# What is Prompt Cache? Definition & Technical Mechanics

> **Source:** [SPIDITS AI AI Glossary](https://spidits.com/ai-glossary)  
> **Category:** Hardware & Infrastructure  
> **Canonical Citation:** [https://spidits.com/ai-glossary/prompt-cache](https://spidits.com/ai-glossary/prompt-cache)

## Definition
Prompt Caching is an optimization technique that stores prefix token representations of long prompts in memory, allowing subsequent API queries with the same prefix to reuse states, reducing latency and cost.

## Architectural Mechanics & Deep Dive
Prompt Caching is an API-level optimization that caches the key-value states of long prompt prefixes in memory. When a new user request shares the same prefix (such as system prompts, documents, or early chat history), the server reuses the cached states, reducing pre-fill compute requirements, latency, and costs.

## Industry Applications & Real-World Use Cases
Long conversation context management, document question-answering systems, and agent tool setups

## Frequently Asked Questions

### How does prompt caching lower API costs?
API providers charge significantly less for cached tokens because they do not require GPU computations for pre-filling attention states.

### What is a typical prompt cache trigger?
Static system instructions, uploaded document context, or early conversation turns that are shared across requests.

## Related Terminology
- [kv-cache](https://spidits.com/ai-glossary/kv-cache)
- [attention-sink](https://spidits.com/ai-glossary/attention-sink)
- [token](https://spidits.com/ai-glossary/token)

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*From the SPIDITS AI Knowledge Base (300+ terms indexed). Explore full technical definitions: [https://spidits.com/ai-glossary](https://spidits.com/ai-glossary)*