Quirx: A Mutation-Based Framework for Evaluating Prompt Robustness in LLM-based Software

Large Language Models (LLMs) increasingly power critical business processes, yet prompt robustness remains under-explored. Small variations—such as synonym changes or instruction reordering—can cause significant output shifts, undermining reliability in domains like customer service and finance. Existing evaluations rely on ad-hoc manual testing, limiting scalability in production environments.We present Quirx, a mutation-based fuzzing framework for systematically evaluating prompt robustness across LLM providers. Quirx applies tri-dimensional mutations (lexical, semantic, structural), executes them against target models, and measures response consistency via multi-level similarity analysis. It produces robustness scores, reveals failure patterns, and supports informed model selection.We evaluate Quirx on four models (GPT-3.5-turbo, GPT-4o-mini, Claude-3.5-Sonnet, Claude-Sonnet-4) across three tasks. Results show sentiment classification is uniformly robust (1.00), summarization is highly provider-sensitive (0.23–0.58) with Claude models 2.5× more robust than OpenAI, and SQL generation is consistently strong (0.80–1.00). Structural mutations cause 50–67% of summarization failures but have minimal effect on other tasks.Demo video: https://youtu.be/Sm3Gk2X2-vk