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Custom GPT Action migration checker
Custom GPTs stop running on 11 December 2026, and their Actions do not carry over to ChatGPT plugins. Paste your GPT's Action schema (the OpenAPI JSON or YAML from the GPT editor) to see what it becomes as an MCP server: every action as an MCP tool, how its authentication moves, what needs fixing, and a free FastMCP starter pre-filled from your schema. Nothing is uploaded.
Checker
Where to find the schema: open the GPT in the editor, go to Configure → Actions, open the action, and copy everything in the Schema box.
Paste the schema, or choose a file. Up to 1.5 MB. Do not paste API keys: they are not needed.
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What changes on 11 December 2026
| Part of the GPT | Moves to a ChatGPT plugin? | What to do |
|---|---|---|
| Instructions | Yes | Review the migrated plugin's skills |
| Knowledge files | Yes | Check file limits after migration |
| Connected apps | Yes | Re-authorise if asked |
| Custom Actions (your API) | No | Serve the same API as an MCP server (this checker shows how) |
| Model choice, sharing settings, conversations | No | Set them again in the plugin; the old GPT becomes read-only |
Reported dates and rules can change: OpenAI says all dates are subject to change. One rule that matters for Actions: plugins listed in the Plugin Directory must use OAuth for sign-in, not a pasted API key, so an Action that used an API key needs an OAuth front door or a server that keeps the key itself.
Three ways to replace an Action
- Run an MCP server yourself. Open-source tools such as FastMCP turn an OpenAPI schema into an MCP server. Free, but you host it, secure the key and add OAuth for ChatGPT and Claude.
- Add MCP to your API directly. If you own the API, the official MCP SDKs let you expose a few well-described tools, which assistants often use better than a 1:1 copy of every endpoint.
- Use a hosted bridge. A service runs the MCP server for you from the same schema (see below).
Hosted option: ActionBridge
ActionBridge, from Tidewell Software, uses the same converter as this checker to host your Action as a remote MCP server, with the API key encrypted server-side, an OAuth front door for ChatGPT and Claude, and free self-host code (Cloudflare Worker or Node) if you would rather run it yourself. Details: ActionBridge. The checker stays free either way.
Frequently asked questions
When do custom GPTs stop working?
OpenAI's retirement notice says custom GPTs stop running on 11 December 2026. Enterprise and FedRAMP workspaces with an approved deferral have until 11 February 2027. Check OpenAI's help centre for your workspace, because dates for deferred workspaces are set per account.
What carries over, and what doesn't?
According to OpenAI's migration FAQ, a GPT's instructions, knowledge files and connected apps can be moved into a ChatGPT plugin. Custom Actions are not migrated. OpenAI's FAQ says they must be rebuilt with a supported connector or a custom MCP server. Anything your GPT did through an Action (calling your API, looking up orders, creating tickets) needs that new home.
Is my schema uploaded?
No. The checker runs in your browser. The schema is parsed and converted on your device, the page has no analytics, and its security policy blocks it from sending data anywhere. API keys are never needed: the checker only reads the schema.
What is MCP, and why move Actions to it?
The Model Context Protocol is the open standard ChatGPT plugins and connectors, Claude, Cursor and most agent tools use to call external tools. One MCP server replaces an Action and works in all of them, not only ChatGPT.
Which schemas work?
OpenAPI 3.0 and 3.1, in JSON or YAML, which is what the GPT Actions editor uses. Swagger 2.0 must be converted first. JSON and form request bodies are supported; file uploads and cookie authentication are not.
Is the FastMCP code ready for production?
It is a working starting point. For production you still need HTTPS hosting, a secret store for the API key, logging that leaves out request bodies, and for per-user APIs an OAuth 2.1 sign-in that ChatGPT and Claude accept.
Third-party code: yaml by Eemeli Aro, ISC licence, bundled into the checker script.
Last updated 8 November 2026. Questions or corrections: [email protected].