Grasshopper is very good to create new splines, but is it possible to restrict it to only generate curves with a constant radius (ie arcs)?

I would like to use the tween curves function in Pufferfish, two generate new curves at an interval between a line and an arc, but I need all the results to be arcs as well.

Take the resulting tween curves, select the two end points and the midpoint (in correct order), and use Arc 3pts. It will look pretty close to the original curve.

As you have discovered. Tweening doesn’t keep arcs. In tweens arcs are treated as curves, otherwise you get weird results like you saw in the other post with the native tween.