Input tokens dominate costs in most AI applications. And the biggest chunk of input tokens is usually the system prompt, the part that never changes between requests.

You are paying the model to read the same instructions over and over again, like handing a restaurant chef the full recipe book every time someone orders a burger.

The good news is that prompt optimization is probably the highest-ROI activity in AI engineering. Unlike switching models or redesigning your architecture, you can often cut costs 40-60% in a single afternoon by making your prompts leaner, smarter, and more cache-friendly.

This chapter gives you a systematic approach to doing exactly that.