Language Model Builder is a free Mac app that guides users through constructing a small language model from the ground up. It functions as an interactive textbook, letting users train a tokenizer, select data, pre-train, fine-tune, and chat with a model they built themselves. The tool requires no coding, math, or machine learning experience, making it accessible to non-programmers. Interactive playgrounds throughout the curriculum let users explore tokenization, embedding space, attention mechanisms, and model training hands-on.