Send a short text message (iMessage/SMS) to the user's own phone, the number they verified on the iMessage or SMS integration. Sends right now by default; pass send_at to schedule it for later (a reminder for tomorrow morning, a nudge before a meeting). send_at must be ISO 8601 WITH the user's UTC offset, e.g. 2026-09-18T09:00:00-04:00: work out their timezone from the conversation or ask, never guess UTC. Scheduled texts can be listed with list_scheduled_texts and cancelled with cancel_scheduled_text. The recipient is always the user themself; no other number can be reached. If no verified number is on file, the response says so and carries the link to connect one. Keep messages short and concrete; there is a rate cap, so never use this for routine chatter. If a send returns setup instructions, relay them and wait for the user to complete that step before retrying. Never report a failed send as delivered.