Every word in the system prompt affects tens of thousands of daily generations across eight models. The cost of a bad change is high, and you can't intuit what a prompt tweak does to Claude versus GPT versus Gemini. I built a comparison pipeline to make it visible. Run a test prompt against all models simultaneously, get side-by-side output at desktop and mobile viewports, see exactly what that change did.Testing revealed that models respond very differently to identical instructions. Claude benefits from explicit design principles. GPT responds better to example-driven guidance. I separated the prompt into model-specific paths.For mobile, the instinct is to add more layout rules. I went the other direction. Fewer constraints, more spatial awareness. The result was better mobile output across every model.