Slack generates “unfurl” previews when someone pastes a URL in a channel, reading Open Graph and Twitter Card meta tags to build an inline card with a title, description, and image. A key gotcha is that Slackbot only fetches the first 32KB of your page – so your meta tags need to appear early in the HTML.
This guide covers how Slackbot crawls pages, which tags it reads (including oEmbed and the bonus twitter:label/twitter:data fields), image sizing, the 30-minute global cache, and the most common issues that prevent unfurls from appearing.