Fix Kotlin Serialization Version Incompatibility Error

The error occurs because kotlinx-serialization-json 1.9.0 requires Kotlin 2.2.0, while your project is using Kotlin 1.9.0. According to the Kotlin documentation and GitHub releases, version 1.9.0 of kotlinx-serialization requires Kotlin 2.2.0, which introduces metadata version 2.2.0 that your current Kotlin compiler cannot read.

Contents

• Understanding the Version Incompatibility
• Solution 1: Update Kotlin to 2.2.0
• Solution 2: Use Compatible Serialization Version
• Solution 3: Verify Plugin Configuration
• Additional Troubleshooting Steps
• Best Practices for Version Management

Understanding the Version Incompatibility

The error occurs due to a metadata version mismatch between your Kotlin compiler and the kotlinx-serialization-json library. According to Stack Overflow discussions, when you use kotlinx-serialization-json 1.9.0, it pulls in dependencies compiled with Kotlin 2.2.0, but your project uses Kotlin 1.9.0 which can only read metadata up to version 2.0.0.

The actual metadata version is 2.2.0, but the compiler version 1.9.0 can read versions up to 2.0.0.

This incompatibility manifests as the “Class ‘kotlin.Unit’ was compiled with an incompatible version of Kotlin” error.

Solution 1: Update Kotlin to 2.2.0

The recommended approach is to update your Kotlin version to 2.2.0 to match the requirements of kotlinx-serialization 1.9.0.

Update your libs.versions.toml:

[versions]
kotlin = "2.2.0"
# Update other versions if needed for Kotlin 2.2.0 compatibility

Update your app build.gradle.kts:

android {
    // ... existing configuration ...
    
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }
    composeOptions {
        // Update Compose compiler version for Kotlin 2.2.0
        kotlinCompilerExtensionVersion = "1.5.8" // or compatible version
    }
}

Add the Kotlin serialization plugin:

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.jetbrains.kotlin.android)
    kotlin("plugin.serialization") version "2.2.0" // