Cornell study: JSON mode cuts answer diversity across 44 language models
omarsar0 · x · 2026-07-23
Structured output can narrow model answers more than expected
A Cornell paper tests whether asking for structured output changes not just formatting, but the content distribution itself. Across 44 language models and 31 broad-answer prompts, the authors find that a simple instruction like “Reply with JSON only” measurably reduces diversity.
Key findings
- On an open-ended pick-a-word prompt, asking for JSON shifts the modal answer from 41% to 64% and cuts distinct answers from 52 to 36.
- The collapse appears for JSON and XML, both formats models are trained to emit, but not for YAML and CSV.
- Forcing a schema at the decoder does not compress answers further, suggesting the narrowing happens when the model forms the response, not when the decoder enforces structure.
- In a within-run re-sample, 53% of stable chat defaults shift under JSON mode, often back toward the majority answer.
- The effect survives in 28 of 31 categories and shows a register-specific gradient: compression is present in answer-delivery formats, absent for YAML/CSV, and reversed for a generic bracket wrapper.
Why it matters
If your agent or pipeline relies on JSON mode, you may be sampling from a smaller and more homogeneous pool than the chat surface suggests.
More from coding & agent
- Dev builds interactive 3D product experience with GPT-6 Astra + Hyper3D Rodin — nikola_mr64990 · 2026-09-11
- Codex tip: use Sol with Astra and Luna sub-agents to save usage — pvncher · 2026-09-11
- agents-best-practices: a provider-neutral Agent Skill for designing and auditing agentic harnesses — tom_doerr · 2026-09-11
- Cognition's SWE-2 uses a KKT duality argument in RL to shift the effort Pareto curve — YouJiacheng · 2026-09-11
- First-ever Three.js Conference lands in Paris, with a panel on AI-shortened design workflows — OdinLovis · 2026-09-11
- Data engineering, not agent frameworks, is the real bottleneck for enterprise AI agents — dhruv2038 · 2026-09-11