Mid-level interview question
When does a prompt cache make the input bill higher rather than lower?
Model answer
When writes aren't followed by reads. Anthropic charges 1.25 times the base input price for a 5-minute cache write and 0.1 times for a read (as of 2026-10-21). A 2,000-token prefix sent once costs 24% more than not caching. Rare traffic is the same: the entry expires, so every request pays to write it again. Shared by 10 requests, a 10,000-token prefix saves 77%.
Practise the rest
6 Caching questions, 492 in all, each with a model answer. Free.
More Caching interview questions
- Name three caches that reuse work when serving an LLM, and the scope of each.
- Someone added today's date as the first line of every prompt, and the prompt-cache hit rate dropped to zero. Why, and how should prompts be laid out?
- A code assistant sends a 2,000-token system prompt and writes long answers. Users complain answers take too long to finish. Will a prefix cache fix it?
- Product wants a semantic answer cache in front of your support bot to cut cost. What risks would you raise, and what would you measure first?
- What is system-prompt caching, and what does it actually save?