Prompt Caching: How to Cut Repeat-Prompt Costs
Prompt caching can cut 50–90% of input token cost on repeated context. Learn how it works and when it pays off.
If your app sends the same long system prompt or document on every request, you're paying to re-send it every time. Caching fixes that.
Why repeated context is expensive
Many apps send the same content on every call:
- A long system prompt with instructions
- A retrieved document or knowledge base chunk
- A conversation history that rarely changes
Every one of those tokens is billed again on the next request.
How prompt caching works
Providers let you mark a stable prefix as cached. The first request pays full input price; subsequent requests that reuse the same prefix are billed at a much lower cached-token rate.
Savings commonly land between 50% and 90% of the input cost for that prefix.
When caching pays off
Caching is strongest when:
- The same long context repeats across many requests
- Requests are frequent (high call volume)
- The prefix is stable for minutes or more
It barely helps if every request is unique.
Practical setup
- Put stable content (system prompt, reference docs) at the top of the context
- Keep the cached prefix unchanged between calls
- Measure the cache hit rate, not just the rate discount
Quick checklist
- Identify repeated prefixes in your prompts
- Enable caching where the prefix is stable
- Track cache hit rate weekly
Related reading
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