Your LLM Bill Jumped After You Added Context: Find the Cache Miss Before You Downgrade the Model

If your LLM API spend climbed after you added retrieval, a longer system prompt, or tool definitions, the cause is almost always input tokens being reprocessed at full price on every request — not the model you picked. Check the cache fields in the response usage object first: if cached reads are zero across repeated requests that share a prefix, you have a bug, not a pricing problem. Fixing prefix stability and breakpoint placement is free; downgrading the model costs you quality. The first thing to do is stop reasoning about the bill and start logging per-request token accounting. On the Ant