What do you mean by "optimal"?  Minimum area?  'BBox' typically keeps its edges aligned to WorldXY so it's possible there might be a rectangle with a smaller area if it were rotated a bit.

Why did the 'BBox' fail?  Post some code.

Works for me:

But as you can see, minimal area of the rectangle for this curve happens when the 'BBox' plane is rotated ~66 degrees:

You guessed rightly, i were searching for the minimum area but finally, i dont quite need this optimal rectangle.

Just a fitting one do the job in my case.

So, BBox problem : when your surface is worldXY no problem but there are some orientation you dont have an untrimmed srf, nor a flat box but a box.

I was almost writing a piece of C# when a friend reminded me the existence of "Plane Through Shape" (cutted banana)

Worked at work, 'll work at home

Thx again mister, have a nice evening !