Colors follow NO_COLOR, FORCE_COLOR and are off when output is not a terminal.

Send a signed test event

# Prints the headers and a curl command, with a realistic sample event
whtk sign stripe --secret whsec_… --event checkout.session.completed

# Or send it straight to your handler
whtk sign github --secret my-secret --file push.json --send http://localhost:3000/api/github
whtk sign twilio --secret <auth token> --url https://example.com/sms --send 3000

Debug "invalid signature"

whtk verify stripe --secret whsec_test_secret --body-file body.json \
  -H "Stripe-Signature: t=1726000000,v1=a530ce3d81556a0eced506e1d1cad777bc55bc1e408e54dd6bfef08e1dde216c"

✗ Invalid Stripe signature (body_trailing_newline_added)
  The signature matches the body without its trailing newline: a newline was appended after signing (proxy, logger, copy-paste). Verify the raw bytes exactly as received.
  expected  t=1726000000,v1=1e0085bf1054f89ac8882e2e5fcbfdf301b1d6445e8ed7a316120cd5dc651ba9
  received  t=1726000000,v1=a530ce3d81556a0eced506e1d1cad777bc55bc1e408e54dd6bfef08e1dde216c
  timestamp 1726000000 (738 days ago)

The verifier replays the usual mistakes (whitespace in the secret, added or stripped trailing newline, CRLF conversion, expired timestamp, and for Twilio: http vs https, port, query string, trailing slash, repeated parameters) and tells you which one matches. The same logic powers the online signature validator.

Test webhooks in your test suite

The library gives your tests a real public URL and a way to wait for what lands on it. It works anonymously; set WEBHOOK_TOOLKIT_KEY in CI for permanent URLs and higher limits.

Vitest (or Jest in ESM mode)

import { expect, test } from "vitest";
import { WebhookToolkit, verify } from "webhook-toolkit";

const wt = new WebhookToolkit(); // anonymous, or reads WEBHOOK_TOOLKIT_KEY

test("creating an order notifies the customer's webhook", async () => {
  const endpoint = await wt.createEndpoint({ name: "ci-orders" });

  // The app under test sends its outgoing webhooks to the capture URL.
  await api.post("/settings/webhooks", { url: endpoint.url, secret: "whsec_MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw" });
  await api.post("/orders", { sku: "tshirt", quantity: 2 });

  const req = await wt.waitForRequest(endpoint.token, {
    timeoutMs: 30_000,
    filter: (r) => JSON.parse(r.body).type === "order.created",
  });

  expect(JSON.parse(req.body).data.quantity).toBe(2);
  // Your outgoing signatures are correct, too:
  expect(verify("svix", { secret: "whsec_MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw", rawBody: req.body, headers: req.headers }).valid).toBe(true);
});

waitForRequest checks requests that already arrived since the endpoint was created, then long-polls until timeoutMs, so the order of "trigger" and "wait" does not matter. It rejects with a WebhookToolkitError (code: "timeout") when nothing matched.

To test your handler without any network at all, sign the payload locally:

import { sign } from "webhook-toolkit";

const { headers, body } = sign("stripe", { secret: process.env.STRIPE_WEBHOOK_SECRET!, event: "checkout.session.completed" });
const res = await fetch("http://localhost:3000/api/webhooks/stripe", { method: "POST", headers, body });
expect(res.status).toBe(200);

Playwright

import { expect, test } from "@playwright/test";
import { WebhookToolkit } from "webhook-toolkit";

const wt = new WebhookToolkit();

test("'Send test event' reaches the configured URL", async ({ page }) => {
  const endpoint = await wt.createEndpoint();

  await page.goto("/settings/integrations");
  await page.getByLabel("Webhook URL").fill(endpoint.url);
  await page.getByRole("button", { name: "Save" }).click();
  await page.getByRole("button", { name: "Send test event" }).click();

  const req = await wt.waitForRequest(endpoint.token, { timeoutMs: 15_000 });
  expect(req.method).toBe("POST");
  expect(req.headers["content-type"]).toContain("application/json");
});

The package is ESM-only. CommonJS test runners work on Node ≥ 20.19 / 22.12 (native require(esm)); Jest needs its ESM mode.

Use it from AI agents (MCP)

webhook-toolkit mcp is a Model Context Protocol server. Your coding agent can create a webhook URL, wait for the delivery after triggering it, read the payload, then replay it or send freshly signed events to your localhost handler while it fixes the code.

The API key is optional: everything except list_webhook_urls and explain_webhook_request works anonymously.

