REVIEW 4 major objections 4 minor 28 references
Prior Prompt Engineering for Reinforcement Fine-Tuning
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Prior prompt choices during reinforcement fine-tuning steer model behavior, and a null-example prompt matches or beats reasoning prompts on average.
desk verdict A genuinely new empirical axis for RFT—systematic prior prompt comparison with a surprising null-example result—but the design bundles prompt wording with format reward, so the causal claim outruns the evidence. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
The main result is that all five trained models beat their non-trained counterparts that are simply prompted at inference time. More surprisingly, the model trained with an examples instruction, which asks the model to list illustrative examples before solving, achieved the highest average gain, slightly ahead of the usual reasoning prompt. The models also developed visibly different styles: the planning-trained model produces a numbered plan, the code-trained coder model writes Python, and so on. However, the training setup includes a formatting reward that requires the exact output tag structure, so the observed behavioral differences are not purely due to the prompt wording. All numbers come from a single run with no error bars, and the headline gaps are small, within a few questions on benchmarks with hundreds or fewer items. Smaller models also failed to replicate the benefit, sometimes collapsing to tag-only outputs.
The paper positions prior prompt engineering as a cheap and overlooked lever in RFT.
Extended reading notes
Core claim
The central assertion is that different prior prompts used during reinforcement fine-tuning lead language models to internalize distinct behavioral styles, and that the null-example (examples) prior prompt achieves the largest average performance gain and the highest improvement on AIME2024 and GPQA-Diamond, surpassing the commonly used reasoning prompt. If correct, pPE is an effective and understudied axis for RFT.
Load-bearing premise
The load-bearing premise is that the observed behavioral and performance differences are caused by the semantic content of the prior prompt, not by the co-varying format reward that gives 0.5 reward for producing exactly the expected XML tags. The experiments vary pPE and format reward jointly (Section 3.2), so if the format reward alone drives the tag-structured behaviors, the conclusion that prompt wording shapes post-RFT behavior would not follow.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces prior prompt engineering (pPE) for reinforcement fine-tuning, adapting five inference-time prompting strategies (chain-of-thought, plan-and-solve, program-of-thought, generated knowledge, and null-shot) into prior prompts with corresponding XML tags. It trains Qwen2.5-7B with GRPO on a math dataset under each pPE, compares to iPE-prompted baselines and a No PP RFT baseline, and evaluates on AIME, AMC, MATH-500, HumanEval+, and GPQA-Diamond. The paper reports that all pPE models improve over iPE on average, that the null-example pPE achieves the largest average gain and best GPQA performance, and that different pPE approaches induce distinct behavioral styles as measured by an LLM-based classifier. Generalization experiments on Qwen2.5-3B, Llama 3.1-8B, and Qwen2.5-Coder-7B are also reported.
Significance. If the causal attribution is established, the paper identifies a practical and understudied axis for RFT, with a counterintuitive result (null-example prior outperforms reasoning prior) and implications for steering model behavior and test-time compute. The paper is transparent: it ships the training script, hyperparameters, full prior-prompt templates, and qualitative examples, and it reports a negative result (iPE trends do not transfer to pPE) that is useful. However, the central claim currently rests on a confounded design and unreplicated small-benchmark differences, so the significance is conditional on additional control experiments.
major comments (4)
- [3.2, D.2] The experimental design varies the prior-prompt instruction and the format reward jointly. Section 3.2 defines a format reward of 0.5 for exactly one pair of the expected XML tag (<think>, <plan>, <code>, <knowledge>, or <examples>), and Section D.2 states this reward is 'crucial' for structured outputs. Each pPE condition therefore differs from the others both in instruction wording and in the specific tag that is rewarded, and the No PP baseline removes both components. The behavioral differences in Figures 6 and 7 could consequently be caused by RL pressure to emit a particular tag token rather than by internalization of the instruction. The reward-hacking collapse in Section 4.3 (Qwen2.5-3B with <plan> pPE producing 9-token responses that collect only the format reward; Table 6 and Figures 25–26) shows the format reward alone can shape output structure. Please add control conditions that fix the format reward and tag structure while varying only the instruction wording, or fix the instruction while varying the tag and reward.
