Agent Skills: ElevenLabs Webhooks & Events

ElevenLabs Webhooks & Events

Overview

ElevenLabs webhooks send HTTP POST notifications when async operations complete: transcription completion, post-call data from Conversational AI agents, and call initiation failures. Every delivery is signed with an HMAC-SHA256 signature you must verify before processing. This skill builds a secure endpoint that verifies signatures, routes events by type, and acks fast to avoid auto-disable.

Prerequisites

• ElevenLabs account (webhooks configured in Settings > Webhooks)
• HTTPS endpoint accessible from the internet
• Webhook secret (generated during webhook creation in dashboard)

Instructions

The full, copy-ready code for each step lives in references/implementation.md; per-event handlers live in references/examples.md. The high-level workflow:

1. Know the event types — subscribe only to what you handle (table below).
2. Create the webhook in the dashboard (Settings > Webhooks) and copy the HMAC secret.
3. Verify the signature with HMAC-SHA256 over "<timestamp>.<raw_body>", using a timing-safe compare and a 5-minute replay window. See the full verifier.
4. Handle the request with a raw body parser, ack 200 immediately, then process asynchronously. See the Express handler.
5. Route events to per-type handlers. See handler examples.
6. Guard against duplicates with idempotency keyed on the event ID. See idempotency.
7. Test locally by tunneling with ngrok. See local testing.

Webhook event types

| Event Type | Payload | When Triggered | |------------|---------|----------------| | post_call_transcription | Full conversation transcript, analysis, metadata | After Conversational AI call ends | | post_call_audio | Base64-encoded call audio, minimal metadata | After call ends (if audio recording enabled) | | call_initiation_failure | Failure reason, metadata | When an outbound call fails to connect | | speech_to_text.completed | Transcription result, word timestamps | Async STT job completes |

Signature verification skeleton

// src/elevenlabs/webhook-verify.ts — Header: t=<unix_ts>,v1=<hex_sig>
export function verifyWebhookSignature(rawBody, signatureHeader, secret) {
  const parts = new Map(signatureHeader.split(",").map(p => {
    const [k, ...v] = p.split("="); return [k, v.join("=")];
  }));
  const timestamp = parts.get("t"), signature = parts.get("v1");
  if (Math.floor(Date.now() / 1000) - parseInt(timestamp) > 300) {
    return { valid: false, reason: "Timestamp too old" };   // replay guard
  }
  const expected = crypto.createHmac("sha256", secret)
    .update(`${timestamp}.${rawBody.toString()}`).digest("hex");
  return { valid: crypto.timingSafeEqual(
    Buffer.from(signature, "hex"), Buffer.from(expected, "hex")) };
}

See references/implementation.md for the production-hardened version with full error handling.

Output

Applying this skill produces:

• src/elevenlabs/webhook-verify.ts — reusable HMAC-SHA256 verifier with replay protection and timing-safe comparison.
• src/api/webhooks/elevenlabs.ts — Express route that verifies signatures, acks 200 immediately, and routes events to per-type handlers.
• Per-event handler functions (handleTranscription, handleCallAudio, handleCallFailure, handleSTTCompleted) extracting the fields each payload carries.
• An idempotency wrapper keyed on event ID so retried deliveries are processed once.

At runtime a verified delivery returns { "received": true } with HTTP 200; a bad signature or expired timestamp returns HTTP 401 { "error": "Invalid signature" }.

Webhook Reliability

| Behavior | Detail | |----------|--------| | Retry policy | ElevenLabs retries failed deliveries | | Auto-disable | After 10 consecutive failures AND 7+ days since last success | | Timeout | Your endpoint must respond within a few seconds | | Re-enable | Manually re-enable in dashboard after fixing the endpoint | | Authentication | HMAC-SHA256 via ElevenLabs-Signature header |

Error Handling

| Issue | Cause | Solution | |-------|-------|----------| | Signature mismatch | Wrong secret or body parsing | Use express.raw(), verify secret matches dashboard | | Webhook auto-disabled | 10+ consecutive failures | Fix endpoint, re-enable in dashboard | | Duplicate events | Retried delivery | Implement idempotency with event ID tracking | | Handler timeout | Slow processing | Return 200 immediately, process async | | Replay attack | Old timestamp reused | Check timestamp age (reject > 5 min) |

Examples

Route a decoded event to the right handler:

switch (event.type || event.event_type) {
  case "post_call_transcription": await handleTranscription(event); break;
  case "post_call_audio":         await handleCallAudio(event);     break;
  case "call_initiation_failure": await handleCallFailure(event);   break;
  case "speech_to_text.completed": await handleSTTCompleted(event); break;
  default: console.log("Unhandled event type:", event.type);
}

Simulate a delivery locally with curl:

curl -X POST http://localhost:3000/webhooks/elevenlabs \
  -H "Content-Type: application/json" \
  -H "ElevenLabs-Signature: t=$(date +%s),v1=test" \
  -d '{"type":"speech_to_text.completed","data":{"text":"Hello world"}}'

Full per-event handlers (transcript, audio, call-failure, STT) with the exact fields each payload carries are in references/examples.md.

Resources

• Post-Call Webhooks
• Webhook API Reference
• references/implementation.md — full verifier, Express handler, idempotency, ngrok testing
• references/examples.md — per-event handler examples

Next Steps

For performance optimization, see the elevenlabs-performance-tuning skill, which covers connection pooling and batching to keep webhook handlers fast enough to ack within the ElevenLabs timeout window.