Lesson 1 · 9 min
The context window is the computer
Why the context window is the fundamental resource constraint in LLM engineering — and the cost model that governs every decision.
The model only knows what you put in front of it
An LLM has no persistent memory. No database. No state between calls. Every message, every document, every fact the model needs to reason about must fit inside the context window — the fixed-size input buffer that the model processes as one unit.
Everything you put in costs tokens. Everything you leave out is invisible to the model. Context window engineering is the discipline of deciding what goes in, what stays out, and how to make the most of the space you have.