Supabase MCP Not Working?

Support Bug Reports
,
1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

I’m using the latest version of Cursor on macOS. When I try to connect the Supabase plugin, it returns:

HTTP ERROR 431
http://localhost:8787/callback?code=

Steps to Reproduce

Go to Cursor Settings, then MCP Servers → Supabase → Connect.

Expected Behavior

Establishing a connection with supabae

Operating System

MacOS

Version Information

Version: 3.10.20

Does this stop you from using Cursor

No - Cursor works, but with this issue

6

That HTTP ERROR 431 is coming from the local sign-in callback, not from Supabase. When Cursor finishes the OAuth handoff it opens a page at localhost:8787, and your browser sends every cookie it has stored for localhost along with that request. If those cookies have grown large (common when you run local dev servers on localhost), they go over the size the callback accepts, so it returns 431 before sign-in can finish.

To get unblocked:

  1. Clear cookies for localhost in your default browser (this only signs you out of things running locally).
  2. In Cursor, go to Settings → MCP Servers → Supabase → Connect and try again.

Alternatively, open the Supabase auth link in a private/incognito window and finish sign-in there, since that window starts with no localhost cookies. Let me know if it still trips up after that!

8

I have cleared all cookies from the localhost and everything worked. Thank you.