Hookflo + Tern Webhook Skill

This skill covers two tightly related tools in the Hookflo ecosystem:

Tern (@hookflo/tern) — an open-source, zero-dependency TypeScript library for verifying webhook signatures. Algorithm-agnostic, supports all major platforms.
Hookflo — a hosted webhook event alerting and logging platform. Sends real-time Slack/email alerts when webhooks fire. No code required on their end; you point your provider at Hookflo's URL and configure alerts in the dashboard.

Mental Model

Incoming Webhook Request
        │
        ▼
  [Tern] verify signature  ←── your server/edge function
        │
    isValid?
        │
   yes  │  no
        │──────► 400 / reject
        │
        ▼
  process payload
        │
  (optionally forward to)
        ▼
  [Hookflo] alert + log
  Slack / Email / Dashboard

Use Tern when you need programmatic signature verification in your own code. Use Hookflo when you want no-code / low-code alerting and centralized event logs. They can be used together or independently.

Part 1 — Tern (Webhook Verification Library)

Installation

npm install @hookflo/tern

No other dependencies required. Full TypeScript support.

Core API

`WebhookVerificationService.verify(request, config)`

The primary method. Returns a WebhookVerificationResult.

import { WebhookVerificationService } from '@hookflo/tern';

const result = await WebhookVerificationService.verify(request, {
  platform: 'stripe',
  secret: process.env.STRIPE_WEBHOOK_SECRET!,
  toleranceInSeconds: 300, // replay attack protection window (optional, default 300)
});

if (result.isValid) {
  console.log('Verified payload:', result.payload);
  console.log('Metadata:', result.metadata); // timestamp, id, etc.
} else {
  console.error('Rejected:', result.error);
  // return 400
}

`WebhookVerificationService.verifyWithPlatformConfig(request, platform, secret, tolerance?)`

Shorthand that accepts just a platform name + secret.

const result = await WebhookVerificationService.verifyWithPlatformConfig(
  request,
  'github',
  process.env.GITHUB_WEBHOOK_SECRET!
);

`WebhookVerificationService.verifyTokenBased(request, webhookId, webhookToken)`

For token-based platforms (Supabase, GitLab).

const result = await WebhookVerificationService.verifyTokenBased(
  request,
  process.env.SUPABASE_WEBHOOK_ID!,
  process.env.SUPABASE_WEBHOOK_TOKEN!
);

`WebhookVerificationResult` type

interface WebhookVerificationResult {
  isValid: boolean;
  error?: string;
  platform: WebhookPlatform;
  payload?: any;             // parsed JSON body
  metadata?: {
    timestamp?: string;
    id?: string | null;
    [key: string]: any;
  };
}

Built-in Platform Configs

| Platform | Algorithm | Signature Header | Format | |---|---|---|---| | stripe | HMAC-SHA256 | stripe-signature | t={ts},v1={sig} | | github | HMAC-SHA256 | x-hub-signature-256 | sha256={sig} | | clerk | HMAC-SHA256 (base64) | svix-signature | v1,{sig} | | supabase | Token-based | custom | — | | gitlab | Token-based | x-gitlab-token | — | | shopify | HMAC-SHA256 | x-shopify-hmac-sha256 | raw | | vercel | HMAC-SHA256 | custom | — | | polar | HMAC-SHA256 | custom | — | | dodo | HMAC-SHA256 (svix) | webhook-signature | v1,{sig} |

Always use the lowercase string name (e.g., 'stripe', 'github').

Custom Platform Configuration

For any provider not in the list, supply a full signatureConfig:

import { WebhookVerificationService } from '@hookflo/tern';

// Standard HMAC-SHA256 with prefix
const result = await WebhookVerificationService.verify(request, {
  platform: 'acmepay',
  secret: 'your_secret',
  signatureConfig: {
    algorithm: 'hmac-sha256',
    headerName: 'x-acme-signature',
    headerFormat: 'prefixed',
    prefix: 'sha256=',
    payloadFormat: 'raw',
  },
});

// Timestamped payload (signs "{timestamp}.{body}")
const result2 = await WebhookVerificationService.verify(request, {
  platform: 'mypay',
  secret: 'your_secret',
  signatureConfig: {
    algorithm: 'hmac-sha256',
    headerName: 'x-webhook-signature',
    headerFormat: 'raw',
    timestampHeader: 'x-webhook-timestamp',
    timestampFormat: 'unix',
    payloadFormat: 'timestamped',
  },
});

