Most apps that grew an "AI" feature in the last two years talk to one of a handful of cloud APIs, and almost all of them speak the same dialect: the OpenAI Chat Completions format. That one detail is the reason you can pull the cloud out and run the whole thing locally on a Mac without the app ever noticing.
Here is the trick, why it works, and the gotchas that bite.
The one interface everything agrees on
OpenAI's /v1/chat/completions endpoint became the de facto standard. So when an app lets you "use your own key" or "set a custom base URL," it is almost always going to POST to {base_url}/chat/completions with a JSON body of messages and read back the same shape. It does not care what is on the other end, only that the response matches.
Local runners leaned into this. Both popular Mac ones expose exactly that endpoint:
