Having used them heavily for 3D model understanding since February I can say it's nuanced.

Opus 4.6 and 4.7 were bad, but GPT 5.2 and above were very usable. Opus 4.8 was usable, but the GPT 5.x series was better.

Fable is great.

Opus 5.0 was interesting. It could solve some problems that Sol 5.x couldn't solve (applying a G2 curve on a 3 way corner where one face was a Bezier curve) but you had to be super prescriptive ("only answer the question"/"only do what I tell you and stop when done") or it would go on a hugely involved validation journey that didn't really achieve a lot.

Opus 5.5 is better than that was in that respect.

Sol 6.x is great, and my daily driver for this (I use Opus for coding though)

Astra can solve problems that Sol can't but for some reason on easy stuff makes uglier solutions.

For all models it's very interactive though - we aren't at the "agentic design" phase for most things yet.

Here's a sample of what I've been able to get them to design with me: https://x.com/nlothian/status/2099023496794018067

I think that this is an unsolved problem in the same way that mangled fingers in image generation was an unsolved problem.

Through at least Opus 4, LLMs were practically useless for authoring any sort of coherent procedural closed-curve geometry (I know this with strong confidence because of the little animated guys at https://letterspractice.com).

Opus 5.5 can bang it all out. Possibly a deliberate RL sort of thing or maybe another surprise emergent capability.

Is it that LLMs are weak with spatial reasoning (and memory) or is it that we are unusually good at it?

When I need to use a program I seldomly use I'm far more likely to remember where I need to click to open it than the word I need to search for to open it.