Retell AI Webhooks Skill

When to Use This Skill

• How do I receive Retell AI webhooks?
• How do I verify Retell webhook signatures (X-Retell-Signature)?
• How do I handle call_ended or call_analyzed events?
• Why is my Retell webhook signature verification failing?
• How do I handle transcript, transfer, and chat events from Retell?

Verification (core)

Retell signs each webhook with HMAC-SHA256 using your Retell API key as the secret (only an API key with the webhook badge in the dashboard works). The signature arrives in the X-Retell-Signature header formatted as v={unix_ms_timestamp},d={hex_digest}, where the digest is computed over the raw request body concatenated with the timestamp. Always verify against the raw body, never a re-serialized JSON string.

The Python SDK ships a verify helper (client.verify(body, api_key, signature) — positional params, so the keyword form used in the FastAPI example also works). The Node SDK has no verify helper, so Node handlers verify manually.

Verified against retell-sdk 5.56.0 (PyPI) and 5.46.0 (npm): the Python package exposes retell.lib.webhook_auth.verify, re-exported as client.verify; the npm package has no webhook/verify export. Check your installed version — the helper's presence and argument order have moved between releases.

const crypto = require('crypto');

// Reject signatures older than 5 minutes to prevent replay attacks
const FIVE_MINUTES_MS = 5 * 60 * 1000;

function verifyRetellSignature(rawBody, signatureHeader, apiKey) {
  const match = /^v=(\d+),d=(.*)$/.exec(signatureHeader || '');
  if (!match) return false;
  const [, timestamp, digest] = match;
  if (Math.abs(Date.now() - Number(timestamp)) > FIVE_MINUTES_MS) return false;

  const expected = crypto
    .createHmac('sha256', apiKey)
    .update(rawBody + timestamp) // raw body + timestamp, in that order
    .digest('hex');
  try {
    return crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(digest));
  } catch {
    return false; // length mismatch = invalid
  }
}

Python (FastAPI) uses the official SDK instead:

from retell import Retell

client = Retell(api_key=os.environ["RETELL_API_KEY"])
# client.verify enforces the ~5 min timestamp window and returns a bool
is_valid = client.verify(raw_body.decode("utf-8"),
                         api_key=os.environ["RETELL_API_KEY"],
                         signature=signature_header)

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

Common Event Types

Payloads carry an event field plus a call object (voice/transfer events) or a chat object (chat events). Dedupe on event + call.call_id (or chat.chat_id) — Retell retries up to 3 times if it doesn't get a 2xx within 10 seconds.

See references/overview.md for full payload structure.

Environment Variables

# The Retell API key (must have the webhook badge). Used as the HMAC secret.
RETELL_API_KEY=key_xxxxxxxxxxxxxxxxxxxxxxxx
PORT=3000

Local Development

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

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

No account required — the CLI creates a guest account on first run and provides a local tunnel + web UI for inspecting requests.

Resources

• Overview - What Retell webhooks are, event types, payloads
• Setup - Configure account-level and agent-level webhooks
• Verification - Signature verification details and gotchas
• Express Example - Complete Express.js implementation
• Next.js Example - Next.js App Router implementation
• FastAPI Example - Python FastAPI implementation (uses the Retell SDK)