[bug] Can’t connect Chromatic-hosted Storybook MCP server to claude.ai as a custom connector

[samgold2020]

Summary

I’m trying to use our Chromatic-hosted Storybook MCP server as a custom connector inside the claude.ai web app, so my team can reference our Storybook components while working in Claude. The OAuth flow fails before I can sign in.

Steps to reproduce

1. In claude.ai web, go to Settings → Connectors → Add custom connector.
2. Enter a MCP URL
3. Click Connect.
4. Instead of redirecting to Chromatic to start the OAuth flow we get this error:

{"error":"invalid_request","error_description":"redirect_uri must be a loopback address or a pre-registered redirect URI"}

The flow stops here — I never reach the sign-in screen, so I can’t authorize the connector.

Expected Behavior:

The same flow that works in Claude Code CLI should work in claude.ai web: I get redirected to Chromatic, sign in, grant access, and the MCP server becomes available in Claude.

Extra context:

• Claude Code CLI connects to the same MCP server without any issue — the problem only happens when adding it as a custom connector in claude.ai web.
• Happy to share anything you need for debugging

cc - @austinlparker for additional info