Usually, Kotlin adoption is a smooth process: do some initial configuration and then follow the typical flow "writing code → building code → shipping code". But turns out this experience does not scale well for a really big project. Two Sergeys will walk you through the Kotlin adoption journey at Meta. You will learn what problems speakers have encountered while trying to bring a new programming language into the biggest mobile codebase: from infrastructure support to hardcode JVM bytecode optimizations. Contains bloody DEX code.