Retell AI Webhooks

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 callended or callanalyzed 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={unixmstimestamp},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/express/), [examples/nextjs/](examples/nextjs/), [examples/fastapi/](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.callid (or chat.chatid) — Retell retries up to 3 times if it doesn't get a 2xx within 10 seconds.

See [references/overview.md](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](references/overview.md) - What Retell webhooks are, event types, payloads
• [Setup](references/setup.md) - Configure account-level and agent-level webhooks
• [Verification](references/verification.md) - Signature verification details and gotchas
• [Express Example](examples/express/) - Complete Express.js implementation
• [Next.js Example](examples/nextjs/) - Next.js App Router implementation
• [FastAPI Example](examples/fastapi/) - Python FastAPI implementation (uses the Retell SDK)

Recommended: webhook-handler-patterns

We recommend installing the webhook-handler-patterns skill alongside this one for handler sequence, idempotency, error handling, and retry logic. Key references (open on GitHub):

• Handler sequence — Verify first, parse second, handle idempotently third
• Idempotency — Dedupe Retell retries on event + call_id
• Error handling — Return codes, logging, dead letter queues
• Retry logic — Provider retry schedules, backoff patterns

Related Skills

• elevenlabs-webhooks - ElevenLabs voice AI webhook handling
• deepgram-webhooks - Deepgram transcription callback handling
• openai-webhooks - OpenAI webhook handling
• twilio-webhooks - Twilio voice and messaging webhook handling
• stripe-webhooks - Stripe payment webhook handling
• github-webhooks - GitHub repository webhook handling
• webhook-handler-patterns - Handler sequence, idempotency, error handling, retry logic
• hookdeck-event-gateway - Webhook infrastructure that replaces your queue — guaranteed delivery, automatic retries, replay, rate limiting, and observability for your webhook handlers