Prompt Caching

Prompt caching is the technique that stores reusable prompt processing artifacts so repeated prompts can skip full prefill computation - it accelerates inference for recurring instructions and template-based workloads.

What Is Prompt caching?

• Definition: Caching mechanism for prompt-level states such as tokenization outputs and KV prefixes.
• Cache Granularity: Can cache full prompts, shared prefixes, or structured prompt fragments.
• Validity Constraints: Entries depend on model, tokenizer, and prompt template versions.
• Pipeline Placement: Applied before decode token generation in serving runtimes.

Why Prompt caching Matters

• First-Token Speed: Cached prefills reduce delay before streamed output begins.
• Compute Efficiency: Removes repeated prefill work for frequently used prompts.
• Scalability: High cache-hit traffic supports larger request volumes on fixed hardware.
• Cost Management: Lower duplicate compute improves inference economics.
• UX Consistency: Repeated workflows become faster and more stable for users.

How It Is Used in Practice

• Key Strategy: Use canonicalized prompt fingerprints and context metadata as cache keys.
• Invalidation Rules: Evict or refresh entries on model updates and policy changes.
• Performance Tracking: Measure hit rate, stale incidents, and latency impact by endpoint.

Prompt caching is a practical acceleration layer in production LLM serving - effective prompt caching reduces prefill overhead and improves interactive responsiveness.