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

You added your Linkilo connection address to an AI assistant, clicked Connect, and got one of:

• Claude: “Couldn’t register with the plugin’s sign-in service. You can try again, or add an OAuth Client ID in the connector settings.”
• ChatGPT: “Couldn’t discover OAuth settings.”

Both mean the assistant could not read your site’s sign-in settings. From Linkilo 3.0.2 the MCP Access page diagnoses this for you. Check the causes in this order.

Cause 1 — Access is turned off

MCP access is off by default. Go to Linkilo → Manage → MCP Access, tick Allow AI assistants to connect, click Turn on, then delete the connector in the AI app and add it again. While access is off, the approval page itself says “MCP access is turned off. Enable it under Linkilo → MCP Access first.”

Cause 2 — You’re on Linkilo 3.0.1 or older

The sign-in flow exists from Linkilo 2.9, but the fixes for blocked hosts (the sign-in check, the relay and the backup addresses) shipped in 3.0.2. Update the plugin, then retry.

Cause 3 — Your host blocks the discovery addresses

Every assistant reads a site’s sign-in settings from an address containing /.well-known/. Some hosts, BigScoots among them, answer those addresses with their own 404 before WordPress runs, so the lookup fails no matter what the plugin does.

Run the Sign-in check on the MCP Access page. It tests the addresses from the outside and shows one of:

• Ready — not this cause; look at the other causes.
• Ready (through Linkilo’s sign-in relay) — already handled. Linkilo detected the block and sends assistants to its relay at linkilo.co, which points them back to your site. You still approve on your own site and the access key never passes through the relay. Delete and re-add the connector and it connects.
• Claude ready · ChatGPT needs one extra step — Claude, Claude Code and other apps on the official MCP kit fall back to backup addresses at the root of your site (/register, /authorize, /token), which Linkilo now answers (only for real sign-in requests, so a page of your own at /register or an SEO plugin’s redirect keeps working). ChatGPT does not fall back; fill in its Advanced OAuth settings with the values listed on the setup screen (Registration method: Dynamic Client Registration; Token endpoint auth method: none; Default scopes: linkilo; the Auth, Token and Registration URLs; PKCE S256 ticked; OIDC off), or fix the block.
• Blocked by your web host — set How assistants find your sign-in settings to Automatic (the default) or Always through Linkilo’s sign-in relay, click Save, then Check again. That is the easiest fix and needs nothing from your host.

If you would rather fix the block at the source, the setup screen gives you both forms of the fix with copy buttons:

• Cloudflare: Rules → URL Rewrite Rule. Filter expression: (http.request.uri.path contains "/.well-known/oauth-") or (http.request.uri.path contains "/.well-known/openid-configuration"). Path: rewrite to Static /index.php. Query: rewrite to Dynamic concat("linkilo_wk=", http.request.uri.path).
• Any other host: ask them to let /.well-known/oauth-authorization-server/*, /.well-known/oauth-protected-resource/* and /.well-known/openid-configuration/* fall through to WordPress (try_files $uri /index.php?$args). They are standard OAuth metadata addresses (RFC 8414 and 9728) served by the plugin.

After either fix, click Check again, then set the discovery setting to Always from my site if you prefer not to use the relay, and re-add the connector.

Still failing? Read the Sign-in activity

Sign-in activity on the MCP Access page logs each step the assistant took. A healthy connection reads: read the connection details → read the sign-in settings → registered or identified → waiting for your approval → you approved → connected. The last line of a failed attempt shows where it stopped, which is the detail to include if you contact support.

Things that are NOT the cause

• Pasting a key into the URL. Web connectors never use a key. Paste only the connection address exactly as the setup screen shows it, and leave the OAuth Client ID and Secret fields empty.
• Wordfence or other security plugins. The sign-in traffic goes through the normal WordPress REST API; these plugins do not block it.
• Your AI provider key. MCP never uses the key saved in AI Settings and never spends provider credits.
• Claude’s newer sign-in method. Claude now identifies itself with a published identity document instead of registering first. Linkilo 3.0.2 supports both, so this is not a cause.

Need it working right now?

Claude Code signs in through the browser and falls back to the backup addresses, so it connects even on a blocked host. Any client can also use a key, which skips discovery entirely. See How to Connect Claude Code and Desktop MCP Clients.

Last reviewed for Linkilo 3.0.2 (October 2026).