LLMs often generate seemingly valid answers to flawed or ill-posed inputs. This is not due to missing knowledge: under discriminative prompting, the same models can mostly identify such issues, yet fail to reflect this in standard generative responses. This reveals a fundamental know-act gap between discriminative recognition and generative behavior. Prior work largely characterizes this issue in narrow settings, such as math word problems or question answering, with limited focus on how to integrate these two modes. In this work, we present a comprehensive analysis using FaultyScience, a newly constructed large-scale, cross-disciplinary benchmark of faulty scientific questions. We show that the gap is pervasive and stems from token-level autoregression, which entangles task selection (validate vs. answer) with content generation, preventing discriminative knowledge from being utilized. To address this, we propose DeIllusionLLM, a task-level autoregressive framework that explicitly models this decision. Through self-distillation, the model unifies discriminative judgment and generative reasoning within a single backbone. Empirically, DeIllusionLLM substantially reduces answer-despite-error failures under natural prompting while maintaining general reasoning performance, demonstrating that self-distillation is an effective and scalable solution for bridging the discriminative-generative know-act gap
On a balanced dataset, these metrics (accuracy = 95%, precision = 90%, recall = 85%) cannot all hold simultaneously. At least one is invalid, so any F1 derived is unreliable.
Table 1: Example where GPT4 fails in generative mode without hints but recognize the issues in discriminative mode.
Count
Ratio (%)
Discipline
Physical Science
3014
19.07
Biological Science
2776
17.57
Earth Science
2404
15.21
Mathematics
2129
13.47
Social Science
2070
13.10
Table 2: Statistics of FaultyScience .
Model
P−hintGen
P+hintGen
PDis
Mixtral-8x7B
10.2
27.8
75.2
Llama3.3 70B
14.0
24.5
90.8
Qwen2.5-72B
10.8
40.1
95.0
GPT4
34.9
48.6
88.1
Table 3: Answer to Q1.1 –performance (%) of existing LLMs in recognizing the issues in the questions of FaultyScience under different prompt style.
Figure 1: Overview of the proposed DeIllusionLLM method.
Model
FaultyScience ( P−hintGen )
BBH
GPQA
Mixtral-8x7B
10.2
16.58
30.13
Llama3.3 70B
14.0
17.11
21.88
Qwen2.5-72B
10.8
42.78
34.15
DeIllusionLLM (self-distilled from Qwen)
67.8
41.71
33.93
GPT-4
34.9
69.50
34.20
DeIllusionLLM (distilled from GPT)
42.7
68.98
38.80
Table 4: DeIllusionLLM vs. baseline LLMs in three benchmarks.
Model
Earth
Physics
Social
Math
Bio
Eng.
Chem.
CS
mean
GPT-4
37.6
18.4
56.0
33.6
31.2
58.4
24.0
7.2
33.3
DeIllusionLLM
60.8
19.2
56.8
30.4
46.4
72.0
22.4
8.8
39.6
Table 5: Model performance, under P−hintGen , on different disciplines of FaultyScience .
Figure 2: Mode [D] percentages for each dataset.
Category
FaultyScience ( P−hintGen )
BBH
GPQA
Aligned (mode = response)
[D] + response_D
51.70%
5.35%
0.00%
[G] + response_G
20.80%
60.43%
80.13%
Misaligned (mode = response)
[D] + response_G
2.40%
26.74%
19.87%
[G] + response_D
25.10%
7.48%
0.00%
Table 6: Mode–response alignment distribution across datasets.