Cache Economics: The Line Item That Decides Your AI Bill

On production AI bills, the line item that decides your costs is no longer model choice — it is prompt caching. Understanding cache economics has become the highest-leverage skill in AI engineering.

Cache Economics: The Line Item That Decides Your AI Bill

Ask an engineering team what determines their AI costs and most will name their model. Increasingly, they are wrong. On production workloads, the decisive factor is prompt caching — and the teams that internalize this are repricing their systems by an order of magnitude without changing models at all.

The shape of real traffic. Production prompts are prefix-heavy: the same system prompt, the same tool definitions, the same document context, sent again and again with a small varying question at the end. List pricing charges that prefix every time. Caching stores it after the first request and serves subsequent reads at a fraction of input price — a mechanism every major vendor now supports, with read rates far below list and retention windows from minutes to an hour.

The arithmetic. A coding assistant sending a 100,000-token repository context with each request illustrates the stakes. Uncached, that prefix dominates cost at full input price. At a 90% cache-hit rate, the effective price on the prefix falls by roughly an order of magnitude — a saving no model switch can match, because no model switch changes the fact that the prefix repeats.

The engineering discipline. Cache economics reward structure: stable prompt prefixes rather than dynamically shuffled ones; context ordered so the changing parts sit at the end; awareness of retention windows so repeated requests arrive inside them. Teams that treat prompt design as cache-aware architecture see hit rates of 60–90%; teams that regenerate their prefix per request see none of it.

The strategic consequence. As vendors compete on cache pricing — and they increasingly do — the gap between cache-aware and cache-naive implementations widens into the difference between a sustainable product and an unsustainable one. Audit your prompts this month. The cheapest token is the one you never have to pay for twice.