- [D.4.1] All evaluations are single runs with a fixed random seed (Section D.4.1), and the benchmarks are small: AIME has 30 problems, AMC has 83, and GPQA-Diamond has 198. The headline differences are within plausible noise: the Examples pPE average (47.60) exceeds Think (46.99) by 0.61 points; AIME is tied at 20.00 for Think, Plan, and Examples; and the GPQA difference (30.81 vs. 28.28) corresponds to about 5 questions out of 198. Without repeated seeds or bootstrap confidence intervals, the superiority of the null-example pPE over the reasoning pPE, and the ranking of pPE variants generally, are not established. Please report multiple seeds or statistical uncertainty estimates for the main Qwen2.5-7B comparison.
- [D.4.2, Figures 18–19] The classification prompts for the four fundamental behaviors are swapped in Section D.4.2. Figure 18, captioned 'Subgoal Settings Classification Prompt,' asks the model to detect backward chaining and gives a backward-chaining example, while Figure 19, captioned 'Backward Chaining Classification Prompt,' asks for explicit subgoal setting and gives a subgoal example. As a result, the subgoal-setting and backward-chaining counts in Figure 6 and Table 7 measure the opposite constructs. This undermines the qualitative evidence for distinct behavioral styles and must be corrected and the classification re-run.
- [D.4.2] The adapted LLM-based behavior classifier (gpt-4.1-mini) is used to quantify behavior differences that are a central claim of the paper, but no accuracy, agreement, or calibration check is reported for the new prompts. Given the prompt-swap error above and the very small counts in the generalization tables (e.g., Table 8), the reliability of the behavior counts is uncertain. Please validate the classifier on a hand-labeled sample or report inter-annotator agreement.
minor comments (4)
- [Abstract, Introduction] The abstract and introduction state that 'all pPE-trained models surpass their iPE-prompted counterparts,' but Table 1 shows this holds only for average performance; per-benchmark exceptions exist (e.g., HE+ Think pPE 70.10 vs. iPE 75.00; GPQA Knowledge pPE 21.72 vs. iPE 24.24). Please qualify the claim.
- [Abstract] The abstract states that null-example pPE achieves 'the highest improvement on AIME2024,' but Table 1 shows Think, Plan, and Examples all reach 20.00 on AIME, and the No PP baseline reaches 26.67. The claim should be restricted to average performance and GPQA.
- [Figure 4] The reward-level comparison in Figure 4 between No PP and pPE is confounded by different reward scales: No PP uses an accuracy reward maxed at 1.0, while pPE uses accuracy 0.5 plus format 0.5. Normalize or rescale before interpreting the lower reward level of No PP.
- [3.2] Section 3.2 contains several language errors, including 'math-only training datasets provides' and 'which inconclusive in implementation standards of the reward function'; these should be copy-edited.
Assumptions & free parameters
free parameters (3)
- format reward weight =
0.5
- training sample count =
10000
- KL coefficient =
1e-6
assumptions (3)
- domain assumption Math-only RFT with verifiable rewards is a representative setting for studying pPE.
- domain assumption gpt-4.1-mini behavior classifications are reliable proxies for model behavior.
- domain assumption The format reward does not confound the pPE effect.
Cite this review
Pith. "Pith review of Prior Prompt Engineering for Reinforcement Fine-Tuning." pith.science (2026). https://pith.science/paper/YUNIGC5C
@misc{pith2026250514157,
author = {Pith},
title = {Pith review of: Prior Prompt Engineering for Reinforcement Fine-Tuning},
year = {2026},
howpublished = {\url{https://pith.science/paper/YUNIGC5C}},
note = {Machine review of arXiv:2505.14157}
}
read the original abstract
This paper investigates prior prompt engineering (pPE) in the context of reinforcement fine-tuning (RFT), where language models (LMs) are incentivized to exhibit behaviors that maximize performance through reward signals. While existing RFT research has primarily focused on algorithms, reward shaping, and data curation, the design of the prior prompt--the instructions prepended to queries during training to elicit behaviors such as step-by-step reasoning--remains underexplored. We investigate whether different pPE approaches can guide LMs to internalize distinct behaviors after RFT. Inspired by inference-time prompt engineering (iPE), we translate five representative iPE strategies--reasoning, planning, code-based reasoning, knowledge recall, and null-example utilization--into corresponding pPE approaches. We experiment with Qwen2.5-7B using each of the pPE approaches, then evaluate performance on in-domain and out-of-domain benchmarks (e.g., AIME2024, HumanEval+, and GPQA-Diamond). Our results show that all pPE-trained models surpass their iPE-prompted counterparts, with the null-example pPE approach achieving the largest average performance gain and the highest improvement on AIME2024 and GPQA-Diamond, surpassing the commonly used reasoning approach. Furthermore, by adapting a behavior-classification framework, we demonstrate that different pPE strategies instill distinct behavioral styles in the resulting models. These findings position pPE as a powerful yet understudied axis for RFT.
