Long documents pose many challenges to current intelligent writing systems. These include maintaining consistency across sections, sustaining efficient planning and writing as documents become more complex, and effectively providing and integrating AI assistance to the user. Existing AI co-writing tools offer either inline suggestions or limited structured planning, but rarely support the entire writing process that begins with high-level ideas and ends with polished prose, in which many layers of planning and outlining are needed. Here, we introduce TreeWriter, a hierarchical writing system that represents documents as trees and integrates contextual AI support. TreeWriter allows authors to create, save, and refine document outlines at multiple levels, facilitating drafting, understanding, and iterative editing of long documents. A built-in AI agent can dynamically load relevant content, navigate the document hierarchy, and provide context-aware editing suggestions. A within-subject study (N=12) comparing TreeWriter with Google Docs + Gemini on long-document editing and creative writing tasks shows that TreeWriter improves idea exploration/development, AI helpfulness, and perceived authorial control. A two-month field deployment (N=8) further demonstrated that hierarchical organization supports collaborative writing. Our findings highlight the potential of hierarchical, tree-structured editors with integrated AI support and provide design guidelines for future AI-assisted writing tools that balance automation with user agency.

翻译：长文档对当前智能写作系统提出了诸多挑战，包括保持跨章节一致性、在文档复杂度增加时维持高效的规划与写作，以及为用户提供并整合有效的AI辅助。现有的AI协同写作工具仅提供行内建议或有限的结构化规划，鲜有系统能支持从高层构思到精修成文的完整写作流程——该流程通常需要多层次的规划与大纲设计。本文提出TreeWriter，一种将文档表示为树结构并整合上下文AI支持的分层写作系统。TreeWriter允许作者在多个层级创建、保存和优化文档大纲，促进长文档的起草、理解与迭代编辑。内置的AI代理能够动态加载相关内容、浏览文档层级结构，并提供上下文感知的编辑建议。一项受试者内对照研究（N=12）将TreeWriter与Google Docs + Gemini在长文档编辑和创意写作任务中进行比较，结果表明TreeWriter在观点探索/发展、AI辅助效用和作者感知控制力方面均表现更优。为期两个月的实地部署（N=8）进一步证明，分层组织结构能有效支持协作写作。我们的研究结果凸显了集成AI支持的分层树状结构编辑器的潜力，并为未来平衡自动化与用户自主权的AI辅助写作工具提供了设计准则。