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WEBHOOK
fax.sending
{
  "id": "evt_8aZqRm4yT3vK7pNxJ2bH9c",
  "created": 123,
  "data": {
    "object": {
      "id": "<string>",
      "to": "<string>",
      "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"
    }
  }
}

Documentation Index

Fetch the complete documentation index at: https://mintfax.com/docs/llms.txt

Use this file to discover all available pages before exploring further.

​
When it fires

At the start of each delivery attempt against the destination. A fax that retries on a transient failure will fire fax.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.delivered or fax.failed.
The number of 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

​
Authorization
string
header
required

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.

​
id
string
required

Event identifier (evt_-prefixed). Stable across retry attempts.

Example:

"evt_8aZqRm4yT3vK7pNxJ2bH9c"

​
type
enum<string>
required

Event type. Always fax.sending for this delivery.

Available options:
fax.sending
​
created
integer
required

Unix epoch seconds when the event was generated.

​
data
object
required

Response

2XX

Acknowledged. mintfax treats any 2xx response as a successful delivery.

Last modified on May 14, 2026