EvoReel: Evolving Harness Engineering for Long-Horizon Video Generation

Long-horizon video generation turns a brief into a minute-scale, multi-shot film. Since video models typically render clips under 30 seconds, it relies on agentic harnesses that must write a coherent script and render it with characters and places consistent across cuts. Existing harnesses, however, remain static. Across films, nothing learned carries over without fine-tuning or curation, and within a film, every shot is conditioned on references fixed in advance, although each shot needs different characters, places and earlier frames. To address these shortcomings, we propose EvoReel, an evolving harness for long-horizon video generation that learns from its own productions without updating any model. Specifically, EvoReel writes and renders every shot from memory and checks it with a self-judge. Across films, it distills its runs on training films into an experience memory of the revisions that made shots more specific and the content that drafts tended to lose. Within a film, role-specific agents write the screenplay from story to scene to shot over a structured story memory, with the shot writer consulting that experience as it first drafts each scene, and a self-judge scores each draft on professional screenwriting dimensions and sends weak shots back for revision. Rendering follows the same pattern: to keep characters and places consistent across shots, a multimodal visual memory supplies each shot with the reference images and earlier frames it calls for, and a second self-judge checks each rendered shot against its script and re-renders those that fall short. With these designs, EvoReel outperforms the strongest of eight baselines on ViStoryBench and held-out MovieBench by 28.6% and 84.1% in script composite and by up to 20.2% in how much of the brief its films show, and ties for the highest character consistency under the proprietary renderer. We also introduce ReelBench, 104 original premises across 13 genres with checkable constraints, which tests agents beyond shot lists adapted from existing productions. Code and demos are available at https://anonymous.4open.science/r/EvoReel-D2EF.