Short answerA Custom GPT is a configured assistant inside ChatGPT. An API build is software you run yourself. OpenAI says the Assistants API was sunset on August 26, 2026, so new API rebuilds should use the Responses API.
The core difference
| Custom GPT | API build | |
|---|---|---|
| Where it runs | Inside ChatGPT | Wherever you host your app |
| Who uses it | People with ChatGPT access | Anyone you give your own interface |
| Setup | Configure in a builder | Write and maintain code |
| Billing | Part of the user's ChatGPT plan | You pay per use through the API |
| Connections | Actions | Tool calls you implement |
Where the Assistants API fits now
OpenAI's migration page says the Assistants API was sunset on August 26, 2026 and recommends the Responses API. Details and the concept mapping are in rebuilding with the OpenAI API.
Which should you choose?
- Your users live in ChatGPT: look at the plugin path.
- You need a public bot or product feature: an API build or a hosted page.
- You need Actions and control: an API build.
Not sure it is simple enough to do yourself?
Tell us what your GPT does, whether it uses Actions and how many files it has. We will say what will break and what a rebuild involves. The audit is free.
Request a free auditRelated guides
Sources
- OpenAI developers: Assistants to Responses migration
- ChatGPT Learn: Moving your custom GPT workflows to plugins
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