Merge 2 closed curve using boolean union didn't work

I am trying to merge two curves as one using BooleanUnion function in rhino common.

Curve.CreateBooleanUnion(Curves, doc.ModelAbsoluteTolerance);

It works for most scenarios, but for some reasons its not working for some curves.

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Control that both curves are in the same plane and intersecting each other. Otherwise, can you share a 3d file with the curves that are not working? :relaxed:

curve.3dm (33.0 KB)
sample file

added sample file in the thread

Hi @hariprazath,

This simple Python script seems to work on the curve in your sample file.

import Rhino
import scriptcontext as sc

def test_curve_booolean_union():
    filter = Rhino.DocObjects.ObjectType.Curve
    rc, objrefs = Rhino.Input.RhinoGet.GetMultipleObjects("Select curves", False, filter)
    if rc != Rhino.Commands.Result.Success: 
    if not objrefs:
    in_curves = []
    for objref in objrefs:
        curve = objref.Curve()
        if curve:
    if len(in_curves) < 2:
    tolerance = sc.doc.ModelAbsoluteTolerance
    out_curves = Rhino.Geometry.Curve.CreateBooleanUnion(in_curves, tolerance)
    if out_curves:
        for curve in out_curves:
            id = sc.doc.Objects.AddCurve(curve)
            rh_obj = sc.doc.Objects.Find(id)
            if rh_obj:

if( __name__ == "__main__" ):

What are we missing?

– Dale