Yes, there is a complexity limit, but it is fuzzy and not perfectly well defined (and that's ok). Modern LLMs ride that line in a wild and interesting way.

It's really fun having become a biologist in recent years, and knowing that there's an entire field that uses living organisms (yeast or neutered E coli) as protein factories. I am bringing it up a bunch here because not only are these things used as tools, but they're literally alive!

Makes me think of working animals, such as plow horses. Are they tools? Is there a better term for them that would also apply to LLMs?

It's not about complexity but architecture. Neural networks work in a fundamentally different way from imperative programs.

You can modify curl (a "tool") and predict how its behavior will change, with a high degree of certainty (bugs aside).

You can't modify a neural network in the same way. You can train it to behave in a certain way, there are no guarantees that it will consistently behave as intended.

More importantly, neural networks encode far more information and behavior than we can fully understand/identify - this is the fundamental reason why LLMs aren't just tools.