Claude Code

claude mcp add webhook-toolkit -- npx -y webhook-toolkit mcp
# with a key:
claude mcp add webhook-toolkit --env WEBHOOK_TOOLKIT_KEY=whk_… -- npx -y webhook-toolkit mcp

Cursor (.cursor/mcp.json), Windsurf (~/.codeium/windsurf/mcp_config.json), Claude Desktop (claude_desktop_config.json)

{
  "mcpServers": {
    "webhook-toolkit": {
      "command": "npx",
      "args": ["-y", "webhook-toolkit", "mcp"],
      "env": { "WEBHOOK_TOOLKIT_KEY": "whk_…" }
    }
  }
}

VS Code (.vscode/mcp.json)

{
  "inputs": [
    { "type": "promptString", "id": "webhook-toolkit-key", "description": "webhook-toolkit.com API key (optional)", "password": true }
  ],
  "servers": {
    "webhook-toolkit": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "webhook-toolkit", "mcp"],
      "env": { "WEBHOOK_TOOLKIT_KEY": "${input:webhook-toolkit-key}" }
    }
  }
}

Codex CLI (~/.codex/config.toml)

[mcp_servers.webhook-toolkit]
command = "npx"
args = ["-y", "webhook-toolkit", "mcp"]
env = { WEBHOOK_TOOLKIT_KEY = "whk_…" }

ChatGPT, Claude.ai and other remote-capable clients: add a custom connector pointing at the hosted server, nothing to install:

https://webhook-toolkit.com/mcp

(Streamable HTTP, optional Authorization: Bearer whk_….) The hosted server cannot reach your machine, so replaying to localhost and send_signed_webhook need the local server above.

Tool	Use it to
create_webhook_url	Get a public URL to receive a webhook from a third-party service or the app under test
wait_for_webhook	Block until the delivery arrives (filters: provider, event, method, path, body); safe to call after triggering
list_webhook_requests / get_webhook_request	See what was actually sent: headers, raw body, detected provider and event
replay_webhook_request	Re-send a captured request from the local machine to e.g. http://localhost:3000/api/webhooks
send_signed_webhook	POST a validly signed event (Stripe, GitHub, Shopify, Slack, Twilio, Mailgun, Svix, Paddle, Discord) to your handler
sign_webhook_payload	Compute signature headers and a curl command without sending
verify_webhook_signature	Explain why a handler rejects a signature
set_webhook_response	Change what the URL answers (status, body): simulate failures, answer challenges
list_webhook_urls	Find an existing permanent URL (key required)
explain_webhook_request	AI explanation of a payload, or a generated handler (paid, 3 free trials)

Outputs are compact text plus the key JSON; bodies over 20 KB are truncated with a note pointing to the inspector. To embed the server in your own process: import { createMcpServer } from "webhook-toolkit/mcp".

Signing and verifying

sign() and verify() run locally with node:crypto: secrets never leave your machine. Signatures are byte-for-byte identical to the webhook-toolkit.com signer, and the tests check them against the providers' published examples (GitHub, Slack, Twilio, Svix).

Provider	id	Signature	Algorithm	secret
Stripe	stripe	Stripe-Signature	HMAC-SHA256 hex of t.body	endpoint secret whsec_…
GitHub, Gitea, Forgejo	github	X-Hub-Signature-256 (+ X-Hub-Signature)	HMAC-SHA256 hex of the body	webhook secret
Shopify	shopify	X-Shopify-Hmac-Sha256	HMAC-SHA256 base64 of the body	app client secret
Slack	slack	X-Slack-Signature, X-Slack-Request-Timestamp	HMAC-SHA256 hex of v0:ts:body	signing secret
Twilio	twilio	X-Twilio-Signature	HMAC-SHA1 base64 of URL + sorted params (bodySHA256 for JSON)	auth token, plus url
Mailgun	mailgun	signature object in the body	HMAC-SHA256 hex of timestamp + token	webhook signing key
Svix / Standard Webhooks (Clerk, Resend)	svix	svix-* and webhook-* headers	HMAC-SHA256 base64 of id.ts.body	whsec_…
Paddle Billing	paddle	Paddle-Signature	HMAC-SHA256 hex of ts:body	pdl_ntfset_…
Discord interactions	discord	X-Signature-Ed25519, X-Signature-Timestamp	Ed25519	verify: app public key; sign: a test private key

import { sign, verify } from "webhook-toolkit";

// In an Express handler (use express.raw so the body stays byte-exact)
app.post("/webhooks/stripe", express.raw({ type: "*/*" }), (req, res) => {
  const result = verify("stripe", { secret: process.env.STRIPE_WEBHOOK_SECRET!, rawBody: req.body, headers: req.headers });
  if (!result.valid) return res.status(400).send(result.message);
  // …
});

verify() returns { valid, reason?, message, expected?, received?, timestamp? }. Failure reasons: missing_signature, malformed_signature, missing_timestamp, missing_url, invalid_secret, timestamp_out_of_tolerance (default 300 s, toleranceSeconds: 0 disables), signature_mismatch, secret_whitespace, body_trailing_newline_added, body_trailing_newline_removed, body_line_endings_changed, url_mismatch (Twilio), body_hash_mismatch (Twilio JSON). Comparisons are constant-time.

Provider detection

detectProvider(headers, body) returns { provider, event } from a data-driven table (DETECTION_RULES), for example { provider: "stripe", event: "invoice.paid" } or { provider: "twilio", event: "message.received" }. It knows Stripe, GitHub, GitLab, Bitbucket, Shopify, WooCommerce, Slack, SendGrid, Twilio, Paddle, Svix (Clerk, Resend), GoCardless, PayPal, Discord, Linear, Typeform, Calendly, Square, HubSpot, Zoom, Lemon Squeezy, Coinbase Commerce, Twitch, Mux, Vercel, Netlify, Sentry, Trello, Plaid, Mandrill, Jira, Postmark, Mollie, Adyen, Mailgun and Redsys. Adding one is a table entry plus a test fixture.

