{
"id": "evt_8aZqRm4yT3vK7pNxJ2bH9c",
"type": "fax.sending",
"created": 123,
"data": {
"object": {
"id": "fax_8aZqRm4yT3vK7pNxJ2bH9c",
"status": "queued",
"to": "+12125551234",
"pages": 123,
"error_code": "<string>",
"error_subcode": "<string>",
"error_message": "<string>",
"max_attempts": 3,
"attempts": [
{
"attempt_number": 2,
"started_at": "2023-11-07T05:31:56Z",
"completed_at": "2023-11-07T05:31:56Z",
"duration_ms": 123,
"error_code": "<string>",
"error_subcode": "<string>",
"error_message": "<string>"
}
],
"webhook_url": "<string>",
"idempotency_key": "<string>",
"tags": {},
"csid": "<string>",
"submitted_at": "2023-11-07T05:31:56Z",
"completed_at": "2023-11-07T05:31:56Z",
"created_at": "2023-11-07T05:31:56Z",
"updated_at": "2023-11-07T05:31:56Z"
}
}
}Event types
fax.sending
Sent when a delivery attempt against the destination begins.
WEBHOOK
fax.sending
{
"id": "evt_8aZqRm4yT3vK7pNxJ2bH9c",
"type": "fax.sending",
"created": 123,
"data": {
"object": {
"id": "fax_8aZqRm4yT3vK7pNxJ2bH9c",
"status": "queued",
"to": "+12125551234",
"pages": 123,
"error_code": "<string>",
"error_subcode": "<string>",
"error_message": "<string>",
"max_attempts": 3,
"attempts": [
{
"attempt_number": 2,
"started_at": "2023-11-07T05:31:56Z",
"completed_at": "2023-11-07T05:31:56Z",
"duration_ms": 123,
"error_code": "<string>",
"error_subcode": "<string>",
"error_message": "<string>"
}
],
"webhook_url": "<string>",
"idempotency_key": "<string>",
"tags": {},
"csid": "<string>",
"submitted_at": "2023-11-07T05:31:56Z",
"completed_at": "2023-11-07T05:31:56Z",
"created_at": "2023-11-07T05:31:56Z",
"updated_at": "2023-11-07T05:31:56Z"
}
}
}When it fires
At the start of each delivery attempt against the destination. A fax that retries on a transient failure will firefax.sending again before the next attempt.
The current attempt is recorded in attempts[] with started_at set and completed_at null. Previous attempts (if any) are also present with their full detail.
What your handler should do
- Use this for in-flight visibility - “fax is currently being sent.”
- If you display fax status to your users, this is the event that moves the display from “queued” to “in progress.”
- Do not finalize success or failure on this event. Wait for
fax.deliveredorfax.failed.
fax.sending events for a single fax depends on how many delivery attempts run, which is bounded by max_attempts (1 to 5, default 3).Authorizations
API key as Bearer token. Use an account key (mfx_acct_...) for
/account/* endpoints, and an environment key (mfx_test_... for
sandbox, mfx_live_... for live) for /environment/*, /faxes,
/webhooks, and /events.
Body
application/json
Event delivery to your subscribed webhook endpoint.
Webhook delivery for fax.sending.
Event identifier (evt_-prefixed). Stable across retry attempts.
Example:
"evt_8aZqRm4yT3vK7pNxJ2bH9c"
Event type. Always fax.sending for this delivery.
Available options:
fax.sending Unix epoch seconds when the event was generated.
Show child attributes
Show child attributes
Response
2XX
Acknowledged. mintfax treats any 2xx response as a successful delivery.
Last modified on July 14, 2026