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When Thinking Fails: The Pitfalls of Reasoning for Instruction-Following in LLMs

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arxiv 2505.11423 v3 pith:CUUG3JOF submitted 2025-05-16 cs.CL

classification cs.CL
keywords reasoningconstraintsinstruction-followingperformancestrategiesattentionclassifier-selectivecomplex
verification ladder T0 review T1 audit T2 compute T3 formal
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Reasoning-enhanced large language models (RLLMs), whether explicitly trained for reasoning or prompted via chain-of-thought (CoT), have achieved state-of-the-art performance on many complex reasoning tasks. However, we uncover a surprising and previously overlooked phenomenon: explicit CoT reasoning can significantly degrade instruction-following accuracy. Evaluating 15 models on two benchmarks: IFEval (with simple, rule-verifiable constraints) and ComplexBench (with complex, compositional constraints), we consistently observe performance drops when CoT prompting is applied. Through large-scale case studies and an attention-based analysis, we identify common patterns where reasoning either helps (e.g., with formatting or lexical precision) or hurts (e.g., by neglecting simple constraints or introducing unnecessary content). We propose a metric, constraint attention, to quantify model focus during generation and show that CoT reasoning often diverts attention away from instruction-relevant tokens. To mitigate these effects, we introduce and evaluate four strategies: in-context learning, self-reflection, self-selective reasoning, and classifier-selective reasoning. Our results demonstrate that selective reasoning strategies, particularly classifier-selective reasoning, can substantially recover lost performance. To our knowledge, this is the first work to systematically expose reasoning-induced failures in instruction-following and offer practical mitigation strategies.

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Cited by 7 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. When Reasoning Hurts Legal Drafting: The Verbalization Bottleneck in Patent Claim Generation

    cs.CL 2026-07 conditional novelty 6.5 of 10

    Implicit CoT consistently outperforms explicit CoT on patent claim generation because verbalized reasoning abstracts details, disrupts drafting patterns, and cascades early errors.

  2. ProactiveEval: A Unified Evaluation Framework for Proactive Dialogue Agents

    cs.CL 2025-08 conditional novelty 6.0 of 10

    A unified evaluation framework for proactive dialogue agents, built with 328 synthetic environments across six domains, shows that thinking modes improve target planning but not dialogue guidance in a 22-model comparison.

  3. Structured Thoughts For Improved Reasoning And Context Pruning

    cs.CL 2026-07 conditional novelty 5.5 of 10

    Structured try/outcome SFT improves math reasoning by up to 8% over standard SFT and enables pruning ~85% of context with ~9% accuracy drop.

  4. M3CoTBench: Benchmark Chain-of-Thought of MLLMs in Medical Image Understanding

    eess.IV 2026-01 conditional novelty 5.0 of 10

    A medical-image benchmark that scores the step-by-step reasoning chains of multimodal LLMs shows current models explain poorly and chain-of-thought prompting frequently reduces diagnostic accuracy.

  5. "GenAI Defaults to Bias!" Gamify AI Literacy Through Reflections on Prompts

    cs.HC 2025-09 conditional novelty 5.0 of 10

    Playing ImaginAItion, a prompt-minimization party game, helped 30 adults recognize GenAI default biases and adjust their prompting strategies, according to pre-post survey coding.

  6. AutoTIR: Autonomous Tools Integrated Reasoning via Reinforcement Learning

    cs.CL 2025-07 conditional novelty 5.0 of 10

    AutoTIR applies GRPO with a hand-designed action reward so a 7B instruct model learns to mix search and code tools, beating tool-using baselines on ten benchmarks.

  7. On the Surprising Efficacy of LLMs for Penetration-Testing

    cs.CR 2025-07 conditional novelty 3.0 of 10

    A critical review arguing that LLMs are surprisingly effective for penetration testing because the task is largely pattern-matching, while noting serious reliability, safety, and cost barriers to autonomous use.

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