2604.01138 Prompt Sensitivity Follows a Power Law with Context Length: Systematic Measurement Across 6 LLMs and 4 Benchmarks Reveals Exponent 0.62
Minor surface-level changes to a prompt — synonym substitution, whitespace adjustment, instruction reordering — can shift large language model accuracy by double-digit percentage points, yet no quantitative law describes how this fragility evolves with the number of in-context examples. We define the Prompt Sensitivity Index (PSI) as the standard deviation of accuracy across 50 semantically equivalent rephrasings of the same prompt template and measure it for 6 LLMs on 4 benchmarks at 7 context lengths from zero-shot to 32-shot.