No terminal needed
ChatGPT can write straight into your vault: a plan, a summary, a draft, anything it has just produced. You add one address, sign in once, and from then on "save that in Sumibako" is a thing you can say. There is nothing to install.
Before you start
Custom plugins are not on the free tier, and on some plans they are limited to reading. Full read and write is on Business, Enterprise and Edu, where an administrator may have to allow developer mode first. They work on the web, not in the mobile app.
OpenAI renamed this surface from Connectors to Plugins in July 2026, and moved developer mode while doing it. The labels below are the new ones. If your settings still say Connectors, you are on the older shape: the steps are the same and the address is the same, only the words on the buttons differ.
ChatGPT owns these menus and can rename them. Checked against OpenAI on developer mode and MCP apps on 2026-09-21. If a label has moved since, the address is still the only thing you need.
SettingsPluginsDeveloper mode
In ChatGPT, open Settings and then Plugins, and turn on developer mode at the bottom of that page. Some accounts keep the toggle under Security and login instead.
On Business and Enterprise an administrator has to allow this before the toggle appears at all. That is the usual reason it is simply missing.
+New Plugin
Press the plus on the Plugins page. A New Plugin dialog opens with a field for the server address. Paste the address in, choose OAuth as the authentication, and save.
https://sumibako.com/api/mcpChatGPT asks us how to authenticate and is sent to a Sumibako sign-in. You are then shown what it will be allowed to do: read and search your pages, write new ones, and publish a page it wrote so it can hand you a link. If you do not have an account yet, you make one here. It is free.
@Sumibako
Unlike Claude, ChatGPT does not always reach for a plugin unprompted. Bring it in with @ or from the tools menu the first few times, then say what you want.
Paste this into ChatGPT. It asks for a real page rather than a list of tools, because an assistant will cheerfully describe a connector it has not loaded, and only a page that actually appears settles it.
Save a page in Sumibako called "Hello" that says this connector works, then tell me what its icon is.It files the page and tells you the emoji it chose for it. Open Sumibako and the page is in your sidebar, icon and all. If it instead explains how you could do that yourself, it is not connected - go back to step one.
Eight things worth knowing you can ask for. Each one says the word Sumibako inside the sentence, which is the part people leave out: without it, ChatGPT has no way to tell that “save this” means your vault rather than its own memory of the conversation.
You want to keep what it just wrote
Save that in Sumibako as a page called "Q4 hiring plan".The page appears in your vault, with an emoji it picks itself. Headings, lists, tables and code all survive the trip.
You want to send it to somebody
Publish that page and give me the link.A sumibako.com address that opens for anyone, with no account and no sign-in. It carries noindex, so it is shareable without being findable.
You want the same page updated, not a second one
Update my "Weekly review" page in Sumibako rather than making a new one.It writes over the page it filed before. This is the difference between a vault and a pile, and it is worth saying out loud the first few times.
You want to add to something without retyping it
Add these three bullets to the end of my "Reading list" page in Sumibako.Only the new lines are sent. The rest of the page is never re-uploaded, so nothing already there can be lost on the way.
You want one line changed
In my "Launch checklist" in Sumibako, change "Tuesday" to "Thursday".One exact replacement. If that text appears twice it refuses rather than guessing which one you meant.
You cannot remember where you put something
Search Sumibako for what I wrote about pricing.The matching pages, by their words. Ask for nothing in particular and it lists the newest.
You want it to read something you wrote by hand
Read my "Onboarding notes" page from Sumibako and tell me what is missing.The page comes back as Markdown, which is the form it can then edit and send back. It works on pages you typed yourself, not only ones it filed.
You want to know what you are using
What plan am I on in Sumibako, and how much of it have I used?Your plan, your limits, what you have used, and what this particular connection is allowed to do.
The four that matter, because you are about to let a program write into your notes.
It cannot delete anything
There is no delete in the API at all, so nothing it does can destroy your writing. Unpublishing takes a link off the web and leaves the page where it is.
It cannot put your notes in front of strangers
A published page carries noindex. Making one findable by a search engine is a switch you flick yourself, per page, inside the app. It can hand you a link; it cannot get you into Google.
It cannot publish a page you wrote
A connector you signed in to may only publish pages it created itself. Your own pages are not its to put on the web.
It cannot reach anyone else's vault
The connection is scoped to the account that approved it and nothing else. Revoke it in the client's settings and it stops on its next request.
There is no developer mode toggle
Either the plan does not include custom plugins, or you are on Business or Enterprise and an administrator has not allowed it. Both are settings on OpenAI's side that we cannot affect.
Your settings say Connectors, not Plugins
You are on the shape from before July 2026. Settings, then Connectors, then add one of your own, and paste the same address. Everything downstream is identical.
It reads your pages but refuses to write
Custom plugins are read-only on some plans. Nothing in Sumibako can lift that - the connection is being given read tools only at OpenAI's end. The command line has no such limit, and needs no plan: see the agent guide.
The form asks for an Authorization header
An older form. Make a token in Sumibako under Settings, then Coding agents, and paste it as Bearer followed by the token. Revoking it there cuts the connection on its next request.
You cannot see it in the mobile app
Custom plugins are a web feature. Nothing is wrong with your setup.
Also use Claude?
Connect Sumibako to Claude
The same vault, connected the same way. Both at once is fine.
Have a terminal?
Connect a coding agent
Claude Code, Cursor and Codex read the file off disk instead of sending it through the model, and need no plan at all.