iMessage link previews: how Apple's Messages app uses Open Graph tags
Apple's Messages app (iMessage) generates rich link previews using og:title, og:description, and og:image, similar to other messaging platforms. The preview shows an inline bubble with the image at the top, title in bold, and domain name below. iMessage fetches the preview when the sender pastes the URL, not when the recipient views it.
How this is calculated
iMessage previews are smaller than other platforms: the image renders at roughly 220x115 in the message bubble. A 1200x630 image will be downscaled significantly, so don't put small text in your OG image and expect it to be readable in iMessage. Apple's LinkPresentation framework (which powers iMessage previews) also respects the website's favicon and displays it as a small icon in the corner of the preview bubble. There's no public debugger or cache-busting mechanism. The preview is generated when the sender pastes, and if it fails (slow server, missing tags), iMessage shows a plain text URL with no preview.
Verdict
iMessage previews are low-effort to support. Standard og: tags work. The image will be small, so keep it clean and bold. Test by pasting your URL in a Messages conversation with yourself.