How to get Cycles viewport output in compositing I'm rendering with Cycles in the viewport (not with F12) and would like to get that viewport output as an input node into the Compositor.

How can this be done?
Well there is a way but it really is a cart before the horse thing. If you really need the image you can only get it by a screenshot, which will be limited to an 8 bit depth and the size of your viewport, obviously.

When it is saved you can then use a file node to import it into the node editor.
As I said this is a nuts way to do it but it can be done.

Don't do it though use F12.