Simon Willison’s Weblog

19th December 2025

Sam Rose explains how LLMs work with a visual essay. Sam Rose is one of my favorite authors of explorable interactive explanations - here's his previous collection.

Sam joined ngrok in September as a developer educator. Here's his first big visual explainer for them, ostensibly about how prompt caching works but it quickly expands to cover tokenization, embeddings, and the basics of the transformer architecture.

The result is one of the clearest and most accessible introductions to LLM internals I've seen anywhere.