>feed the same input to the old Go app and the new Rust then diff the outputs.

Yes, I completely forgot, this is exactly my case. Rune is a TUI editor, so I feeded the same terminal sequences to the old and new apps.

It didn't translated 1:1 (I ported core editor first, there was side panels, and different chrome elements) so I instructed LLM to use ttyd (tty -> browser render), Fable then could open both apps with playwright, make and compare screenshots.

To rephrase, one critical component that I completely forgot to mention is to establish a feedback loop for the model. This new generation of models: Opus 5, Fable, GLM-5.2, even Qwen3.8-27B can self-correct, provided they know whether they are progressing or not.

A month ago, especially smaller model would fall into a rabbit hole it dug for itself and would never recover. This generation can easily run tens of hours without losing track.