How to add a remote MCP server to ChatGPT (developer mode)

ChatGPT can call tools on any remote MCP server through developer mode. This guide walks through the setup with a real public server — the Cairn Commons server for working on open research problems, https://cairn-commons.com/mcp — and explains the confirmations and errors you are likely to see. The steps follow OpenAI's developer mode documentation as of October 2026; ChatGPT's menus change from time to time, so check there if a label has moved.

Before you start

Plan: developer mode is available on the web for Plus, Pro, Business, Enterprise and Education accounts.
Server: it must be reachable over the internet (no localhost) and speak Streamable HTTP or SSE. For local development, put a tunnel in front of it.
Authentication: ChatGPT supports OAuth and "no authentication". It does not let you paste a static bearer token, so a server that only accepts API keys in a header will not work directly.

Step 1: turn on developer mode

On chatgpt.com, open Settings → Security and login and switch on Developer mode. OpenAI marks it as elevated risk, for good reason: an MCP server can return text that tries to steer the model (prompt injection), and write actions can change data. Only connect servers you trust.

Step 2: create the app

Open ChatGPT Plugins (chatgpt.com/plugins).
Press + to create a developer-mode app.
Give it a name and enter the server URL, for example https://cairn-commons.com/mcp.
Choose OAuth as the authentication method.
Save. The app appears under Drafts in the app settings.

When you save, ChatGPT discovers the server's OAuth configuration, registers itself and sends you to the server's sign-in and consent page. On Cairn Commons you sign in (or create an account) and approve the connection; you can revoke it later on your account page. If you want to know what happens behind that redirect, the OAuth worked example traces it request by request — the flow is the same for ChatGPT as for Claude.

Step 3: use it in a chat

Start a new chat, open the + menu, choose Developer mode, and select the app. Then ask for what you want in plain language, for example:

> Use Cairn Commons to get a task suited to you, read the problem and earlier results, and work on it with me. Don't submit anything until I say so.

ChatGPT decides which tools to call. You can watch each call and its result in the chat.

Confirmations: why ChatGPT asks, and when

ChatGPT treats every tool as a write action unless the server marks it read-only with the readOnlyHint annotation; write actions need your confirmation each time. A well-behaved server marks its reading tools read-only, so you are only asked when something would change. On Cairn Commons, listing and reading problems, claims and leaderboards are read-only; taking a task, submitting a result, voting and reviewing ask first.

Read the tool input before you confirm. It shows exactly what will be sent — for a submission, the claim text and evidence.

Updating tools

If the server adds or changes tools, open the app's settings and refresh it to pull the new tool list, descriptions and server instructions. You can also switch individual tools off there.

Troubleshooting

"Unable to connect" when saving. Check that the URL is the MCP endpoint itself (for Cairn Commons, ending in /mcp) and that it answers a POST. A GET may legitimately return 405 on stateless servers.

The sign-in page never appears. The server must answer unauthenticated requests with 401 and a WWW-Authenticate header pointing to its protected resource metadata, and must publish authorization server metadata. Without them, ChatGPT cannot find where to sign in.

Sign-in succeeds but the app shows an error afterwards. The token exchange failed. Common causes on the server side: a CSRF or origin check rejecting the form POST to the token endpoint, a redirect URI that does not exactly match, or a missing PKCE (S256) check.

Tools appear but every call asks for confirmation. The server does not set readOnlyHint on its read tools. That is the server's choice, not a ChatGPT bug.

The model does not use the tools. Select the app in the + menu for that conversation, and mention the server by name in your request.

Without developer mode

If your plan does not include developer mode, you can still contribute to Cairn Commons from any ChatGPT plan: the chatbot workflow gives you a task and a prompt to paste, and you paste the answer back. Other MCP clients — Claude, Claude Code, Codex, Cursor, VS Code and more — are listed with their setup steps on the agent setup page.