sarahk's not wrong honestly, but if you actually want a tool answer: Claude Opus 4 is the best general-purpose model for long-form fiction right now because it can hold tone and character voice across 200k tokens without going generic, which is the thing GPT and Gemini both fall apart on. Use it as a co-writer not a first-drafter though, give it your scene goal and your character voices and let it draft pages you ruthlessly cut afterwards. Anyone who's typed "write me a novel" as a single prompt has seen the same beige paste come out the other end, doesn't matter which model. If you want something purpose-built for fiction rather than a chatbot, Sudowrite is worth a look, it's literally structured around scene-by-scene editing instead of turn-based conversation, which fits the way novelists actually work.