Caching is one of the fastest ways to reduce LLM serving cost, but the word hides two different controls. Semantic caching and prompt caching solve related problems at different points in the inference path.
For technical buyers and operators, the short answer is simple: semantic caching looks at whether a user request is meaningfully similar to previous work, while prompt caching reuses repeated prompt prefixes and context inside model execution.
What semantic caching does
Semantic caching stores answers or intermediate results for requests that are meaningfully similar. It can catch repeated questions even when the wording changes.
This is useful for support workflows, internal knowledge assistants, policy questions, documentation lookups, and repeated operational analysis where many users ask for the same thing in different words.
What prompt caching does
Prompt caching reuses stable prompt prefixes such as system instructions, policy blocks, tool descriptions, retrieval context, or repeated document context.
It is especially valuable when requests share long context windows. The user question may be new, but the expensive setup around that question does not need to be recomputed each time.
Where each cache can fail
Semantic caching can be risky when freshness, permissions, or exact wording changes the correct answer. It needs policy checks, invalidation rules, and confidence thresholds.
Prompt caching can underperform when prompts are unstable, retrieval context changes constantly, or application code builds prompts in inconsistent order.
How to combine them
A strong serving layer checks semantic cache eligibility first, verifies policy and freshness, then uses prompt caching when model execution is still needed.
The combined goal is to minimize repeated GPU work while keeping answer quality, access control, and auditability intact.
Key takeaways
- Semantic caching reduces repeated answers.
- Prompt caching reduces repeated context computation.
- Enterprise cache policy must include freshness, permissions, and audit rules.