Silent Production Breaks: Why LLM Model Upgrades Need Regression Testing

Model upgrades pose a silent risk to production systems: tests pass and syntax validates, but downstream logic breaks invisibly. This article dissects how a GPT-4.1 migration caused silent field renames that corrupted every processed ticket for two weeks undetected. It identifies three failure modes (format drift, reasoning regression, tone shift) and introduces promptfork, a dedicated regression testing tool to catch AI-specific failures before they reach customers.