Four words before you start

Most confusion in the first month comes from four words used loosely. The model is the trained network that turns text into more text. It keeps nothing between calls. On every turn it sees only what is sent to it. The harness is the program around the model. Claude Code, the Claude app and ChatGPT are harnesses. They decide what the model sees, which tools it can use and what gets saved between sessions. The context window is everything the model can see on one turn, counted in tokens: your messages, its replies, the files it read, tool results and standing instructions. It has a ceiling, and answers get worse well before the ceiling as it fills. An agent is a model in a harness using tools in a loop. It acts, looks at the result and picks its own next step instead of only replying. These four words tell you where to look when something goes wrong. It forgot what you said yesterday? The model keeps nothing and the harness did not save it. Answers got worse late in a long session? The context is too full. It did something you did not ask for? The agent chose its own next step. Everything else in this track builds on these four.