On August 11, 2026, Modular released Mojo 1.0 as part of the broader Modular 26.5 platform update. This is not a minor version bump dressed up with marketing language. It closes a three-year period during which the language's syntax, standard library, and core semantics changed release over release, often breaking source compatibility for anyone maintaining a nontrivial codebase on top of it. This article examines what changed at the language level, what the stability guarantee actually covers, ...