Developer Documentation
Receive Delivery Events with Signed Webhooks
Webhooks let Alert21 notify your server when a message changes state so you do not need to continuously poll the API.
{
"event": "sms.delivered",
"message_id": "AL21SMS260824001234",
"to": "919876543210",
"status": "delivered",
"delivered_at": "2026-08-24T14:00:05+05:30"
}Example Delivery Event
{
"event": "sms.delivered",
"message_id": "AL21SMS260824001234",
"to": "919876543210",
"status": "delivered",
"delivered_at": "2026-08-24T14:00:05+05:30"
}Example signature header:
X-Alert21-Signature: <signature>Webhook Handler Rules
- Use HTTPS.
- Verify the signature before trusting the payload.
- Return a 2xx response quickly after durable acceptance.
- Process heavy work asynchronously.
- Make handlers idempotent because an event can be retried.
- Do not rely on arrival order; use event/message timestamps and state logic.
- Log the Alert21 event ID/message ID, not secrets.
Retry Policy
Alert21 should document its retry schedule before public launch. A recommended design is bounded exponential backoff with a maximum retry window, webhook-attempt visibility and a manual replay function in the customer panel.
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