# ChatGPT shows “Unknown OAuth client.” on a site with Rank Math

> Rank Math and some other plugins include a built-in MCP sign-in that can answer your site's sign-in addresses, so ChatGPT shows "Unknown OAuth client." or Claude connects to the wrong server. Here is how to fix it with Royal MCP 1.5.12 or later.

- Canonical: <https://royalplugins.com/support/royal-mcp/rank-math-sign-in/>
- Last updated: 2026-11-05
- HTML version: <https://royalplugins.com/support/royal-mcp/rank-math-sign-in/>

---

Recent versions of Rank Math SEO include a built-in MCP sign-in, and a few other plugins include the same one. It switches on by itself, with no setting, on sites where the WordPress Abilities API and the MCP Adapter are available, and it answers the standard sign-in addresses that AI clients read. When it answers instead of Royal MCP, ChatGPT stops with “Unknown OAuth client.” and Claude may connect to that plugin instead of to Royal MCP. This guide shows how to recognise the clash, fix it by updating Royal MCP and reconnecting, and, if you do not use it, switch the bundled sign-in off.

## What you will see

- **ChatGPT** stops on a page that says “Unknown OAuth client.” when you add the connector or try to sign in.
- **Claude** may connect without an error, but the tool list shows another plugin’s tools rather than Royal MCP’s, whose names look like `wp_get_posts` and `wp_update_post`.
- **The sign-in page** the client opens belongs to a plugin you did not mean to connect, or says “OAuth Error”.
- **In WordPress**, Royal MCP shows the notice “Royal MCP: another plugin is answering OAuth sign-in for this site.” and names the plugin, often with the words “which has a built-in MCP sign-in”.

Nothing is wrong with your host, your AI client or Royal MCP’s settings. Two plugins are answering the same sign-in address, and the AI client followed the other one.

## Why it happens

An AI client finds out how to sign in by reading standard addresses on your site, such as `/.well-known/oauth-authorization-server`. Only one plugin can answer each address. The built-in MCP sign-in in Rank Math, and in other plugins that include the same sign-in library, claims those site-root addresses and only recognises the AI clients it was built for. ChatGPT is not one of them, so it is turned away with “Unknown OAuth client.”

Royal MCP 1.5.12 and later sign AI clients in at Royal MCP’s own address under `/wp-json/royal-mcp/v1`, which the other plugin never answers. When another plugin owns the site-root addresses, Royal MCP leaves them to it and keeps its own. Both plugins then work side by side, as long as your AI client is pointed at Royal MCP’s server address.

## Fix it

#### Update Royal MCP

Go to **Plugins** and update Royal MCP to 1.5.14 or later (1.5.12 is the minimum; 1.5.14 also moves clients that connected before the update to Royal MCP’s own address).

#### Remove the connector in your AI client

In ChatGPT or Claude, open the connector settings and remove the connector for your site. This clears the sign-in details the client saved from the other plugin.

#### Add it again with Royal MCP’s server address

Add a new connector with `https://your-site.com/wp-json/royal-mcp/v1/mcp`, not the bare site address. The client now signs in on Royal MCP’s consent screen. The guides for [ChatGPT](https://royalplugins.com/support/royal-mcp/connecting-to-chatgpt/) and [Claude](https://royalplugins.com/support/royal-mcp/connecting-to-claude/) walk through this screen by screen.

#### Check the tools

Ask the client to list its tools, or to run `royal_mcp_connection_health`. You should see Royal MCP’s tools.

That is enough on most sites. You do not have to switch the other plugin’s sign-in off, and doing so is optional.

## Optional: switch the bundled sign-in off

If nobody on your site connects AI clients to the other plugin’s own MCP server, you can switch its bundled sign-in off. The site-root sign-in addresses then return to Royal MCP, and AI clients that only read the site root also reach Royal MCP. The library provides a switch for this, set with one line of code.

#### Create a must-use plugin

Using your host’s file manager or SFTP, open `wp-content`, create a folder called `mu-plugins` if it does not exist, and add a file such as `turn-off-bundled-mcp-sign-in.php` with the code below. Must-use plugins load automatically and cannot be deactivated by mistake.

#### Re-save permalinks

Go to **Settings > Permalinks** and click **Save Changes** without changing anything. This clears the sign-in addresses the bundled sign-in had registered.

#### Re-check

Open **Royal MCP > Settings** and click **Re-check now** on the notice, or ask your AI client to run `wp_get_mcp_coexistence`, which shows who answers each sign-in address.

```
<?php
/**
 * Plugin Name: Turn off bundled MCP sign-in
 * Description: Switches off the MCP sign-in bundled with some plugins so
 *              Royal MCP answers the site's sign-in addresses.
 */
defined( 'ABSPATH' ) || exit;

add_filter( 'wpmedia_mcp_oauth_server_enabled', '__return_false' );
```

What this switches off

The line turns off only the bundled MCP sign-in. AI clients that were connected to the other plugin’s MCP server through that sign-in will stop working. To turn it back on, delete the file and re-save permalinks again.

## If it still fails

### ChatGPT still says “Unknown OAuth client.”

ChatGPT is probably still using the connector it saved before. Remove it completely and add it again with the full Royal MCP address ending in `/wp-json/royal-mcp/v1/mcp`. Check under Plugins that Royal MCP is 1.5.12 or later.

### The Royal MCP notice names a different plugin

Another plugin is answering part of the sign-in. Follow [Another plugin is answering OAuth sign-in](https://royalplugins.com/support/royal-mcp/another-mcp-plugin-answers-oauth-discovery/), which covers every plugin the self-check can name.

### The sign-in page loops or ends in a 404

A cache or security layer may be serving an old copy of the sign-in addresses. Clear your page cache and any CDN cache, then re-save permalinks. The [troubleshooting checklist](https://royalplugins.com/support/royal-mcp/troubleshooting-start-here/) covers host-level causes.

## Frequently asked questions

### Do I have to switch off Rank Math’s MCP sign-in?

No. With Royal MCP 1.5.12 or later, AI clients connected to Royal MCP’s own address work next to it. Switching it off is optional and only useful if nobody uses that plugin’s MCP server.

### Does the code change affect Rank Math’s SEO features?

The filter switches off only the bundled MCP sign-in. It does not change Rank Math’s settings.

### Why does Claude connect when ChatGPT does not?

The bundled sign-in recognises Claude but not ChatGPT. Claude may therefore connect to the other plugin’s server instead of Royal MCP’s. Reconnect it with the Royal MCP address so it reaches Royal MCP’s tools.

### Do clients I connected earlier need to reconnect after updating?

Not from Royal MCP 1.5.14, which moves earlier connections to Royal MCP’s own address. A client that was signed in to the other plugin by mistake does need to be removed and added again.

## Related

[Coexistence

Royal MCP Next to Other MCP Servers

The Abilities API, the MCP Adapter, Elementor MCP and shared sign-in.](https://royalplugins.com/support/royal-mcp/other-mcp-servers/)
[Troubleshooting

Unknown Client ID Error

Other reasons an AI client is not recognised at sign-in.](https://royalplugins.com/support/royal-mcp/unknown-client-id-error/)

[Back to Royal MCP Support](https://royalplugins.com/support/royal-mcp/)
[Form Entry Privacy](https://royalplugins.com/support/royal-mcp/form-entry-privacy/)