Slack Link Preview Checker
Slack's classic unfurl is a crawl of the page you pasted. Check the title, description, and image before the link goes into a channel.
What the slack link preview checker checks
- Open Graph, then Twitter tags. With no Slack app registered for the domain, Slack crawls the URL and reads Open Graph and X (Twitter) Card tags. The unfurl is built from og:title, og:description, og:image, and og:site_name. The same crawler looks for twitter:title, twitter:description, and twitter:image, so a page with only Twitter tags can still unfurl — set the og: tags anyway. This checker does not treat twitter:image as a stand-in for a missing og:image.
- Why the unfurl is blank. Slackbot-LinkExpanding fetches a byte range of the HTML and reads the meta tags in that slice, and it does not run JavaScript. Put the tags at the top of the document the server sends. og:image has to be an absolute URL the crawler can fetch; a path with no host produces no image. A login wall, or a firewall blocking that user agent, leaves nothing to unfurl. Slack does not consult robots.txt. A link whose label is the URL without the protocol is skipped on purpose.
- Stale cards. Slack caches the crawl for around 30 minutes, for the same URL, across the service. There is no public Slack debugger. Edit the tags, then share a URL Slack has not cached — add a query string such as ?v=2 — or wait for that cache to expire.
Tags covered: og:title · og:description · og:image · og:site_name · twitter:card · twitter:title · twitter:image
More checkers
Twitter Card ValidatorOpen Graph CheckerLinkedIn Link Preview CheckerDiscord Embed PreviewWhatsApp Link Preview CheckeriMessage Link Preview CheckerFacebook Link Preview CheckerOpen Graph Image SizeAll platforms
What each platform reads
- X / Twittertwitter:card, twitter:* → og:* fallback1200×630 (min 300×157), < 5 MB
- Facebookog:title, og:description, og:image1200×630, 1.91:1
- LinkedInog:title, og:image (og: only)1200×627, 1.91:1
- Discordog:site_name, og:*, theme-color, twitter:cardLarge image needs summary_large_image
- Slackog:site_name, og:title, og:description, og:image1200×630
- WhatsAppog:title, og:description, og:imageKeep < 300 KB
Guidance based on each platform's public documentation and observed behavior. Platforms change their rules without notice.
FAQ
+Is Peekly free?
Yes. Checking a single URL is free forever — no signup and no limit on one-off checks. Batch watching and monitoring may arrive later as a paid Pro plan; the free single-URL tool stays.
+Why does my link show no image when I share it?
Usually one of four things: there's no og:image tag, the image URL is relative instead of absolute, the image is behind a login, firewall or bot block, or the file is too big or in an unsupported format like SVG. Peekly checks all four and tells you which one it is.
+What size should my og:image be?
1200×630 pixels (a 1.91:1 ratio) as a PNG or JPG under about 300 KB. That size works for large cards on X, Facebook, LinkedIn, Discord, Slack and WhatsApp.
+Do I need twitter: tags if I already have Open Graph tags?
X reads og:title, og:description and og:image as fallbacks, but you still need twitter:card set to summary_large_image if you want the big image card. Without it, X shows a small thumbnail.
+I fixed my tags but the old preview still shows. Why?
Platforms cache link previews, sometimes for days. Facebook and LinkedIn let you force a refresh with their own debugging tools. On X, Slack and Discord the cache usually expires on its own, or you can add a query string such as ?v=2 to the URL.
+Do you store the pages I check?
No. We fetch the page once to read its <head>, show you the result, and don't save the page or the result. We only fetch public http(s) URLs, and private or internal addresses are blocked.
+Are the previews pixel-perfect?
They're close approximations of each platform's current card layout, built from the same tags the platforms read. Platforms change their designs from time to time, so treat them as a very good guide rather than a screenshot.
$9
One time
A written fix for the batch.
Up to 25 URLs. We check og:image, Twitter cards, size, redirects, and cache, then send copy-paste meta tags within 48 hours. The free single-URL check stays free.