i don't know if spirit could do API to interact with flutter
What are you trying to do? I suspect you are trying to integrate Spirit into some mobile app. Depending on the kind of integration, this ranges from difficult to pretty much impossible. When I say "impossible", I mean you are better off choosing a forum that provides an API for everything outside the box, if that's the kind of integration you need. However, a very simple integration, like showing "read only comments" would be doable.