Fix “Couldn’t Register with the Sign-In Service” When Connecting MCP

You added your Linkilo MCP endpoint to claude.ai (or ChatGPT), clicked Connect, and got:

"Couldn't register with the plugin's sign-in service. You can try again, or add an OAuth Client ID in the connector settings."

This error means the AI app couldn't complete the automatic sign-in discovery with your site. It's almost always one of the three causes below — check them in this order.

Cause 1 — MCP access is turned off

MCP is off by default in Linkilo. Go to Linkilo → MCP Access, tick Enable MCP access, Save, then delete the connector in the AI app and add it again.

Cause 2 — You're on Linkilo 2.8 or older

The sign-in (OAuth) flow that web connectors require shipped in Linkilo 2.9. Update the plugin, then retry.

Cause 3 — Your host blocks the discovery URLs (nginx and ".well-known")

Web connectors look up your site's sign-in details at URLs containing /.well-known/. On some managed hosts running nginx (BigScoots, Kinsta, and similar), the server answers those URLs itself with a 404 — WordPress never runs, so the lookup fails no matter what the plugin does.

The fix is a one-line change only your host can make. Send them this:

A plugin on my site serves OAuth discovery endpoints, but nginx currently returns 404 for URLs containing /.well-known before WordPress can handle them. Please make these fall through to WordPress (try_files $uri /index.php?$args): /.well-known/oauth-authorization-server, /.well-known/oauth-protected-resource, and /wp-json//.well-known/. These are OAuth metadata URLs (RFC 8414/9728) served dynamically by the plugin.

After the host confirms, delete the connector in the AI app, re-add the URL, and Connect again. (Apache hosts don't have this problem — WordPress handles those URLs out of the box.)

Things that are NOT the cause

• Pasting a token into the URL. Web connectors never use a token — paste only the endpoint URL, exactly as shown in Linkilo, and leave the OAuth Client ID/Secret fields empty.
• Wordfence or other security plugins — they don't block this flow; the sign-in traffic goes through the normal WordPress REST API.

Need it working right now?

Claude Code and other desktop clients skip discovery entirely and connect with a token immediately — see How to Connect Claude Code and Desktop MCP Clients with a Token.