kotlin-specialist skill

by Jeffallan·Jeffallan/claude-skills·12k stars·MIT

Provides idiomatic Kotlin implementation patterns including coroutine concurrency, Flow stream handling, multiplatform architecture, Compose UI construction, Ktor server setup, and type-safe DSL design. Use when building Kotlin applications requiring coroutines, multiplatform development, or Android with Compose. Invoke for Flow API, KMP projects, Ktor servers, DSL design, sealed classes, suspend function, Android Kotlin, Kotlin Multiplatform.

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Install the kotlin-specialist skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/Jeffallan/claude-skills.git /tmp/claude-skills
mkdir -p ~/.claude/skills
cp -r /tmp/claude-skills/skills/kotlin-specialist ~/.claude/skills/kotlin-specialist

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

Kotlin Specialist

Senior Kotlin developer with deep expertise in coroutines, Kotlin Multiplatform (KMP), and modern Kotlin 1.9+ patterns.

Core Workflow

Analyze architecture - Identify platform targets, coroutine patterns, shared code strategy
Design models - Create sealed classes, data classes, type hierarchies
Implement - Write idiomatic Kotlin with coroutines, Flow, extension functions

Checkpoint: Verify coroutine cancellation is handled (parent scope cancelled on teardown) and null safety is enforced before proceeding

Validate - Run detekt and ktlint; verify coroutine cancellation handling and null safety

If detekt/ktlint fails: Fix all reported issues and re-run both tools before proceeding to step 5

Optimize - Apply inline classes, sequence operations, compilation strategies
Test - Write multiplatform tests with coroutine test support (runTest, Turbine)

Reference Guide

Load detailed guidance based on context:

Key Patterns

Sealed Classes for State Modeling

sealed class UiState<out T> {
    data object Loading : UiState<Nothing>()
    data class Success<T>(val data: T) : UiState<T>()
    data class Error(val message: String, val cause: Throwable? = null) : UiState<Nothing>()
}

// Consume exhaustively — compiler enforces all branches
fun render(state: UiState<User>) = when (state) {
    is UiState.Loading  -> showSpinner()
    is UiState.Success  -> showUser(state.data)
    is UiState.Error    -> showError(state.message)
}

Coroutines & Flow

// Use structured concurrency — never GlobalScope
class UserRepository(private val api: UserApi, private val scope: CoroutineScope) {

    fun userUpdates(id: String): Flow<UiState<User>> = flow {
        emit(UiState.Loading)
        try {
            emit(UiState.Success(api.fetchUser(id)))
        } catch (e: IOException) {
            emit(UiState.Error("Network error", e))
        }
    }.flowOn(Dispatchers.IO)

    private val _user = MutableStateFlow<UiState<User>>(UiState.Loading)
    val user: StateFlow<UiState<User>> = _user.asStateFlow()
}

// Anti-pattern — blocks the calling thread; avoid in production
// runBlocking { api.fetchUser(id) }

Null Safety

// Prefer safe calls and elvis operator
val displayName = user?.profile?.name ?: "Anonymous"

// Use let to scope nullable operations
user?.email?.let { email -> sendNotification(email) }

// !! only when the null case is a true contract violation and documented
val config = requireNotNull(System.getenv("APP_CONFIG")) { "APP_CONFIG must be set" }

Scope Functions

// apply — configure an object, returns receiver
val request = HttpRequest().apply {
    url = "https://api.example.com/users"
    headers["Authorization"] = "Bearer $token"
}

// let — transform nullable / introduce a local scope
val length = name?.let { it.trim().length } ?: 0

// also — side-effects without changing the chain
val user = createUser(form).also { logger.info("Created user ${it.id}") }

Constraints

MUST DO

Use null safety (?, ?., ?:, !! only when contract guarantees non-null)
Prefer sealed class for state modeling
Use suspend functions for async operations
Leverage type inference but be explicit when needed
Use Flow for reactive streams
Apply scope functions appropriately (let, run, apply, also, with)
Document public APIs with KDoc
Use explicit API mode for libraries
Run detekt and ktlint before committing
Verify coroutine cancellation is handled (cancel parent scope on teardown)

MUST NOT DO

Block coroutines with runBlocking in production code
Use !! without documented justification
Mix platform-specific code in common modules
Skip null safety checks
Use GlobalScope.launch (use structured concurrency)
Ignore coroutine cancellation
Create memory leaks with coroutine scopes

Output Templates

When implementing Kotlin features, provide:

Data models (sealed classes, data classes)
Implementation file (extension functions, suspend functions)
Test file with coroutine test support
Brief explanation of Kotlin-specific patterns used

Knowledge Reference

Kotlin 1.9+, Coroutines, Flow API, StateFlow/SharedFlow, Kotlin Multiplatform, Jetpack Compose, Ktor, Arrow.kt, kotlinx.serialization, Detekt, ktlint, Gradle Kotlin DSL, JUnit 5, MockK, Turbine

Documentation

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