Overview
When you paste a link in Slack, it "unfurls" into a rich preview — an image, title, description, and a colored sidebar. This is generated by Slackbot's link expansion system, which uses two separate crawlers working in sequence.
Slackbot-LinkExpanding fetches the smallest useful part of your page with HTTP Range headers and reads oEmbed, Twitter Card, and Open Graph tags. Slack-ImgProxy separately fetches and caches image URLs. This two-request path means your HTML and image need to be independently accessible.
Slack is widely used in professional environments, making it one of the most important platforms for link previews — especially for SaaS products, documentation, blog posts, and internal tools.
Slack Bot User-Agent Strings
Slack Bot identifies itself with the following user-agent strings:
Slackbot-LinkExpanding 1.0 (+https://api.slack.com/robots)Slack-ImgProxy 0.19 (+https://api.slack.com/robots)Slackbot 1.0 (+https://api.slack.com/robots)Slack Bot robots.txt
Slack's robots page says Slack does not currently honor robots.txt for link unfurling. Use server rules, bot protection settings, or contact Slack if you need to block or allow Slack embeds.
# robots.txt is not the control point for Slack unfurls.
# If you block bots at your CDN or firewall, allow these user agents there instead:
Slackbot-LinkExpanding 1.0 (+https://api.slack.com/robots)
Slack-ImgProxy 0.19 (+https://api.slack.com/robots)How Slack Bot Handles OG Images
Slack uses a two-phase fetch: one bot reads OG tags from HTML, another fetches the image. Both must succeed for a complete preview.
Meta Tags Read
og:imageog:titleog:descriptionog:site_nametwitter:imagetwitter:titletwitter:description| Preferred Size | 1200 × 630 |
| Minimum Size | No strict minimum |
| Cache Duration | ~30 minutes for the same URL |
Caching Behavior
Slack says LinkExpanding responses are cached globally around 30 minutes for the same URL. There is no public cache-clear tool, so changing the page URL or image URL is the practical clean test.
Fallback Behavior
Without og:image, Slack shows a text-only unfurl with the title and description. If neither OG nor Twitter tags are found, Slack attempts to use the page's <title> tag.
Things to Know
- Slackbot-LinkExpanding may use HTTP Range requests, so servers need to return enough head metadata in partial responses.
- Slack reads oEmbed, Open Graph, and Twitter Card tags for classic unfurls.
- Slack currently says it does not honor robots.txt for link unfurling because it is acting on behalf of a human sharing a link.
- Workspace admins can disable link unfurling entirely or for specific domains.
- Private/internal URLs that require authentication will not unfurl unless the workspace has an app integration configured for that domain.
How to Test
Slack has no public debugger tool. Test by pasting your URL in a Slack channel or DM. Check both that the text preview and image appear correctly.
- Use a test Slack workspace or DM to yourself to preview unfurls without notifying teammates.
- If only the text appears but no image, check that your CDN or firewall allows requests from the Slack-ImgProxy user-agent.
- If the cached version is wrong, wait for Slack's cache window or change the page URL or og:image URL with a version query.
You can also use the MyOG OG Preview tool to check how your OG tags are configured before testing with Slack Bot.
FAQ
What is Slackbot-LinkExpanding?
Slackbot-LinkExpanding is one of Slack's two crawlers for generating link previews. It fetches the HTML of a shared URL and reads Open Graph meta tags. The second crawler, Slack-ImgProxy, separately fetches the og:image URL. Together they produce the rich preview (unfurl) shown in Slack channels.
Why isn't my link unfurling in Slack?
Common causes: your CDN or firewall blocks Slack's user agents, the OG tags are missing or malformed, the og:image URL is inaccessible from Slack's servers, Slack cached an older fetch, or your workspace admin disabled unfurling for your domain. Check your server logs for requests from Slackbot-LinkExpanding and Slack-ImgProxy.
Does Slack read Twitter Card tags?
Yes. Slack is one of the few platforms that reads both Open Graph and Twitter Card meta tags. If you have both, Slack will use the available metadata from either source to generate a preview.
Related Bots
Check what crawlers see
Paste your URL to test OG tags, image fetches, crawler access, and the preview card before you share it.
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