It began as an obscure bug-checking program at Microsoft Research developed by a lone computer engineer named Leo de Moura. Then an unlikely crew of mathematical misfits caught wind of it and began to adopt it with messianic zeal. Their goal was to create a truth machine that could provide the rarest of all commodities in life: a complete, 100 percent guarantee that something is true. Its name: Lean. As the movement grew and strengthened the program’s capabilities, it drew in two of the world’s most prominent mathematicians: Peter Scholze and Terence Tao. Google DeepMind, Meta AI, and other tech firms started using the program to supercharge computer reasoning. Now it’s remaking the multi-thousand-year history of how mathematicians work, collaborate, and assess truth, while charting a new path in the march toward machine intelligence. In The Proof in the Code, Kevin Hartnett tells the definitive story of the birth and rise of Lean, and how a growing movement is transforming the