Figures
Figures from the paper (28 more)
Reference graph
Works this paper leans on
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[1]
Start by dividing 196 by the smallest prime number, which is 2: 196÷2 = 98 So, 196 = 2 x 98
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[2]
Next, divide 98 by 2: 98÷2 = 49 So, 196 = 2 x 2 x 49
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[3]
However, 49 is not divisible by 3 either
Now, 49 is not divisible by 2, so we move to the next smallest prime number, which is 3. However, 49 is not divisible by 3 either. The next prime number is 5, and 49 is not divisible by 5. The next prime number is 7: 49÷7 = 7 So, 196 = 2 x 2 x 7 x 7
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[4]
We can write this in exponential form: 196 = 22 ×7 2 To find the number of divisors, we use the formula for the number of divisors based on the prime factorization. If a number n has a prime factorization of pe1 1 ×p e2 2 × · · · ×pek k , then the number of divisorsD(n)is given by: D(n) = (e1 + 1)(e2 + 1)· · ·(ek + 1) For 196, the exponents in the prime f...
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Let’s think step by step
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Let’s work backwards from the target. 24/2 = 12. So, 12*2=24
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Identify the prime factorization of 196
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</plan> <answer> To find the number of positive whole-number divisors of 196, we first need to determine its prime factorization
Use the formula for finding the number of divisors from the prime factorization. </plan> <answer> To find the number of positive whole-number divisors of 196, we first need to determine its prime factorization
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[18]
- 98 is also even, so it is divisible by 2: 98÷2 = 49
Prime factorization of 196: - 196 is even, so it is divisible by 2:196÷2 = 98. - 98 is also even, so it is divisible by 2: 98÷2 = 49. - 49 is not even, but it is divisible by 7:49÷7 = 7. - 7 is a prime number. So, the prime factorization of 196 is2 2 ×7 2
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×pek k , then the number of divisors ofnis given by(e 1 + 1)(e2 + 1)
Using the formula for finding the number of divisors from the prime factorization: If a number n has a prime factorization of pe1 1 ×p e2 2 ×. . .×pek k , then the number of divisors ofnis given by(e 1 + 1)(e2 + 1). . .(ek + 1). For 196, the exponents in the prime factoriza- t...
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196 is even, so it is divisible by 2: 196÷2 = 98 So, we have196 = 2×98
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98 is also even, so it is divisible by 2: 98÷2 = 49 So, we have196 = 2×2×49
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[22]
49 is not even, but it is divisible by 7: 49÷7 = 7 So, we have196 = 2×2×7×7
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[23]
The prime factorization of 196 is2 2 ×7 2
7 is a prime number, so we stop here. The prime factorization of 196 is2 2 ×7 2. To find the number of positive divisors, we use the formula that involves adding 1 to each of the exponents in the prime factorization and then multiplying the results: (2 + 1)(2 + 1) = 3×3 = 9 Th...
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[24]
Start by dividing 196 by the smallest prime number, which is 2: 196÷2 = 98 So, 2 is a prime factor
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Divide 98 by 2 again: 98÷2 = 49 So, 2 is a prime factor again
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[26]
However, 49 is not divisible by 3 either
Now, 49 is not divisible by 2, so we move to the next smallest prime number, which is 3. However, 49 is not divisible by 3 either. The next prime number is 5, but 49 is not divisible by 5. The next prime number is 7: 49÷7 = 7 So, 7 is a prime factor
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[27]
Finally, 7 is a prime number itself, so we divide it by 7: 7÷7 = 1 So, 7 is a prime factor again. Thus, the prime factorization of 196 is: 196 = 22 ×7 2 To find the number of positive whole-number divisors, we use the formula for finding the number of divisors from the prime f...
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[28]
</examples> <answer> 9 </answer> Figure 42: Response from Qwen2.5 7B trained with the <examples> prior prompt
Therefore, the number of positive divisors is: (2 + 1)(2 + 1) = 3×3 = 9 So, 196 has9 positive whole-number divisors. </examples> <answer> 9 </answer> Figure 42: Response from Qwen2.5 7B trained with the <examples> prior prompt. The model introduces illustrative examples to sup...
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Reviewed August 7, 2026 · model on record in the stance chip above.
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