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Paste a URL and see the link preview Tumblr will build from it.
Tumblr turns a link post into a dashboard card about 540px wide: the og:image full-width on top, a bold og:title below it, a short og:description, and the bare domain as a muted label. The image carries the card in the feed, so a picture that reads at dashboard width matters more than fine detail.
The crawler reads the standard Open Graph tags and does not execute JavaScript, so the tags must be present in the initial HTML response.
When a link post shows a bare URL instead of a card, the fetch failed rather than a tag being absent. Tumblr's crawler has been historically inconsistent with its user agent string, which has caused issues with prerender services and user-agent-based routing that serve it a different response. A login wall or an og:image it cannot reach has the same visible result.
The scan above requests the page the way Tumblr's crawler does, so a check that passes here is a fetch that works in a link post.
Set og:title, og:description, and og:image in the page head, server-rendered. Use an HTTPS image at 1200×630 when possible, keep it under 5 MB, and serve it without an avoidable redirect. Keep the title and the first line of the description self-contained — that is what people scan in the dashboard.
The scanner fetches both the page and its image with Tumblr's user agent, so it can show whether the tags are reachable before you publish the post.