Instagram error 100: Failed to fetch the file from the URL

When you send media by URL, Meta's servers fetch the file from your URL before delivering the message - and subcode 2018008 means that fetch failed. The documented message enumerates the checklist itself: the URL must be valid and publicly reachable, the SSL certificate must be valid, the file size must be within limits, and your server must respond "fast enough to avoid timeouts."

The failure is on the hosting side of the transaction, not in your API call. Classic culprits: URLs behind authentication or IP allowlists (Meta's fetchers are not your office IP), self-signed or expired TLS certificates, redirects to protected locations, hotlink protection, and slow dynamic endpoints that generate the asset on demand. Signed URLs with short expiry windows are a subtle variant - valid when you built the payload, expired by the time Meta fetched. The related timeout for video specifically is 2018294, and size violations surface as 2018109.

Treat media hosting as part of the integration: public CDN URLs, monitored certificates, and a pre-send validation that fetches each asset once from outside your network. For frequently reused media, upload once and reuse the attachment ID, removing the URL fetch from the hot path entirely. The documented media formats and size ceilings (8 MB images, 25 MB audio/video) are in the limits reference, and the sibling media errors are 2018109 and 2018294.