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WhatsApp decides what you may send. Nodium tells you before Meta refuses.

On WhatsApp you may send free text — plain messages, buttons, lists — only within 24 hours of the customer's last message. Outside that window, only an approved template goes through. This is Meta's rule; Nodium enforces it before sending, so a message never looks sent when it was not.

• Every conversation carries window: open, expiresAt, minutesLeft. Read it before choosing between text and template.
• Free text outside the window is refused with window_closed (409). Send a template instead: POST /messages with type: "template".
• A template never reopens the window. Only a message from the customer does.
• Writing first to someone who never wrote to you is always outside the window: it must be a template.

Marketing consent

A customer can tap Stop under a marketing message. Nodium records it — also when Meta refuses a send for that reason (error 131050) — sets optOutAt on the contact, emits contact.opted_out, and refuses further marketing templates to them with consent_missing. Utility and authentication templates still go through. You can also set consent yourself with PATCH /contacts/{id}.