---

description: Enforce TDD workflow for Kotlin. Write Kotest tests first, then implement. Verify 80%+ coverage with Kover.

Kotlin TDD Command

This command enforces test-driven development methodology for Kotlin code using Kotest, MockK, and Kover.

What This Command Does

1. Define Types/Interfaces: Scaffold function signatures first
2. Write Kotest Tests: Create comprehensive test specs (RED)
3. Run Tests: Verify tests fail for the right reason
4. Implement Code: Write minimal code to pass (GREEN)
5. Refactor: Improve while keeping tests green
6. Check Coverage: Ensure 80%+ coverage with Kover

When to Use

Use /kotlin-test when:

• Implementing new Kotlin functions or classes
• Adding test coverage to existing Kotlin code
• Fixing bugs (write failing test first)
• Building critical business logic
• Learning TDD workflow in Kotlin

TDD Cycle

RED     -> Write failing Kotest test
GREEN   -> Implement minimal code to pass
REFACTOR -> Improve code, tests stay green
REPEAT  -> Next test case

Example Session

User: /kotlin-test I need a function to validate user registration requests

Agent:
# TDD Session: Registration Validator

## Step 1: Define Interface

```kotlin
// validator/RegistrationValidator.kt
package com.example.validator

data class RegistrationRequest(
    val name: String,
    val email: String,
    val password: String,
)

sealed class ValidationResult {
    data object Valid : ValidationResult()
    data class Invalid(val errors: List<String>) : ValidationResult()
}

fun validateRegistration(request: RegistrationRequest): ValidationResult {
    TODO("not implemented")
}
```

## Step 2: Write Kotest Tests (RED)

```kotlin
// validator/RegistrationValidatorTest.kt
package com.example.validator

import io.kotest.core.spec.style.FunSpec
import io.kotest.matchers.shouldBe
import io.kotest.matchers.types.shouldBeInstanceOf

class RegistrationValidatorTest : FunSpec({
    test("valid registration returns Valid") {
        val request = RegistrationRequest(
            name = "Alice",
            email = "alice@example.com",
            password = "SecureP@ss1",
        )

        val result = validateRegistration(request)

        result.shouldBeInstanceOf<ValidationResult.Valid>()
    }

    test("blank name returns Invalid") {
        val request = RegistrationRequest(
            name = "",
            email = "alice@example.com",
            password = "SecureP@ss1",
        )

        val result = validateRegistration(request)

        val invalid = result.shouldBeInstanceOf<ValidationResult.Invalid>()
        invalid.errors shouldBe listOf("Name is required")
    }

    test("invalid email returns Invalid") {
        val request = RegistrationRequest(
            name = "Alice",
            email = "not-an-email",
            password = "SecureP@ss1",
        )

        val result = validateRegistration(request)

        val invalid = result.shouldBeInstanceOf<ValidationResult.Invalid>()
        invalid.errors shouldBe listOf("Invalid email format")
    }

    test("short password returns Invalid") {
        val request = RegistrationRequest(
            name = "Alice",
            email = "alice@example.com",
            password = "short",
        )

        val result = validateRegistration(request)

        val invalid = result.shouldBeInstanceOf<ValidationResult.Invalid>()
        invalid.errors shouldBe listOf("Password must be at least 8 characters")
    }

    test("multiple errors returns all errors") {
        val request = RegistrationRequest(
            name = "",
            email = "bad",
            password = "short",
        )

        val result = validateRegistration(request)

        val invalid = result.shouldBeInstanceOf<ValidationResult.Invalid>()
        invalid.errors.size shouldBe 3
    }
})
```

## Step 3: Run Tests - Verify FAIL

```bash
$ ./gradlew test

RegistrationValidatorTest > valid registration returns Valid FAILED
  kotlin.NotImplementedError: An operation is not implemented

FAILED (5 tests, 0 passed, 5 failed)
```

✓ Tests fail as expected (NotImplementedError).

## Step 4: Implement Minimal Code (GREEN)

```kotlin
// validator/RegistrationValidator.kt
package com.example.validator

private val EMAIL_REGEX = Regex("^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$")
private const val MIN_PASSWORD_LENGTH = 8

fun validateRegistration(request: RegistrationRequest): ValidationResult {
    val errors = buildList {
        if (request.name.isBlank()) add("Name is required")
        if (!EMAIL_REGEX.matches(request.email)) add("Invalid email format")
        if (request.password.length < MIN_PASSWORD_LENGTH) add("Password must be at least $MIN_PASSWORD_LENGTH characters")
    }

    return if (errors.isEmpty()) ValidationResult.Valid
    else ValidationResult.Invalid(errors)
}
```

## Step 5: Run Tests - Verify PASS

```bash
$ ./gradlew test

RegistrationValidatorTest > valid registration returns Valid PASSED
RegistrationValidatorTest > blank name returns Invalid PASSED
RegistrationValidatorTest > invalid email returns Invalid PASSED
RegistrationValidatorTest > short password returns Invalid PASSED
RegistrationValidatorTest > multiple errors returns all errors PASSED

PASSED (5 tests, 5 passed, 0 failed)
```

✓ All tests passing!

## Step 6: Check Coverage

```bash
$ ./gradlew koverHtmlReport

Coverage: 100.0% of statements
```

✓ Coverage: 100%

## TDD Complete!

Test Patterns

StringSpec (Simplest)

class CalculatorTest : StringSpec({
    "add two positive numbers" {
        Calculator.add(2, 3) shouldBe 5
    }
})

BehaviorSpec (BDD)

class OrderServiceTest : BehaviorSpec({
    Given("a valid order") {
        When("placed") {
            Then("should be confirmed") { /* ... */ }
        }
    }
})

Data-Driven Tests

class ParserTest : FunSpec({
    context("valid inputs") {
        withData("2026-01-15", "2026-12-31", "2000-01-01") { input ->
            parseDate(input).shouldNotBeNull()
        }
    }
})

Coroutine Testing

class AsyncServiceTest : FunSpec({
    test("concurrent fetch completes") {
        runTest {
            val result = service.fetchAll()
            result.shouldNotBeEmpty()
        }
    }
})

Coverage Commands

# Run tests with coverage
./gradlew koverHtmlReport

# Verify coverage thresholds
./gradlew koverVerify

# XML report for CI
./gradlew koverXmlReport

# Open HTML report
open build/reports/kover/html/index.html

# Run specific test class
./gradlew test --tests "com.example.UserServiceTest"

# Run with verbose output
./gradlew test --info

Coverage Targets

Code Type	Target
Critical business logic	100%
Public APIs	90%+
General code	80%+
Generated code	Exclude

TDD Best Practices

DO:

• Write test FIRST, before any implementation
• Run tests after each change
• Use Kotest matchers for expressive assertions
• Use MockK's coEvery/coVerify for suspend functions
• Test behavior, not implementation details
• Include edge cases (empty, null, max values)

DON'T:

• Write implementation before tests
• Skip the RED phase
• Test private functions directly
• Use Thread.sleep() in coroutine tests
• Ignore flaky tests

Related Commands

• /kotlin-build - Fix build errors
• /kotlin-review - Review code after implementation
• verification-loop skill - Run full verification loop

Related

• Skill: skills/kotlin-testing/
• Skill: skills/tdd-workflow/