LongBench v2: Towards Deeper Understanding and Reasoning on Realistic Long-context Multitasks
Written for the Research library (claude-opus-5-5) from the paper text, in our own words: the license "arXiv — non-exclusive licence to distribute" does not allow redistributing the text itself. Original — arxiv.org/pdf/2412.15204
Summary
Most long-context benchmarks reward shallow skills. Answers can often be pulled straight from the text, tasks are frequently synthetic, and scoring relies on fragile metrics like F1 or ROUGE. LongBench v2 targets genuine comprehension and reasoning over very long inputs instead. The authors gathered 503 English multiple-choice questions, written by 97 university-educated annotators, with contexts between 8k and 2M words. The questions span six realistic categories split into 20 subtasks. Each item went through two filters. It was dropped if three long-context LLMs all solved it, and it also had to defeat an expert reviewer working under a three-minute limit. Even so, human experts reached only 53.7% given 15 minutes. GPT-4o answering directly scored 50.1%, while o1-preview, which reasons at length before answering, reached 57.7% and beat the human baseline. The benchmark therefore suggests that spending more compute at inference time is a promising path for long-context reasoning. It also shows that retrieval alone cannot solve these questions.
Key points
- The benchmark has 503 four-option questions, with a median length of about 54k words and a mean of about 104k. They cover six categories: single-doc QA, multi-doc QA, long in-context learning, long-dialogue history, code repository, and long structured data.
- Explicit reasoning helps:
- Chain-of-thought prompting adds about 3.4% on average for open-source models.
- o1-preview beats GPT-4o by 7.6%, and o1-mini beats GPT-4o-mini by 8.5%.
- Context length matters for the model–human gap:
- On inputs under 32k, the best model exceeds humans by 15.4%.
- In the 32k–128k range, the best model trails humans by 5.6%.
- The biggest shortfall appears on structured data, such as tables and knowledge graphs.
- RAG was tested with 512-token chunks ranked by Zhipu Embedding-3:
- Qwen2.5 and GLM-4-Plus stop improving once retrieved context exceeds 32k.
- Only GPT-4o benefits from 128k of retrieved text, and it still falls 0.6% short of its full-context score.
- Without the context, most models score around 25–30%, close to random guessing, which indicates little reliance on memorized content.
Cited by
- DeepSeek-V4: Towards Highly Efficient Million-Token Context Intelligence
- GLM-4.5: Agentic, Reasoning, and Coding (ARC) Foundation Models
- Kimi K2: Open Agentic Intelligence
- Siren's Song in the AI Ocean: A Survey on Hallucination in Large Language Models
Cite
@misc{bai2024longbench,
title = {{LongBench v2: Towards Deeper Understanding and Reasoning on Realistic Long-context Multitasks}},
author = {Bai, Yushi and Tu, Shangqing and Zhang, Jiajie and Peng, Hao and Wang, Xiaozhi and Lv, Xin and Cao, Shulin and Xu, Jiazheng and Hou, Lei and Dong, Yuxiao and Tang, Jie and Li, Juanzi},
year = {2024},
doi = {10.48550/arxiv.2412.15204},
eprint = {2412.15204},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2412.15204}
}References
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