Messenger error 10: Message sent outside of allowed window

Code 10 with subcode 2018278 is the Messenger Platform's central rule being enforced: a Page may send messages only within the standard messaging window, the 24-hour period that starts each time the person messages your Page or takes an equivalent action. Your token, permissions and payload are all fine; the timing is not. Meta's error-codes page lists the message as "This message is sent outside of allowed window. Learn more about the new policy here," a string dating from the 2020 policy change that shortened the window to a flat 24 hours.

The window is more generous than most developers assume. It opens, and re-opens, when a person sends your Page a message, clicks a call-to-action like Get Started, messages you from a Click-to-Messenger ad, uses the Send to Messenger or checkbox plugin, opens an existing conversation from an m.me link (the kind you can build with the Messenger link generator), reacts to a message, comments on a Page post, or publishes a visitor post. Every one of those actions resets the clock; an active conversation never expires. It closes quietly 24 hours after the last such action.

Watch out: code 10 is heavily overloaded. Without a subcode it is a plain permission error, and with subcode 2534022 or 2018065 it is still the window but under a different rule. Read the subcode before routing the failure. Outside the window your sanctioned options are limited: the HUMAN_AGENT tag for a human reply within 7 days (App Review required), one-time notifications the person opted into, or sponsored messages. The old tag escape hatches are mostly gone; since April 27, 2026, CONFIRMED_EVENT_UPDATE, ACCOUNT_UPDATE and POST_PURCHASE_UPDATE fail with error code 100. The full window mechanics are tabulated in the Messenger limits reference.

{
  "error": {
    "message": "(#10) This message is sent outside of allowed window. Learn more about the new policy here: https://developers.facebook.com/docs/messenger-platform/policy-overview",
    "type": "OAuthException",
    "code": 10,
    "error_subcode": 2018278,
    "fbtrace_id": "Fz54k3GZrio"
  }
}

Gate every outbound send on a window check: latest qualifying inbound timestamp plus 24 hours, evaluated immediately before the API call, with the tagged or opted-in fallback chosen automatically when the window is closed. Design reminder-style flows on one-time notifications from day one instead of retrofitting. Keep the conversation alive legitimately, quick replies and menus invite interactions that reset the clock. The window rules sit alongside every other cap in the limits table, and the troubleshooting guide walks the symptom backwards.