Semantic Caching for LLMs: Stop Paying for the Same Answer Twice

A customer-support bot answers "what is your return policy?" thousands of times a month. Without a cache, every one of those is a full-price model call for an answer you already generated. Caching turns repeated and near-repeated questions into near-zero-cost responses — and on the right workload it is the single easiest cost win in an LLM stack.

Exact cache vs. semantic cache

• Exact cache — the same prompt returns the stored response instantly, at zero upstream token cost. Best for deterministic, repeated calls.
• Semantic cache — a near-duplicate prompt (above a similarity threshold) reuses a previous answer. "What's your refund policy?" and "how do returns work?" can share one cached response.

Which workloads actually benefit

Caching pays off where questions repeat and answers are stable:

• Customer support and FAQ deflection (high repeat rate).
• RAG over a slow-changing corpus (similar queries, same context).
• Classification and extraction on recurring input shapes.

It does not pay off — and you should skip it — for personalized responses, real-time data, or anything where two similar prompts must not share an answer.

The similarity-threshold trade-off

Semantic cache lives and dies by its threshold. Too loose and you serve a subtly wrong answer to a different question; too tight and you miss real duplicates. Start conservative, measure false reuse on a sample of traffic, then loosen deliberately.

One line to turn it on

In Autrace, caching is opt-in per request via the plugins array — no code refactor:

{
  "model": "gpt-4o-mini",
  "messages": [{ "role": "user", "content": "..." }],
  "plugins": [{ "id": "cache" }, { "id": "semantic-cache" }]
}

Every hit is tracked on the dashboard as tokens (and money) not spent. Caching is skipped automatically when reversible PII tokens are present, so a redacted value never leaks across cache entries. Semantic cache is available on the Growth plan and above.

If you run a support or FAQ workload, this is the first thing to switch on. Start free → or see how it fits support copilots.