Calls off a message you scheduled with scheduled_at, before it is released for sending. The message moves to CANCELLED, which is terminal, and it is never sent.

Only a SCHEDULED message can be cancelled. A message that has already been released — queued, in flight, or finished — returns 409. The release sweeper runs continuously, so a cancellation sent within about a minute of the scheduled instant can lose that race and be refused; cancel with time to spare.

One message per call. A send to many recipients, or on several channels, becomes one message per recipient × channel, each with its own id; cancel the ones you want stopped. There is no bulk cancellation.

Cancelling is free, and nothing was charged for the hold in the first place — a scheduled message is only priced and billed at release.

A cancellation fires message.cancelled on your webhook, exactly once, carrying the scheduled_at that was called off so a consumer can un-record the send it was expecting. Cancelling the same message again returns this same response and fires no second event. Idempotency-Key is honoured.

A cancelled message cannot be resent — send a new one.

The response is the message itself, in the same shape GET /v3/messages/{id} returns, so the status you read back is the one now stored on the row, and scheduled_at is the time it would have gone out. GET /v3/messages/{id} keeps reporting both afterwards.

events is the message lifecycle as it stood immediately before the cancellation: the CANCELLED entry is written asynchronously and lands a moment later, so read status for the outcome. If you need the entry itself, re-fetch the message after a few seconds — GET /v3/messages/{id} is cached briefly, so an immediate re-fetch can still show the pre-cancellation state.