For the complete documentation index, see llms.txt. Markdown variants are available by appending .md to any URL or sending an Accept: text/markdown header. An agent skill is available at /.well-known/agent-skills/site-skill.md.
Paste a URL and see the tile Reddit will build from it in the feed.
A Reddit link post is mostly the thumbnail and the domain. The headline above the tile is whatever the person submitting typed, not your og:title, so the one piece of the card you control on the feed is the image pulled from og:image.
That makes Reddit the platform where the image has to carry the message on its own. In the feed it is shown small and cropped, so fine print in a corner disappears; on the post page it opens wider and the full frame is visible.
When a post shows the grey placeholder tile instead of your image, the fetch failed rather than the tag being absent. Reddit's crawler needs to reach the image without a login, without a redirect chain, and without being served a different response by user agent. The scan above requests the page the same way, so a check that passes here is a fetch that works there.
Some subreddits also strip thumbnails or disallow link posts altogether, and a link crossposted from an old submission keeps the thumbnail captured at the time it was first submitted.