Patchbook: LLMs are just APIs

Short, simple chapters that serve as a conceptual upgrade for software engineers.

The central thesis is simple: working with LLMs is ordinary software engineering. The model is a stateless engine, the host controls everything, and an agent is just a loop. The chapters build those mental models one at a time, then close with what a single call can do and the gotchas that come with it.

Let us begin with Chapter 0: Introduction.

• Chapter 0: IntroductionWhat this book is, what you will learn, and who it is for.
• Chapter 1: It sounds more complicated than it actually isAI agents are a handful of primitives, not magic.
• Chapter 2: An agent is a for loopChatGPT, Claude Code, and other agents are just while loops.
• Chapter 3: How to send a chat request to an LLMTalking to an LLM is just a POST request with a sender and a message.
• Chapter 4: LLMs are statelessLLMs only remember the information you send in the current request.
• Chapter 5: System PromptsHow to shape LLM behavior with system prompts.
• Chapter 6: Context WindowsHow context windows bound LLM working memory, attention, and cost.
• Chapter 7: What can a single call do?How a single stateless LLM call performs classification, extraction, transformation, and evaluation.
• Chapter 8: GotchasCommon pitfalls when working with single LLM API calls, from hallucinations to format drift.