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PRESTO: Preimage-Informed Instruction Optimization for Prompting Black-Box LLMs

NeurIPS 2025 · Large language models › Language, safety and interpretability · approach: LLM unlearning and benchmarks · 0 citations (OpenAlex)

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Proposes PRESTO, which exploits the many-to-one mapping from soft prompts to instructions in a white-box LLM (preimages) via score sharing, preimage-based initialization, and score consistency regularization to optimize instructions for black-box LLMs more efficiently.

Problem

Optimizing instructions for black-box LLMs using soft prompts fed to a white-box LLM is query-inefficient because many distinct soft prompts map to the same instruction, causing redundant queries.

Approach

Treats the many-to-one soft-prompt-to-instruction mapping (preimages) as prior knowledge: scores of one evaluated instruction are shared with all soft prompts in its preimage, initialization covers the embedding space using preimage sizes and MMD, and an unsupervised loss enforces equal predictions within unscored preimages.

Key results

Measured effects reported in the paper

What was changedWhereMeasuredEffect
use PRESTO instead of ZOPOinstruction optimization · 30 instruction induction tasksaverage rank: 2.90 -> 1.97Helped
use PRESTO instead of the strongest listed baselineCoT prompting · AQUA-RATaccuracy: 0.7520 -> 0.7756Helped
use PRESTO instead of the strongest listed baselineCoT prompting · GSM8Kaccuracy: 0.9143 -> 0.9128Hurt
add score sharing (ablation)vanilla instruction optimizer · 20 instruction induction tasksaverage accuracy: 51.91 -> 59.57 (+7.66)Helped
add score consistency regularization to score sharing (ablation)instruction optimizer with score sharing · 20 instruction induction tasksaverage accuracy: 59.57 -> 61.77Helped
add preimage-based initialization to score sharing (ablation)instruction optimizer with score sharing · 20 instruction induction tasksaverage accuracy: 59.57 -> 61.82Helped
increase preimage size from 0% to 100% (ablation)PRESTO · 20 instruction induction tasksaverage accuracy: 51.91 -> 62.91Helped

Surprising findings

Limitations

Experimental setup

Datasets: instruction induction (30 tasks) GSM8K AQUA-RAT SVAMP

Benchmarks: instruction induction (30 tasks) GSM8K AQUA-RAT SVAMP

Metrics: accuracy average rank number of best-performing tasks

Baselines: APE InstructZero INSTINCT EvoPrompt ZOPO OPRO

Builds on: INSTINCT InstructZero NeuralUCB ZOPO

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