If you can predict when you're going to get new content, just use an HTTP header to tell until when the content can be seen as fresh. Then the client will know when to call the API again.
If not, ask the client to provide an HTTP callback endpoint, and ping it to notify of new fresh data on your side. Using WebSocket is also an option.
If not, ask the client to provide an HTTP callback endpoint, and ping it to notify of new fresh data on your side. Using WebSocket is also an option.