Custom GPTs End on 11 December. Is a Claude Project the Swap for a CA?

I have not built a custom GPT, so everything on that side here comes from OpenAI's announcement as reported in September 2026. The Claude side I did myself. I set up a Claude Project on the free plan for a chartered accountant and tried to upload 10 to 15 of his files: client notices, GST circulars and notifications, some scanned PDFs and a few spreadsheets. The project knowledge bar was full after 4 to 6 of them, so I cut the list down and uploaded fewer files.

What OpenAI is changing

OpenAI announced on 11 September 2026 that custom GPTs will stop running on 11 December 2026. They are being replaced by plugins. Some Enterprise workspaces with an approved deferral get until 11 February 2027. Reports give different dates for when you can no longer create a new GPT, and they vary by plan, so check the notice in your own account.

Part of your GPT	Until 11 Dec 2026	After migrating to a plugin
Instructions	Live in the GPT	Become a skill inside the plugin
Knowledge files	Attached to the GPT	Copied into the plugin's reference files
Connected apps	Work as before	Can be carried over
Custom Actions	Work as before	Do not transfer
Old conversations	In the GPT	Do not move into the plugin
Sharing	As you set it	Plugin starts private

OpenAI also warns that "A migrated plugin may respond differently", so test the plugin on your usual questions before you rely on it.

The Claude Project I set up instead

The accountant is the same one I connected Gmail and Drive for in my earlier piece on his connectors. This time I put his reference documents in one Project. He used the Project for GST and income-tax questions, for drafting routine client emails, and for asking about the documents he had uploaded.

Claude Projects, read 16 Oct 2026	Free plan	Paid plans
Number of Projects	Up to 5	No cap listed
When knowledge fills up	It stops there	Switches to retrieval, up to 10x more capacity
What held in my test	4 to 6 of 10 to 15 files	Not tested

A worked example: one CA's folder

His folder was 10 to 15 files across four kinds: client notices, GST circulars, scanned PDFs and spreadsheets. On the free plan, the bar reached 100% after 4 to 6. Only part of the folder fits, and you pick which part.

Whatever you leave out, Claude cannot see when he asks a question about it.

Anthropic says paid plans switch to retrieval when knowledge nears the limit, which it describes as up to 10 times the capacity. I have not tested a paid plan, so I cannot tell you whether all 15 of his files would fit. If you pay for Claude, test with your own folder before moving everything over.

What I would tell a CA with a GPT today

If your GPT holds a handful of reference files, the free Claude Project is enough to try the move before 11 December. If it holds a full folder, budget for a paid plan or accept that you will be picking files. Do the move early: a migrated GPT and a new Claude Project both need retesting, and you want that done before the old one stops.