---name: kotlin-api-designdescription: Use when designing or reviewing Kotlin function ownership, member or extension functions, factories, single-field domain types, value classes, data classes, Kotlin Multiplatform expect/actual declarations, or platform service boundaries.---

Kotlin API design

Core principle

Place behavior, types, and platform seams where their meaning is clearest to callers; use the smallest public abstraction that preserves domain language and platform independence.

Procedure

1. Name the domain concept, its owning type or module, and the callers that need to depend on it.
2. Choose function ownership before adding an extension, factory, helper, or service layer.
3. When reviewing a public mapping over a sealed result, name every caller-visible outcome. Flag a catch-all else that hides a subtype and recommend explicit subtype branches so the contract stays exhaustive and preserves smart casts.
4. Represent a single-field domain concept with the smallest type that preserves its semantic and interop contract.
5. Keep shared code semantic; put native SDK and platform details behind an interface or a narrowly justified expect/actual boundary.
6. Read the focused reference for the selected decision below.
7. Finish when the public surface states domain intent, platform details remain at leaves, and callers do not depend on convenience abstractions with no clear owner.

Topic router

Signal	Read
Member vs top-level, extension, factory, service, or receiver choice	Function ownership
Primitive obsession, one-field domain type, @JvmInline value class, data class, interop, or Compose stability	Value classes
Source sets, platform services, native SDKs, files, sensors, permissions, Compose Multiplatform interop, or expect/actual	Multiplatform boundaries
Branching, guard-condition shape, sealed-result mapping, or a catch-all else	Kotlin control flow