LINE error 403: Not enough recipients (narrowcast)

The narrowcast endpoint enforces privacy floors on audience size, and this 403 is what tripping the first of them looks like. The documented restrictions: "To specify attribute data as a condition of sending, your LINE Official Account's target reach number must be 100 people or more. Returns a 403 HTTP status code if your target reach is less than 100 people." Beyond that, "the final number of recipients must be 50 or more" when using attributes or audiences, and each audience combined into the condition must itself have at least 50 recipients - with the wrinkle that those two 50-floors don't reject your request up front: the API still returns 202, and "an error will occur when message delivery starts."

That split behavior is the debugging trap. A narrowcast can be accepted (202) and then die asynchronously minutes later; you only see the failure by polling the get-narrowcast-progress endpoint, where the phase turns to failed. So "narrowcast returns 403" and "narrowcast silently doesn't deliver" are usually the same family of problem at different thresholds: under 100 target reach fails fast, under 50 final recipients fails late.

These are policy floors designed to prevent de-anonymizing individuals through narrow targeting, so there is no override. Audiences created by uploading user IDs from your own account and chat tag audiences are documented exceptions to the per-audience floor - but the floors still apply to audiences created by other LINE Official Accounts.

HTTP/1.1 403 Forbidden

{
  "message": "Not enough recipients"
}

Encode the floors into your campaign tooling: refuse to submit attribute-targeted narrowcasts when target reach is under 100, warn when estimated recipients approach 50, and always store the requestId so the progress endpoint can be polled to completion. For developer and staging accounts, standardize on multicast for testing - narrowcast is untestable below the privacy thresholds by design. See sending caps and audience limits.