When to choose Fast
Fast suits short explanations, rewriting and everyday questions. Give a clear goal and a small amount of relevant context. A shorter response is useful when you need an initial draft, not a full investigation. The name does not guarantee a fixed response time.
When to choose Smart
Smart is intended for complex code, analysis, planning and several constraints. Provide the inputs, expected output and important trade-offs. For either mode, turn on search when the question needs current external information. Neither mode guarantees correctness or replaces your tests.