A: Hi!
Thank you for the question :)
We use ChatGPT-4 for all rewrite options, except for Magic, Slightly, Structured, and X Post. For these specific rewrites, we use a fine-tuned GPT-3.5 model, as we find that fine-tuning performs better than prompting for these tasks.
We've tested fine-tuning on GPT-4o mini, but there's still a lot of work to do before we can deploy it to production. We're on it!