Hi ​@NMN 

The key is that you can apply specific styles to individual shapes or sub-shapes, including rounding, without affecting the entire root shape. 

You can apply rounding to specific parts of a custom shape, like using path shapes or controlling individual shape styles.

Refer this thread to learn more - Rounding a rectangle and setting it to no fill in Lucidchart | Community

Hope this helps - Happy to help further!!
Thank you very much and have a great one!
Warm regards

Hi ​@NMN , you might find this example helpful: GroupedShape.shape from our example-shape-library repo. It shows an example of styling with sub-shapes specifically. 

Hope that helps! Let us know if you get stuck or have any further questions.

Hi ​@NMN 

You are right, that styles rounding works at the root level in a custom shape. besides, you can still create sub-shapes with rounded corners by using a path instead of a basic rectangle.

Best Regards

Thank you for your answers.

I’ve tried using the path type. See the example below. However, when you resize the shape, you lose the roundness.

{
    "geometry": :
        {
            "type": "path"
            "path": "M 0.1,0 h 0.8 a 0.1,0.1 0 0 1 0.1,0.1 v 0.8 a 0.1,0.1 0 0 1 -0.1,0.1 h -0.8 a 0.1,0.1 0 0 1 -0.1,-0.1 v -0.8 a 0.1,0.1 0 0 1 0.1,-0.1 z"
        }
    ]
}