Messenger error 100: Attachment size exceeds allowable limit

Code 100 with subcode 2018109 is the size gate: the file behind your attachment is larger than the platform allows. Meta's Send API reference documents the ceiling in the attachment parameter itself, "maximum file size is 25MB," applying across the attachment types (audio, file, image, video). The legacy Send API error table filed this subcode under Limit Errors with the same one-line message the API still returns.

Twenty-five megabytes sounds generous until video enters the picture: a minute of phone-quality footage can exceed it easily, and the interaction with 2018294 makes the practical budget tighter than the nominal one, the platform also has to finish fetching a URL-referenced video within 75 seconds, so a file that squeaks under 25 MB on a slow origin can still die as a timeout. Files that arrive from users (support screenshots re-shared, documents forwarded into other channels) are the other repeat offender, since their sizes are outside your control.

The error is deterministic and cheap to prevent: check sizes before calling the API. A HEAD request's Content-Length on your own asset, or a stat on uploaded files, costs nothing; discovering the limit via a failed send costs a user-visible delay. When content genuinely exceeds the cap, the answer is transformation, compress, transcode, thumbnail-plus-link, rather than hoping: there is no override, and retrying an oversized file is a no-op.

{
  "error": {
    "message": "(#100) Attachment size exceeds allowable limit",
    "type": "OAuthException",
    "code": 100,
    "error_subcode": 2018109,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Put a size budget in the media pipeline with headroom below the hard cap, 20 MB is a sensible internal ceiling, and transform at ingestion, not at send time. Bots that generate media (receipts, reports) should render to constrained formats by construction. The 25 MB cap and its neighbors, the video fetch timeout and per-type behavior, are tabulated in the Messenger limits reference, with debugging steps in the troubleshooting guide.