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 first fetches your page's HTML and reads the OG meta tags. Then Slack-ImgProxy separately fetches the image URL found in og:image. This two-phase approach 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.
User-Agent Strings
Slack Bot identifies itself with the following user-agent strings:
Slackbot-LinkExpanding 1.0 (+https://api.slack.com/robots)Slack-ImgProxy 1.0 (+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 | Variable, hours to days |
Caching Behavior
Workspace-level caching. Slack caches unfurls for a variable period. There is no public tool to clear the cache, but re-posting the URL with a query parameter can force a fresh fetch.
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
- Slack uses two separate bots: Slackbot-LinkExpanding for HTML and Slack-ImgProxy for images. Both must be allowed by your server.
- Slack reads both OG and Twitter Card tags — one of the few platforms that supports both.
- 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.
robots.txt
Respects robots.txt. You need to allow both Slackbot-LinkExpanding (for HTML) and Slack-ImgProxy (for images). Blocking either will result in partial or no unfurling.
User-agent: Slackbot-LinkExpanding
Allow: /
User-agent: Slack-ImgProxy
Allow: /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 server allows requests from the Slack-ImgProxy user-agent.
- Append a query parameter (e.g., ?v=2) to force Slack to re-fetch the URL if the cached version is wrong.
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 server's robots.txt blocks Slackbot, the OG tags are missing or malformed, the og:image URL is inaccessible from Slack's servers, or your workspace admin has 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
Test Your OG Images
Check how your Open Graph images appear to bots and crawlers. Preview your link cards before sharing.
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