Relay: a real tunnel (paid)

listen --forward is free and fine for most webhooks, but the sender receives the capture URL's response, not your app's. When the sender needs your real answer (Slack slash commands, Twilio TwiML, Shopify mandatory responses, OAuth callbacks), use the relay:

whtk login               # once
whtk relay --to 3000

  webhook-toolkit relay · live

  Public URL  https://webhook-toolkit.com/relay/my-app  (any sub-path works)
  Forwarding  → http://localhost:3000

Requests to the public URL are proxied over a WebSocket to your machine and your local response (status, headers, body) goes back to the caller. It reconnects with backoff. The relay comes with the 7-day Pass or Pro.

Free vs paid

	Anonymous	Free account	Pass (7 days, €5 once) / Pro (€9/month)
Capture URLs	up to 30 new per day, each lasts 7 days	1 permanent URL	25 permanent URLs
History	7 days	7 days	30 days
CLI listen --forward, replay, sign, verify, MCP	✓	✓	✓
Relay (real tunnel to localhost)			✓
AI explanations and generated handlers	3 free trials	3 free trials	✓

Details and the Business plan: webhook-toolkit.com/pricing. Everything that runs locally (signing, verifying, detection, forwarding) is free and stays free.

How it compares

• smee.io is a free, open-source service from the GitHub Probot team that forwards webhooks to localhost through its client. If forwarding is all you need, it does the job. webhook-toolkit's listen --forward works the same way and adds a stored inspector, provider/event detection, replay, signing and verification tools, a test-suite API and an MCP server.
• webhook.site is a mature hosted request inspector with custom responses and paid automation features. webhook-toolkit covers the inspect-and-respond part and focuses on the local developer loop: forwarding, signed test events, signature diagnosis, tests and agents.
• ngrok is a general-purpose tunnel for any HTTP or TCP service, with a local traffic inspector. Pick it to expose a whole app. webhook-toolkit's free flow needs no account, and its paid relay is scoped to webhooks.

Library reference

import { WebhookToolkit, WebhookToolkitError, sign, verify, detectProvider, forwardRequest } from "webhook-toolkit";

const wt = new WebhookToolkit({ apiKey, baseUrl }); // both optional (env: WEBHOOK_TOOLKIT_KEY, WEBHOOK_TOOLKIT_URL)

Method	Returns
createEndpoint({ name? })	Endpoint: { token, url, inspectUrl, expiresAt, createdAt, response, … }
listEndpoints()	Endpoint[] (API key required)
getEndpoint(token) / deleteEndpoint(token)	Endpoint / void
updateEndpoint(token, { name?, response?: { status?, body?, contentType? } })	Endpoint
listRequests(token, { limit?, after? })	CapturedRequest[], newest first
getRequest(token, id) / clearRequests(token)	CapturedRequest / void
waitForRequest(token, { timeoutMs?, after?, filter?, signal? })	CapturedRequest (rejects with code: "timeout")
stream(token, onRequest, { signal?, after?, onOpen?, onReconnect? })	Promise<void>: live SSE feed, reconnects and catches up on missed requests
replayRequest(token, id, url)	server-side replay to a public URL
explainRequest(token, id, { mode?, language? })	AI explanation or handler (paid, 3 free trials)
me(), listRelays(), createRelay()	account, plan, relays

forwardRequest(request, target) re-sends a captured request from your machine (the same function behind listen --forward and replay). Errors are WebhookToolkitError with status (HTTP status, 0 for network errors), code (unauthorized, upgrade_required, plan_limit, expired, timeout, network_error, …) and upgradeUrl on 402 answers. Everything is typed; types ship with the package.

Configuration

Variable	Purpose
WEBHOOK_TOOLKIT_KEY	API key (whk_…), created in your dashboard. Takes precedence over the saved key; --key overrides both.
WEBHOOK_TOOLKIT_URL	API origin, e.g. http://localhost:3000 for a self-hosted instance. Same as --base-url.
WEBHOOK_TOOLKIT_RELAY_TOKEN	Relay token for whtk relay.
NO_COLOR / FORCE_COLOR	Terminal colors.

whtk login stores the key in ~/.config/webhook-toolkit/config.json (or $XDG_CONFIG_HOME/webhook-toolkit/config.json) with mode 600. The same file remembers the last URL used by listen.

A capture URL's token is a capability: anyone holding it can read the requests sent to it. Use an account for anything sensitive; the owner's key is then required to change or delete the URL.

Requirements

Node.js 18.17 or later. Runtime dependencies: @modelcontextprotocol/sdk and zod (MCP server) and ws (relay).

Contributing

Issues and pull requests are welcome: see CONTRIBUTING.md. New providers for detection or signing are the easiest place to start.

License

MIT © 2026 Kipdev. Hosted service: webhook-toolkit.com.