OGPreview.io

Slack link preview checker

Slack “unfurls” links by reading the page’s Open Graph and Twitter tags. See the attachment Slack will build before you paste the link into a channel.

How Slack builds an unfurl

Slackbot-LinkExpanding fetches the page, reads og:site_name (or the domain) as the top line, og:title as the blue link, og:description as text and og:image as the picture, with twitter: tags and oEmbed as alternatives. It only reads the first part of the HTML, so keep your meta tags near the top of the <head>.

  • Slack caches unfurls for about 30 minutes per URL.
  • Admins can turn off unfurling for some domains, and users can remove a preview. If nothing appears for anyone, check the tags.
  • Pages behind a login unfurl as the login page. That’s expected.

Frequently asked questions

Why is my Slack link not unfurling?

Slack needs a publicly reachable page that answers quickly with og: or twitter: tags in the HTML. Slow pages, bot protection and tags injected by JavaScript are the usual causes.

How do I refresh a Slack preview?

Slack caches an unfurl for roughly 30 minutes. Wait, or add a harmless query string (?v=2) to the link to get a fresh one.

Does Slack use theme-color?

No. The bar on the left of a link unfurl is always grey. Discord is the app that colours it with theme-color.

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