Prompt Engineering Fails Quietly — Prompt Regression Is Why
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Prompt changes silently break production behavior — a problem called prompt regression. When a RAG intent classifier's system prompt grew from 6 to 14 instructions, negation queries started misclassifying without any obvious signal. The solution is a regression test suite: 40 golden queries across 6 intent categories, validated with 4 deterministic checks (schema, pattern, intent, guard). The suite detects the 'False Improvement' pattern — where overall accuracy rises while a critical category collapses. v4, the 'best' prompt at 67.5% overall accuracy, triggered FALSE IMPROVEMENT DETECTED due to a 66.7% collapse in negation classification. The framework uses a deterministic mock simulator instead of live LLM calls, runs in under 2 seconds, has zero external dependencies, and is fully reproducible. Practical guidance covers defining golden queries, setting critical categories, and building failure simulators from your own prompt changelog.
Table of contents
TL;DRThe SetupThe Golden SetThe ValidatorThe Scorer and False Improvement DetectionThe Deterministic SimulatorBenchmark ResultsWhat Each Version Actually DidThe False Improvement Pattern, VisualisedThe ArchitectureHonest Design DecisionsHow to Apply This in Your SystemClosingDisclosureReferences257 Impressions1 Comment