// Svix/StandardWebhooks compatible (Clerk, Dodo, etc.)
const result3 = await WebhookVerificationService.verify(request, {
  platform: 'my-svix-platform',
  secret: 'whsec_abc123...',
  signatureConfig: {
    algorithm: 'hmac-sha256',
    headerName: 'webhook-signature',
    headerFormat: 'raw',
    timestampHeader: 'webhook-timestamp',
    timestampFormat: 'unix',
    payloadFormat: 'custom',
    customConfig: {
      payloadFormat: '{id}.{timestamp}.{body}',
      idHeader: 'webhook-id',
    },
  },
});

SignatureConfig fields:

algorithm: 'hmac-sha256' | 'hmac-sha1' | 'hmac-sha512' | custom
headerName: the HTTP header that carries the signature
headerFormat: 'raw' | 'prefixed' | 'comma-separated' | 'space-separated'
prefix: string prefix to strip before comparing (e.g. 'sha256=')
timestampHeader: header name for the timestamp (if any)
timestampFormat: 'unix' | 'iso' | 'ms'
payloadFormat: 'raw' | 'timestamped' | 'custom'
customConfig.payloadFormat: template like '{id}.{timestamp}.{body}'
customConfig.idHeader: header supplying the {id} value
customConfig.encoding: 'base64' if the provider base64-encodes the key

Framework Integration

Express.js

import express from 'express';
import { WebhookVerificationService } from '@hookflo/tern';

const app = express();

// IMPORTANT: use raw body parser for webhook routes
app.post(
  '/webhooks/stripe',
  express.raw({ type: 'application/json' }),
  async (req, res) => {
    const result = await WebhookVerificationService.verifyWithPlatformConfig(
      req,
      'stripe',
      process.env.STRIPE_WEBHOOK_SECRET!
    );

    if (!result.isValid) {
      return res.status(400).json({ error: result.error });
    }

    const event = result.payload;
    // handle event.type, e.g. 'payment_intent.succeeded'

    res.json({ received: true });
  }
);

Common mistake: Express's default json() middleware consumes and re-serializes the body, breaking HMAC. Always use express.raw() on webhook endpoints.

Next.js App Router (Route Handler)

// app/api/webhooks/github/route.ts
import { NextRequest, NextResponse } from 'next/server';
import { WebhookVerificationService } from '@hookflo/tern';

export async function POST(req: NextRequest) {
  const result = await WebhookVerificationService.verifyWithPlatformConfig(
    req,
    'github',
    process.env.GITHUB_WEBHOOK_SECRET!
  );

  if (!result.isValid) {
    return NextResponse.json({ error: result.error }, { status: 400 });
  }

  const event = req.headers.get('x-github-event');
  // handle event

  return NextResponse.json({ received: true });
}

// Disable body parsing so Tern gets the raw body
export const config = { api: { bodyParser: false } };

Cloudflare Workers

addEventListener('fetch', (event) => {
  event.respondWith(handleRequest(event.request));
});

async function handleRequest(request: Request): Promise<Response> {
  if (request.method === 'POST' && new URL(request.url).pathname === '/webhooks/clerk') {
    const result = await WebhookVerificationService.verifyWithPlatformConfig(
      request,
      'clerk',
      CLERK_WEBHOOK_SECRET
    );

    if (!result.isValid) {
      return new Response(JSON.stringify({ error: result.error }), {
        status: 400,
        headers: { 'Content-Type': 'application/json' },
      });
    }

    return new Response(JSON.stringify({ received: true }));
  }

  return new Response('Not Found', { status: 404 });
}

Platform Manager (Advanced)

import { platformManager } from '@hookflo/tern';

// Verify using the platform manager directly
const result = await platformManager.verify(request, 'stripe', 'whsec_...');

// Get the config for a platform (for inspection)
const config = platformManager.getConfig('stripe');

// Get docs/metadata for a platform
const docs = platformManager.getDocumentation('stripe');

// Run built-in tests for a platform
const passed = await platformManager.runPlatformTests('stripe');

Testing

npm test                       # run all tests
npm run test:platform stripe   # test one platform
npm run test:all               # test all platforms

Part 2 — Hookflo (Hosted Alerting Platform)

Hookflo requires no library installation. The integration is:

Create a webhook endpoint in the Hookflo Dashboard → get a Webhook URL + Secret
Point your provider (Stripe, Supabase, Clerk, GitHub, etc.) at that URL
Configure Slack/email notifications in the dashboard

