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Connect ChatGPT

On chatgpt.com, add a custom MCP server pointing at mcp.justonce.ai and sign in. Plus what ChatGPT can and can't see.

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Written by Aaron Wittman

ChatGPT adds custom MCP servers on the web at chatgpt.com, not in its desktop or mobile apps. OpenAI decides which plans can do this; at the time of writing it's Pro, Business and Enterprise/Edu. On Business, an admin or owner adds the server and publishes it for the workspace.

  1. On chatgpt.com open Plugins, select +, then Add custom MCP server.

  2. Name it JustOnce and enter the URL https://mcp.justonce.ai exactly, with nothing added to the end (not /mcp). Keep OAuth as the authentication, read OpenAI's risk notice, confirm, and create it.

  3. When ChatGPT sends you to the JustOnce sign-in, sign in (or create your account there), then click Allow.

  4. In a chat, turn on JustOnce in the tools picker and ask "What do you remember about me?".

If your account still shows the older layout, the route is Settings → Apps → Advanced → Developer mode: turn it on, then add the server there with the same URL.

What ChatGPT can and can't do

  • ChatGPT gets most JustOnce tools, plus two ChatGPT-only tools, search and fetch, that return citable results (see What your assistant can do: the JustOnce tools explained).

  • To meet OpenAI's rules, JustOnce keeps some memories out of ChatGPT: anything filed under credentials, identity, health or finance; anything holding a password, PIN, card, bank or ID number; and anything JustOnce hasn't confidently filed yet. ChatGPT is told how many it can't see. They're all still in your vault and in the app.

  • JustOnce won't save passwords, PINs, or card, bank or ID numbers sent from ChatGPT. Add those in the app instead.

  • ChatGPT doesn't get your standing profile, topic context blocks, the "as of a date" lookup or the relationship graph. Other assistants do.

  • On Pro, OpenAI lets ChatGPT recall from custom servers but not save to them.

  • Documents: ChatGPT can read a file you drop into the chat and save its key facts. For the original file, it gives you a one-time upload link (see Save a document from your assistant).

  • The same rules apply to OpenAI's Codex.

Troubleshooting

  • No way to add a server? Your plan or workspace may not allow custom MCP servers. On Business, ask your workspace admin.

  • If the connector shows as disconnected, remove it, add it again and sign in. You can then Forget the old entry on the app's Connections page to tidy the list.

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