You paste a link into a Slack channel, hit Enter — and nothing happens. No preview card, no thumbnail, just a bare URL sitting in the middle of a conversation. Slack is where most dev teams and SaaS founders share product pages every single day, and a missing preview makes your site look broken, unfinished, or worse: untrustworthy.
Slack's preview system works differently from Facebook's or Twitter's in ways that surprise people. It has no public debugger tool, it refuses to expand links that were shared in the same conversation within the past hour, and it gives up entirely on pages that don't include the right embedded data. Most of the "Slack preview not working" reports I see come from sites that have OG tags — but not the ones Slack actually needs, or images Slackbot can't reach.
In this guide, I'll cover how Slack's classic link unfurling reads your Open Graph tags, the exact image specs you need, and a step-by-step fix you can run in about ten minutes — verified with ogimgen's free tools.
Why Slack Shows No Link Preview: Root Causes
Slack's crawler fetches your page when a link is posted, reads the embedded preview data, and attaches an attachment. According to Slack's official help docs, the preview won't expand for several specific reasons. Here are the ones I see most often in the wild, roughly in order of frequency:
- The page has no preview data. If your HTML lacks Open Graph tags (or the right ones), Slack has nothing to build a card from and shows a bare link.
- The image is blocked or unreachable. Slackbot can't load an image behind a login, a bot-blocking robots.txt, or a hotlink protection rule — the text preview may still show, but the card looks broken.
- The same link was already shared recently. Slack deliberately skips expansion if the exact link was posted in the same conversation within the past hour. This one fools everyone: your preview "suddenly disappeared" because a teammate posted it before you.
- Too many links in one message. Messages containing more than five links don't get expanded at all. Slack silently drops previews to keep channels readable.
- Missing protocol. A bare
www.example.comwithouthttps://often won't expand. Always paste the full URL. - Private or password-protected content. Slack won't show previews for pages or files behind authentication, which is why password-protected videos and gated docs never expand.
- Audio/video from a non-allowlisted service. Slack only expands media links from services it has approved. Self-hosted or obscure hosts stay bare.
The good news: almost all of these are fixable on your side. The three that matter most for most sites are the missing OG tags, an unreachable image, and that confusing one-hour duplicate rule.
The OG Image Specs Slack Actually Needs
Slack's classic unfurling reads the standard Open Graph meta tags — og:title, og:description, and og:image. It falls back to Twitter Card tags if OG tags are missing, but a properly set og:image is the reliable path. The requirements are close to Facebook's, which is convenient since most sites already target that:
| Setting | Slack Requirement | Why It Matters |
|---|---|---|
| Recommended size | 1200×630 px | Same as Facebook; scales cleanly in Slack |
| Display crop | 1.91:1 landscape | Slack crops wide; keep text out of the edges |
| Minimum size | Around 200×200 px | Tiny images get dropped or look pixelated |
| Access | Public, no auth | Slackbot must fetch it without credentials |
| HTTPS | Required | HTTP images are rejected by Slack's fetcher |
| File size | Keep under 1 MB | Heavy images delay or fail the preview |
Two technical details trip up developers specifically. First, Slackbot must not be blocked by your robots.txt. Slack's crawler identifies itself as Slackbot-LinkExpanding, and image fetches come from Slack-ImgProxy. If your robots.txt or CDN rules block these agents, the text preview may render while the image silently disappears. Second, redirects can break image loading — use the final direct URL in your og:image tag, not one that 301s through an analytics or tracking layer.
The minimum meta tag set looks exactly like this:
<meta property="og:title" content="Your Page Title">
<meta property="og:description" content="A short, punchy description">
<meta property="og:image" content="https://yoursite.com/og-image.png">
<meta property="og:image:width" content="1200">
<meta property="og:image:height" content="630">
<meta property="og:url" content="https://yoursite.com/page">Slack pulls og:title and og:description for the text part of the card and og:image for the thumbnail. Skip og:image and you get a plain text card; skip all of them and you get a bare URL. A 1200×630 image generated with our free OG image generator covers all of it — the same file works for Slack, Facebook, LinkedIn, and Twitter.
How to Fix Slack Link Previews (Step by Step)
Slack has no public "scrape again" button, so you have to verify and bust the cache yourself. This is the sequence I use — it catches roughly 95% of Slack preview problems:
- Verify your tags first. Paste your URL into our free OG image checker. It shows exactly what a crawler sees — title, description, image, and dimensions. If the checker reports a missing or broken og:image, Slack will never show a thumbnail.
- Fix the underlying issue. Add or correct the og tags on your page, generate a proper 1200×630 image if yours is too small, and confirm the checker shows a valid card. Double-check that the image URL is public, HTTPS, and not behind a redirect.
- Check robots.txt and CDN rules. Make sure
Slackbot-LinkExpandingandSlack-ImgProxyaren't blocked. If you're using a bot-management service (Cloudflare Bot Fight Mode, DataDome, etc.), whitelist Slack's crawler or the image fetch gets challenged. - Bust the cache with a query parameter. Slack caches previews against the URL. Appending
?v=2(or any unique string) to the og:image URL makes Slackbot treat it as a new image. Update the tag, save, and re-verify with the checker. - Test in a fresh conversation. Post the link in a private channel or DM where it hasn't been shared in the last hour. If the preview is still missing, delete the message and post again — and make sure you're not pasting more than five links in one message, which disables expansion entirely.
- Wait out Slack's cache. Even with a new URL, Slack may keep an old preview for a while. If your checker shows a clean card, the preview usually catches up within a few hours.
If you run a Slack workspace and need to inspect what Slack is actually pulling from a URL, Slack's help docs mention a URL debugging tool available to workspace admins — run a link through it to see the exact preview metadata Slackbot receives. That's the closest thing Slack has to Facebook's Sharing Debugger.
Slack vs. Other Platforms: What Changes
If your OG setup already works on Facebook, it will almost certainly work on Slack — but the debugging experience is very different:
| Aspect | Slack | Twitter / X | |
|---|---|---|---|
| Image crop | 1.91:1 landscape | 1.91:1 landscape | 1.91:1 or 2:1 |
| Public debugger | None (admin tool only) | Sharing Debugger | Card Validator |
| Cache refresh | URL query param | "Scrape Again" button | Validator rescrape |
| Duplicate suppression | Same link within 1 hour | None | None |
| Max links per message | 5 | N/A | N/A |
| Crawler UA | Slackbot-LinkExpanding | facebookexternalhit | Twitterbot |
Beyond classic unfurling, Slack also offers app unfurling: if you own the domain, you can build a Slack app that listens for link_shared events and calls the chat.unfurl API to render a fully custom card with interactive blocks. It's the right solution for SaaS products that want rich, real-time previews of their own links — but for a normal website, correct OG tags are 100% of the job.
Summary
Slack link previews fail for four main reasons: missing Open Graph data, an unreachable or too-small image, a duplicate share within the past hour, or more than five links in one message. The fix is straightforward — publish a 1200×630 OG image with a public, HTTPS, redirect-free URL, make sure Slackbot isn't blocked, and append a query parameter to bust the cached preview.
Because Slack is where product links get shared with teammates, investors, and customers, a clean branded card makes your site look professional in every conversation — and a bare URL quietly reads as "this project isn't maintained." Generate your OG image with ogimgen in about 30 seconds, verify the tags with the OG checker, and test in a private Slack channel before you share it anywhere public.