How to Set Up a Hookflo Integration

Step 1 — Go to hookflo.com/dashboard and create a new webhook. You'll receive:

Webhook URL — paste into your provider's webhook settings
Webhook ID — used for token-based platforms
Secret Token — used by Hookflo to verify incoming events
Notification channel settings — configure Slack or email

Step 2 — Set up the provider to send to that Hookflo URL:

| Provider | Where to paste the URL | |---|---| | Stripe | Dashboard → Developers → Webhooks → Add endpoint | | Supabase | Dashboard → Database → Webhooks → Create webhook | | Clerk | Dashboard → Webhooks → Add endpoint | | GitHub | Repo/Org Settings → Webhooks → Add webhook |

Step 3 — In the Hookflo dashboard, configure:

Which event types to alert on (e.g., payment_intent.succeeded, user.created)
Notification channels (Slack workspace/channel, email addresses)
Digest frequency if you want batched summaries instead of per-event alerts

Hookflo Platform Docs

Stripe: docs.hookflo.com/webhook-platforms/stripe
Supabase: docs.hookflo.com/webhook-platforms/supabase
Clerk: docs.hookflo.com/webhook-platforms/clerk
GitHub: docs.hookflo.com/webhook-platforms/github
Slack notifications: docs.hookflo.com/notification-channels/slack

Hookflo + Tern Together

If you want both programmatic verification (Tern) AND logging/alerting (Hookflo), use a proxy pattern:

// Your server receives the webhook, verifies it with Tern, then forwards to Hookflo
app.post('/webhooks/stripe', express.raw({ type: 'application/json' }), async (req, res) => {
  // 1. Verify with Tern
  const result = await WebhookVerificationService.verifyWithPlatformConfig(
    req, 'stripe', process.env.STRIPE_WEBHOOK_SECRET!
  );
  if (!result.isValid) return res.status(400).json({ error: result.error });

  // 2. Process locally
  handleStripeEvent(result.payload);

  // 3. Forward to Hookflo for alerting/logging (optional)
  await fetch(process.env.HOOKFLO_WEBHOOK_URL!, {
    method: 'POST',
    headers: { ...req.headers, 'Content-Type': 'application/json' },
    body: req.body,
  });

  res.json({ received: true });
});

Alternatively, point Stripe directly at your Hookflo URL and keep Tern for a different endpoint.

Common Pitfalls & Best Practices

Raw Body Requirement

HMAC signatures are computed over the exact raw bytes of the request body. Any re-serialization (e.g., by a JSON body parser) will break verification. Always ensure:

Express: use express.raw({ type: 'application/json' }) on webhook routes
Next.js Pages Router: set export const config = { api: { bodyParser: false } }
Next.js App Router: Tern reads the body directly from the Request object

Replay Attack Protection

Always pass toleranceInSeconds (default is 300 = 5 minutes). This rejects requests with timestamps too far in the past, preventing replay attacks.

Secrets Management

Never hardcode secrets in source code
Use environment variables: process.env.STRIPE_WEBHOOK_SECRET
For Cloudflare Workers: use wrangler secret put STRIPE_WEBHOOK_SECRET
For Vercel: add secrets in project settings

Error Responses

Always return HTTP 400 (not 500) for failed verification — this signals to the sender that the request was rejected (not that your server crashed).

HTTPS Only

Webhook endpoints must use HTTPS in production. Never accept webhook traffic over HTTP.

Troubleshooting

| Symptom | Likely Cause | Fix | |---|---|---| | isValid: false, error about signature | Body was parsed before Tern | Use raw body parser | | isValid: false, error about timestamp | Clock skew or replay attack | Check server clock; increase tolerance if dev | | isValid: false for Clerk | Missing svix headers | Ensure svix-id, svix-timestamp, svix-signature are forwarded | | isValid: false for GitHub | Wrong secret | Re-copy secret from GitHub Webhooks settings | | Tern not finding platform | Typo in platform name | Use lowercase: 'stripe', 'github', 'clerk' | | Hookflo not receiving events | Wrong URL pasted | Re-copy URL from Hookflo dashboard |

Key Links

Tern GitHub: https://github.com/Hookflo/tern
Tern npm: https://www.npmjs.com/package/@hookflo/tern
Tern docs: https://tern.hookflo.com
Hookflo homepage: https://hookflo.com
Hookflo dashboard: https://hookflo.com/dashboard
Hookflo docs: https://docs.hookflo.com
Hookflo Discord: https://discord.com/invite/SNmCjU97nr