Send one text or WhatsApp message as the business (sms:write). Email goes out through POST /v1/email-campaigns.

One send endpoint for texts and WhatsApp. sms goes out from the agent's own line (from_agent_id required) with the same gates and error codes as POST /v1/sms/send. whatsapp goes out from the business's number connected to WhatsApp (pass from when there is more than one) and only inside the 24-hour window after the customer last messaged that number; WhatsApp consent and Do-Not-Call are checked first. Email is not sent here: channel email is 400 use_email_campaigns. API email goes out as a campaign the business owner reviews and approves, through POST /v1/email-campaigns. Send Idempotency-Key (or idempotency_key) to make a retry return the original send (replayed: true) instead of sending again; the same key with a different message is 409 idempotency_conflict. New keyed WhatsApp sends claim a durable intent before dispatch; concurrent or uncertain replays never dispatch again. A 409 message_outcome_unknown requires reconciliation, not a new key. Pre-existing lost receipts cannot be recovered. 503 idempotency_unavailable means reservation or replay evidence could not be verified. A 200 means the provider accepted the message, not that it was delivered.