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.
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.
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.