Kotlin Stack Defaults

Language and Runtime

• Kotlin 2.3.x (kotlin("jvm") version "2.3.0") — the latest stable, picks up the type-inference and coroutine fixes that downstream libraries (Koog, Ktor 3.4, kotlinx-coroutines 1.10) build against
• JDK 21 (jvmToolchain(21)) — the current LTS; required by JavaCV 1.5.13's native libs and recommended by Koog
• Avoid Kotlin 2.1.x for new projects — Koog ≥ 0.7 (including the 1.0.0-preview line) artifacts are built with newer Kotlin metadata that 2.1.x consumers can't compile against

Build System

• Gradle with Kotlin DSL (build.gradle.kts, settings.gradle.kts) — Groovy DSL only for legacy projects you didn't start
• Use the Gradle wrapper (./gradlew) committed to the repo so a fresh clone runs without a system Gradle install
• Prefer version catalogs (gradle/libs.versions.toml) when there are more than ~5 dependencies; inline implementation("group:artifact:version") is fine for tiny projects

Project Structure

• Standard layout: src/main/kotlin/, src/main/resources/, src/test/kotlin/
• One main() per file when each file is independently runnable (CLI tools, demo stages); use application plugin + per-stage JavaExec tasks rather than a single fat main

Anti-patterns

• ❌ Java + Kotlin mixed module without a clear reason — pick one, write Kotlin
• ❌ Maven instead of Gradle for new Kotlin projects
• ❌ JDK 17 / 11 / 8 without a forcing constraint — modern Kotlin stacks assume 21
• ❌ apply plugin: 'kotlin' in a build.gradle Groovy file when starting fresh