Godot Engine PR #86195 adds a closed property to the Curve3D class.

Previously, 3D curves in Godot didn’t support a native closed mode. As a workaround, developers had to manually duplicate the first point at the end of the curve.

This approach had several drawbacks:

• Redundant control points, poorer UX, and manual adjustments every time the curve changed.
• Tilt data didn’t interpolate correctly across the closing segment.
• Explicitly marking a curve as closed is useful for many workflows, especially mesh extrusion and procedural generation, where distinguishing between open and closed curves is important.

The implementation involved:

• Changing the color of the start and end points (especially useful for closed curves, where the beginning of the curve is otherwise unclear).
• Displaying the In handle for the start point and the Out handle for the end point when closed = true.
• Showing the In and Out properties in the Inspector when closed = true.
• Replacing the Close Curve button in Path3DEditorPlugin with a toggle that switches the closed property on and off.
• Adding the corresponding documentation.

You can read the full discussion and implementation details in the pull request: Add closed property to Curve3D.