This Deep Think one was so good that I did get suspicious that maybe it was at least rendering the SVG to an image and then "looking" at the image and tweaking it over a few iterations.

But the reasoning trace doesn't hint at that and looks legit to me: https://gist.github.com/simonw/7e317ebb5cf8e75b2fcec4d0694a8...

I also asked Deep Think what tools it has access to and it has Python and Bash but no internet access, and as far as I can tell that environment doesn't have any libraries or tools installed that can render an SVG to an image format that it could view.