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schedule.send_at is the earliest a scheduled message can go. delivery_conditions can hold it past that moment until the conditions are met, up to a deadline you choose.
Once the message is due, the evaluator checks the conditions every minute. While they hold it back the status stays scheduled, and each check is recorded in GET /v1/queue-executions as a constraint_check with result: "deferred" and the reasons in error_message, for example outside time window 09:00–17:00 (Europe/Brussels).

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Time windows

The zone is the condition’s own timezone, otherwise the message’s schedule.timezone, otherwise UTC. A window may cross midnight: "start": "22:00", "end": "06:00" covers the evening of a listed day and the small hours that follow it.

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The deadline

max_delay_at in the response is send_at + max_delay_minutes, the moment the wait ends. It is null when there is no max_delay_minutes. The evaluator recomputes it when you reschedule, and each occurrence of a recurring message gets its own. When max_delay_at passes and the conditions still do not hold:
  • on_miss: "send", or no on_miss at all: the message is sent at that tick regardless.
  • on_miss: "expire": the message becomes expired, a terminal status, with error_message starting expired: followed by the reasons, and nothing is sent. The execution log records a constraint_check with result: "failed".
A message that is expired cannot be rescheduled or sent; schedule a new one.

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Changing the conditions

PATCH /v1/scheduled-messages/{id} with a new delivery_conditions replaces the whole object; with "delivery_conditions": null it removes the hold. POST …/send-now removes the conditions and the deadline as well as making the message due, so it goes on the next tick.

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Presence conditions (reserved)

The contract also names a presence condition: deliver when the recipient is active on Slack.
When a presence condition is present, on_miss is required: say whether the message should send or expire if the target never becomes active before the deadline. Presence delivery is not enabled yet. The API validates the shape but answers 422 with Presence delivery not yet enabled on create and on PATCH; remove the condition to schedule the message. The shape is fixed now so that enabling it later changes nothing in what you send.