Hail Webhooks Skill

When to Use This Skill

• How do I receive Hail webhooks?
• How do I verify the X-Hail-Signature header?
• How do I handle email.received, email.bounced, sms.received, or call.completed events?
• Why does the whsec_ secret not work with the Standard Webhooks or Svix libraries?
• Why is my Hail webhook signature verification failing?

Essential Code (USE THIS)

Every Hail delivery carries X-Hail-Signature: t=<unix>,v1=<hex>. v1 is the HMAC-SHA256 (hex) of "<t>.<raw body>". The key is the whole signing secret string, including whsec_, as UTF-8. Hail returns the secret only when you create the subscription, so save it then.

This is not Standard Webhooks. The secret looks like whsec_..., but do not use the standardwebhooks or svix libraries, and do not strip or base64-decode the whsec_ prefix.

Hail's signer emits a single v1, and rotating the secret (POST /webhooks/{id}/rotate-secret) invalidates the old one immediately, so there is no overlap period to cover. The code below still accepts any matching v1, which costs nothing. Reject a header with no t or no v1.

Verification (Node.js)

const crypto = require('crypto');

// rawBody: Buffer of the exact request bytes (express.raw({ type: '*/*' }), not express.json)
function verifyHail(rawBody, signatureHeader, secret) {
  if (!secret || !signatureHeader) return false;
  let t = null; const v1s = [];
  for (const part of signatureHeader.split(',')) {
    const i = part.indexOf('=');
    if (i <= 0) continue;
    const k = part.slice(0, i).trim(), v = part.slice(i + 1).trim();
    if (k === 't') t = v; else if (k === 'v1') v1s.push(v);
  }
  if (!t || v1s.length === 0) return false;
  // Optional replay window, off by default: Hookdeck retries and replays keep the original timestamp
  const expected = Buffer.from(crypto.createHmac('sha256', secret) // whole secret, as-is
    .update(`${t}.`).update(rawBody).digest('hex'));
  // exactly 64 hex chars first, so a bad value can't truncate or throw
  return v1s.some((v) => /^[0-9a-f]{64}$/i.test(v) &&
    crypto.timingSafeEqual(Buffer.from(v.toLowerCase()), expected));
}

Verification (Python)

import hashlib, hmac

def verify_hail(raw_body: bytes, signature_header: str, secret: str) -> bool:
    if not secret or not signature_header:
        return False
    t, v1s = None, []
    for part in signature_header.split(","):
        key, sep, value = part.partition("=")
        if sep and key.strip() == "t":
            t = value.strip()
        elif sep and key.strip() == "v1":
            v1s.append(value.strip())
    if not t or not v1s:
        return False
    signed = t.encode() + b"." + raw_body
    expected = hmac.new(secret.encode(), signed, hashlib.sha256).hexdigest().encode()
    return any(hmac.compare_digest(v.lower().encode(), expected) for v in v1s)

For complete handlers with tests, see examples/express/, examples/nextjs/, examples/fastapi/.

Timestamp window: off by default

A 5-minute freshness window is common advice, but Hookdeck retries and replays resend the original timestamp, so a 5-minute window rejects late retries. (Hail itself re-signs each of its own attempts with the current time.) The examples therefore skip the freshness check unless you set HAIL_TIMESTAMP_TOLERANCE_SECONDS to a positive number of seconds (default 0 = off). Dedupe on X-Hail-Delivery or the event id instead. references/verification.md shows exactly where the window can be dropped or widened.

Common Event Types

The event name is the body's type field (also the X-Hail-Event header).

See references/overview.md for the payload.

Important Headers

Environment Variables

HAIL_WEBHOOK_SECRET=whsec_your_signing_secret   # returned once by POST /webhooks; use the whole string
HAIL_TIMESTAMP_TOLERANCE_SECONDS=0              # optional; a positive value opts in to the replay window

Hookdeck Setup

Paste the whole signing secret, including any whsec_ prefix, into the Hookdeck source's webhook secret. Hail uses it as a plain HMAC key. See references/setup.md.

Local Development

# Start tunnel (no account needed)
npx hookdeck-cli listen 3000 hail --path /webhooks/hail

Reference Materials

• references/overview.md - Hail webhook concepts, events, payload, retries
• references/setup.md - Creating the subscription through the API and saving the secret
• references/verification.md - Signature details, the timestamp window, and debugging