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arXiv:2408.02442cs.CL

Let Me Speak Freely? A Study on the Impact of Format Restrictions on Performance of Large Language Models

Zhi Rui Tam, Cheng-Kuang Wu, Yi-Lin Tsai, Chieh-Yen Lin, Hung-yi Lee, Yun-Nung Chen

Abstract

Structured generation, the process of producing content in standardized formats like JSON and XML, is widely utilized in real-world applications to extract key output information from large language models (LLMs). This study investigates whether such constraints on generation space impact LLMs abilities, including reasoning and domain knowledge comprehension. Specifically, we evaluate LLMs performance when restricted to adhere to structured formats versus generating free-form responses across various common tasks. Surprisingly, we observe a significant decline in LLMs reasoning abilities under format restrictions. Furthermore, we find that stricter format constraints generally lead to greater performance degradation in reasoning tasks.

arXiv comments
18 pages
Journal reference
DOI
Subjects
cs.CLComputation and Language

On Hacker News

Linked by 6 identifiable commenters in 6 comments across 6 threads, from Aug 2024 to Oct 2026. Found in 5 HN submissions from Aug 2024 to Aug 2024. Open submission discussions.

Comments linking this paper, per year.

Discussed in comments6 threads

  1. Fixing Tool-Call Accuracy in vLLM 1 comment

    • We are also interested in this and may focus on it in a future post. They are two different perspectives: (1) What is the overall quality of LLM+serving stack+harness, and (2) How close does the behavior of our setup match the first-party provider's? In this post, we focused on…

      jesdoView on HNParent context

  2. Why does Opus 5 feel worse to work with? 1 comment

    • Keep in mind all this kind of stuff can make the model less capable. If it has to think in "plain" English, it may well be squashing quality of code etc output. I'm not sure how true this is, but when using "forced" json output it def had a big drop off in quality…

      martinaldView on HNParent context

  3. How to stop Claude from saying load-bearing 1 comment

    • things like that indeed tend to be fraught some relevant links https://arxiv.org/abs/2408.02442 https://arxiv.org/abs/2510.15061 https://arxiv.org/abs/2604.13006

      tensegristView on HNParent context

  4. Sampling and structured outputs in LLMs 1 comment

    • I was hoping to find some insights about why performance drops when using actual structured outputs. It's been a known problem. For example this paper "Let Me Speak Freely? A Study on the Impact of Format Restrictions on Performance of Large Language Models" says: >…

      visargaView on HNParent context

  5. Show HN: Best Practices for Using Structured Output from LLMs 1 comment

    • I appreciate you taking the time to read the article and share your thoughts. Your arguments consider many perspectives and dimensions. Please allow me to address each point: > You’re comparing apples to oranges - structured output (a capability) with structured output + CoT (a…

      xiaofei_View on HNParent context

  6. Structured Outputs in the API 1 comment

    • Do beware on some reasoning task, our recent work[0] actually found it may cause some performance degradation as well as possible reasoning weakening in JSON. I really hope they fix this in the latest GPT-4o version. [0] https://arxiv.org/abs/2408.02442

      blackcat201View on HNParent context

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