gocardless-webhooks

GoCardless Webhooks

GoCardless is a bank debit / recurring payments platform. It sends webhooks as batches of events (up to 250 per request) in an events array, signed with an HMAC-SHA256 signature in the Webhook-Signature header.

When to Use This Skill

• How do I receive GoCardless webhooks?
• How do I verify the GoCardless Webhook-Signature header?
• Why is my GoCardless webhook signature verification failing?
• How do I handle payments confirmed/failed, mandates cancelled, or payouts paid events?
• How do I process the GoCardless events array idempotently?

How GoCardless Signs Webhooks

• Header: Webhook-Signature — a bare 64-char lowercase hex digest (no X- prefix, no sha256= scheme prefix)
• Algorithm: HMAC-SHA256 over the raw request body, keyed with the webhook endpoint secret (from your GoCardless Dashboard)
• Key: use the secret verbatim as a UTF-8 string — do NOT base64-decode it even though it looks base64url-ish; decoding it produces a wrong signature
• Encoding: lowercase hex string
• Comparison: timing-safe equality
• Response: return 204 No Content once the whole batch is accepted; a non-2xx (e.g. 498) marks the delivery failed. GoCardless does not auto-retry — redelivery is manual (POST /webhooks/{id}/actions/retry). Delivery is at-least-once, so keep handlers idempotent on event.id.

(Scheme verified 2026-08 against a live sandbox delivery and the official gocardless-nodejs SDK; GoCardless's prose still doesn't name the algorithm.)

Always verify against the raw body — parsing JSON first and re-serializing will change the bytes and break the signature.

Verification (core)

Use the official gocardless-nodejs SDK where it runs (Node.js). parse() verifies the signature (timing-safe) and returns the events array, throwing InvalidSignatureError when the signature does not match.

// Node.js — official SDK (gocardless-nodejs), req.body is the RAW Buffer
const { parse, InvalidSignatureError } = require('gocardless-nodejs/webhooks');

try {
  const events = parse(
    req.body,                                  // raw body (Buffer/string), NOT parsed JSON
    process.env.GOCARDLESS_WEBHOOK_SECRET,     // webhook endpoint secret
    req.headers['webhook-signature']           // Webhook-Signature header
  );
  // signature valid — process each event, then respond 204
} catch (err) {
  if (err instanceof InvalidSignatureError) {
    // signature mismatch — respond 498 (do not process)
  }
}