Prompt Engineering Fails Quietly —  Prompt Regression Is Why

Changes to system prompts can cause hidden failures, or prompt regressions, where new instructions silently break existing functionalities in LLM applications. A proposed solution is a regression testing framework that validates prompt updates before deployment, inspired by traditional software engineering. This suite uses a golden set of queries, deterministic validation checks, and a scorer to identify "false improvements" where overall metrics improve while critical capabilities degrade. The system avoids expensive and variable LLM-as-a-judge validation, instead treating prompts as dynamic APIs that require rigorous, deterministic testing.