You only need the frontier model for one single edit

Can Bölük analysed token distribution across about two million agent tool calls: 91 per cent of tokens go on reading the codebase, 9 per cent on edits. That kills the intuitive plan-with-a-frontier-model, implement-with-a-cheap-model split, because the cheap executor has to reread everything the frontier model already read; in his tests the split actually cost 14 per cent more than the frontier model working alone. A plan document is “a literal postcard, describing a journey to a model that never took it”.

Their fix, prewalk, hands over the frontier model’s actual context window after its first successful edit: 92 per cent of the performance at 53 per cent of the cost.