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HTTP 409, or error.type in a message.failed event. The channel will not deliver outside its conversation window.

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What it means

On WhatsApp you may send free-form messages only within 24 hours of the contact’s last message. After that, WhatsApp accepts only approved templates; the send is refused with HTTP 409. On iMessage a line can reach only contacts who have messaged it (or opted in). The send is accepted, and the refusal arrives later as a message.failed event with outside_window. On SMS (US, in pilot), a sender does not message contacts who have not texted it in the last hour during its quiet hours (sender.sms.quiet_hours, by default 21:00 to 08:00), in the contact’s local time worked out from their area code. The send is refused with HTTP 409, channel_code quiet_hours, and retry_after in seconds until the quiet hours end. Typing and read receipts go to the channel at once, so they answer HTTP 409 outside_window directly when the channel’s window for them is closed: on iMessage, typing works only within 5 minutes of the contact’s last message, and a read receipt only once the contact has written; on WhatsApp, typing works only inside the 24-hour window, while read receipts go at any time.

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Why it happens

  • WhatsApp: the contact last wrote more than 24 hours ago.
  • WhatsApp: you are starting a conversation with someone who never wrote to the number.
  • iMessage: the contact has never messaged the line, or has not opted in to it.
  • iMessage: you turned typing on more than 5 minutes after the contact’s last message.
  • WhatsApp, in message.failed (channel_code window_closed): the window was open when you sent, but closed while the message waited in the queue (for example behind the sender’s pacing), so Flow did not send it.
  • In message.failed (channel_code queued_too_long): the message waited in the queue longer than its sender allows, for example while the sender was paused or its credentials were rejected, so Flow did not send it late.
  • SMS: it is inside the sender’s quiet hours where the contact lives (quiet_hours), and they have not texted in the last hour. When their area code does not tell their time zone, every US time zone must be outside the quiet hours.

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How to fix it

On WhatsApp, send an approved template (POST /v0/conversations/{conversation_id}/template, or POST /v0/messages with kind template to start), or wait for the contact to write again; their message reopens the window. The conversation’s window_open_until (GET /v0/conversations/{conversation_id}) says until when free-form messages go.
For window_closed, send the message again as an approved template, or wait for the contact to write. For queued_too_long, send it again if it still applies, once the sender is active again (sender.status_changed). On iMessage, ask the contact to message the line first (share its handle, or its opt-in link from the sender’s address.link when the line has one, for example in your app or website), then reply in the conversation their message opens. On SMS, retry after retry_after seconds (schedule the send for then; Flow does not hold it for you), or wait for the contact to text: replies within an hour of their message always go. From typing or a read receipt, ignore it and send your reply: the indicator is a courtesy, and the reply itself is not affected. Every error also carries hint, one sentence specific to your request, and doc_url, this page. Coding agents connected to Flow’s MCP server (https://api.flow.engineer/mcp) can call explain_error with the type to read this page.