Why Writing Formal Specs Before Code Catches Bugs AI and Humans Miss

A software engineering article argues that developers should document every system guarantee in a formal specification before writing a single line of code. Using TLA+, a decades-old formal specification language, the author demonstrates how a simple to-do list app's rules — such as valid task state transitions — can be precisely defined in just a few lines. The approach gained credibility when AWS engineers reported in a CACM paper that writing TLA+ specs uncovered bugs in already-reviewed, production-bound systems. The author warns this practice is especially urgent now that AI agents generate plausible-looking code that can pass human review while violating core system rules. A lightweight Rust-based toolchain called tlatools is presented as a practical way to automatically check whether generated code conforms to a written specification.