Elazar formalized the Kraft-McMillan inequality and uniquely decodable codes within the leanprover-community/mathlib4 repository, expanding the library’s foundation for coding theory. Using Lean 4 and drawing on expertise in mathematics and theoretical computer science, Elazar implemented definitions and proofs that establish the injectivity of codeword concatenation and the probabilistic bound for noiseless codes. The work introduced new namespaces and documentation, enabling reliable formal reasoning about coding bounds and supporting future verification tools. Although no bugs were addressed during this period, the depth of the feature delivered strengthened the library’s capacity for rigorous formalization in information theory and coding.