GoCardless Webhooks Skill

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)
  }
}

For languages without a GoCardless SDK (e.g. Python/FastAPI), verify manually — same algorithm, timing-safe compare:

import hmac, hashlib
expected = hmac.new(secret.encode(), raw_body, hashlib.sha256).hexdigest()
valid = hmac.compare_digest(expected, signature_header)  # timing-safe

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

Common Event Types

GoCardless events combine a resource_type with an action. The most common:

See overview.md for the full action list per resource type.

Environment Variables

# Webhook endpoint secret from the GoCardless Dashboard (Developers → Webhook endpoints)
GOCARDLESS_WEBHOOK_SECRET=your_webhook_endpoint_secret

Local Development

For local webhook testing, run the Hookdeck CLI via npx — no install required:

npx hookdeck-cli listen 3000 gocardless --path /webhooks/gocardless

No account required. The CLI creates a guest account on first run and provides a local tunnel + web UI for inspecting requests. Use port 8000 for the FastAPI example.

Reference Materials

• Overview - What GoCardless webhooks are, full event/action list
• Setup - Create a webhook endpoint and copy the secret
• Verification - Signature verification details and gotchas

Examples

• Express Example - Express 5 handler using the GoCardless SDK, with tests
• Next.js Example - Next.js App Router route using the GoCardless SDK, with tests
• FastAPI Example - Python FastAPI handler with manual HMAC verification, with tests