We explain how the field of Large Language Model (LLM) application development is evolving beyond simple "prompt engineering" to a more comprehensive approach called "context engineering." This shift emphasizes not just crafting user instructions, but systematically designing and managing the entire information payload (the context window) an LLM processes, including dynamic elements like Retrieval-Augmented Generation (RAG), tool definitions, and conversational history. We argue that this complex "thick layer of non-trivial software" for orchestration, model dispatching, verification, and operational controls is where true innovation and competitive advantage lie, distinguishing robust LLM-native applications from mere "ChatGPT wrappers." Ultimately, it redefines building with LLMs as a software architecture challenge for non-deterministic systems, rather than solely a linguistic one.