Messenger error 100: Invalid attachment ID or not owned

Code 100 with subcode 2018074 concerns reusable attachments: "Possible invalid ID or you do not own the attachment," with Meta's table pointing to the Attachment Upload API documentation. When you upload media once and receive an attachment_id, that ID is a reference into Meta's storage, and it is scoped: it belongs to the Page (and app context) that uploaded it. Sending with an ID that does not exist, was mistyped, or was minted by a different Page or app produces this error.

The scoping is what surprises multi-tenant platforms. A bot serving many Pages cannot upload a shared image once under Page A and reference it from Page B; each Page needs its own upload and its own ID. Migrations trip the same wire: move the Page to a new app, or rebuild the integration, and a warehouse of stored attachment IDs can silently become someone else's property from the API's point of view. IDs can also age out, Meta does not document a lifetime, but treating them as cache entries rather than permanent assets matches observed behavior.

Data-handling bugs supply the rest: IDs stored as numbers and mangled by precision loss, IDs confused with message IDs or asset URLs in the database schema, and copy-paste of example IDs from documentation. The correct mental model is: attachment_id is a per-Page cache key you must be able to regenerate, never the only copy of the asset.

{
  "error": {
    "message": "(#100) Possible invalid ID or you do not own the attachment.",
    "type": "OAuthException",
    "code": 100,
    "error_subcode": 2018074,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Design attachment IDs as a regenerable cache: keep the canonical asset in your own storage, key cached IDs by Page, and implement the miss path (upload, store, send) once so every send site inherits it. That single pattern absorbs invalidation, migrations and multi-tenancy. The upload API's role among the media rules is shown in the Messenger limits reference, alongside the URL-fetch failure it helps you avoid.