Prompt Caching Explained: How to Skip Prefill on Every API Call

Indexed: 2026-05-17

Most developers are paying full prefill cost on every single API call — and they don't have to. Here's the problem: your system prompt can be hundreds or thousands of tokens. Without caching, your LLM re-processes every single one of those tokens from scratch on each request. That's wasted compute, wasted money, and slower responses. Prompt caching fixes this. The first time you send a request, the server hashes your prefix and stores the KV tensors it computed during prefill. Every subsequent request with the same prefix hits the cache — and skips prefill entirely for those tokens. The result: up to 85% faster time to first token. Up to 90% cheaper input token cost. Same output quality. Anthropic stores the cache for 5 minutes. OpenAI for up to 1 hour. You just add a flag to your API request — no infra changes needed. If your system prompt doesn't change between calls, you should be caching it. Follow @neural.ai.flair for more AI internals broken down visually.