Prompt Caching at the Edge: Using CloudFront Functions and Lambda to Speed Up Claude Calls

LLM APIs like Claude feel snappy—until latency spikes hit your users. By caching prompt‑response pairs right at the edge, you can cut round‑trip time to milliseconds. This post shows you how to make that happen with CloudFront Functions and a Lambda origin. Why Prompt Caching Matters for LLM‑Powered Apps When a user types a question, your front‑end sends the text to an LLM (large language model) API, waits for the model to generate a reply, and then shows the answer. The user experience is dominated by two things: Network latency – the time it takes for the request to travel from the user’s browser to the API endpoint and back. Model compute time – how long the LLM needs to think. Even if the model itself is fast, the network hop to the provider’s data center can add 100 ms – 300 ms, and sometimes more during traffic spikes. For a chat UI that refreshes every few seconds, those extra milliseconds feel like a noticeable lag. Prompt caching means storing the exact prompt (the user’s message) together with the response (the model’s answer) in a fast lookup table. If the same prompt arrives again within a short window, you can return the cached answer instantly, without touching the LLM provider at all. In plain English: Think of the cache as a “sticky note” on the receptionist’s desk. If someone asks the same question twice, the receptionist can hand them the note instead of calling the manager again. Freshness vs. Speed LLM responses are not immutable—new data, temperature settings, or model updates can change the answer. A short time‑to‑live (TTL) of a few minutes gives you a good trade‑off: most users repeat recent prompts, but you still get new answers after a reasonable window. Setting Up a CloudFront Distribution with an Edge Key‑Value Store The big picture Edge KV store – a tiny key‑value database that lives on every CloudFront edge node. CloudFront Function – a lightweight JavaScript snippet (max 2 MB) that runs on every request before it reaches the origin. It