I don't think we're approaching the limit of deterministic prompt -> source code mapping any time soon. Small variability in prompts produces medium variability in outputs. Building on previous outputs only extends the variability, where behavioral differences start appearing a few prompts down the line.

And the AI companies aren't focused on making this process any more deterministic, where repeated prompts are sampled using different seeds. I don't know if it's because the randomness makes the models explore a larger space and perform better, but it's probably intentionally done because it'd be quite easy to remove the stochasticity.