I want the Capture Length of a Bend SOP to be always the size of the incoming geometry. getbbox_size(0).x and $BBX are not working, however I'm mostly just interessted in the VEX expression and not so much in HScript solution.

What am I doing wrong?

And before you suggest alternative solutions, my actual geometry is a little more complex than the simple Grid here, so it has to be the bounding box instead of just getting the width of the Grid with a ch() function.

Parameters only support HScript (the default language) or Python (if you set the parameter's language to Python), not VEX. bbox(0,D_XSIZE)seems to work.

If you really want to use VEX, you can use an Attribute Wrangle with Run Over set to Detail to write the bounding box size to a detail attribute like @length = getbbox_size(0).x;, then read the detail attribute from the parameter like detail(0,"length",0), or something similar.

Hopefully one of those is what you were looking for.

That was very helpful, thanks.

super useful for me also,
thanks !