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Troubleshooting guide

Slack link preview not showing?

Slack calls a link preview an unfurl. When someone posts your link, Slack fetches the top of the page and reads its Open Graph, Twitter Card or oEmbed tags. Here’s what to check, based on Slack’s own documentation.

The check opens on the Slack preview and flags missing tags and images that can’t be loaded.

What Slack needs

Tags
Open Graph, Twitter Card or oEmbed
Fetch
As little of the page as possible, using HTTP Range requests
Crawler
Slackbot-LinkExpanding 1.0, ignores robots.txt
Cache
About 30 minutes, across all of Slack
From Slack’s crawler docs ↗

Common causes and fixes

  1. 1

    The tags are missing or added by JavaScript

    Slack reads the HTML your server sends. Tags that a JavaScript framework adds after the page loads are never seen.

    Fix: Check your page to see which tags the server sends, then copy the generated code into your <head>.

  2. 2

    The tags sit too far down the page

    Slack downloads as little of the page as it can, so tags placed after large inline scripts or styles may never be read.

    Fix: Move the meta tags to the top of the <head>, before any inline code.

  3. 3

    Bot protection blocks Slack

    Slack ignores robots.txt, so that file isn’t the cause. Firewalls and bot protection can still block the Slackbot-LinkExpanding user agent.

    Fix: Let Slackbot-LinkExpanding through your bot protection.

  4. 4

    The image can’t be loaded

    Slack also fetches the image to check that it’s valid. It has to be a public https address, written out in full.

    Fix: Use an absolute https:// address for og:image and open it in a private window to confirm it’s public.

  5. 5

    Slack is showing an old version

    Slack caches each preview for about 30 minutes, and messages already posted keep the preview they had.

    Fix: Wait half an hour, or post the link with a query string such as ?v=2 to test your fix.

  6. 6

    Previews are turned off for that person or message

    Each person can turn off link previews in their Slack preferences, and the sender can remove the preview from a message.

    Fix: Nothing to fix on your page. Compare with someone who has previews on.

Still seeing the old preview?

Slack caches previews for about 30 minutes. After a fix, wait that long or post the link with a query string such as ?v=2. Messages already posted keep their old preview.

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