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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 /inbox/conversations/{id}/template.
• A template never reopens the window. Only a message from the customer does.
• Writing first to someone (POST /inbox/conversations) is always outside the window: it must be a template.

Marketing consent

A customer can tap Stop under a marketing message. Nodium records it (optOutAt on the contact, contact.opted_out webhook) 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} and optIn.