Mermaid diagrams on GitHub have stopped rendering

[benjikj-cera]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Other

Body

Mermaid diagrams have stopped rendering for me on the GitHub platform. It tries to load for a while and then displays the message "Unable to render rich display" and the mermaid code below. The same diagrams rendered correctly a couple of weeks ago, but have now just stopped.

Is this a bug, or has GitHub stopped supporting mermaid?

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[LuShadowX]

Mermaid support hasn't been removed. GitHub renders these diagrams in a sandboxed iframe served from a different host than github.com:

https://viewscreen.githubusercontent.com/markdown/mermaid

That's still embedded in file pages right now, and the endpoint is up — I get HTTP 200 in about a second. "Unable to render rich display" is what you see when that iframe fails, not a sign the feature is gone.

The detail in your description that matters is "it tries to load for a while and then displays the message." A mermaid syntax error fails almost immediately. A long spin followed by that message means the iframe never loaded at all — so the first thing to check is whether something is blocking viewscreen.githubusercontent.com, not your diagram.

In order:

1. Open https://viewscreen.githubusercontent.com/markdown/mermaid directly in a browser tab. You should get a page, not a hang or a proxy error. If it hangs, that's your answer.

2. Open DevTools → Network, reload a page with a diagram, filter for viewscreen. A blocked, failed or endlessly pending request confirms it.

3. Try an incognito window with extensions disabled, then a different network — a phone hotspot is the quickest test. If the diagram renders on the hotspot, it's your normal network or a browser extension.

4. Only if all of that is clean, test the diagram. Put a trivial one in a gist:

   ```mermaid
   graph TD
     A-->B
   ```
   

   If that renders and yours doesn't, paste yours into https://mermaid.live — it shows the parser error that GitHub swallows behind the generic message.

Given "the same diagrams rendered correctly a couple of weeks ago" with no change on your side, and that they're your own unchanged diagrams, network or extension blocking is where I'd put money. Corporate networks that allowlist github.com but not githubusercontent.com are a common cause, and it's easy to confirm: if that domain is blocked, raw file links like https://raw.githubusercontent.com/... and image attachments in issues will also fail for you. Both being broken points at the network; only diagrams being broken points at the diagram.

Reference: Creating diagrams.

[benjikj-cera]

Failed on step 1. I get "This page is blocked. Your organization doesn’t allow you to view this site" when opening that page. So guess that's it. I'll follow up with my org admins to see what and why they've changed it. Thank you so much!

[LuShadowX]

happy to help!😁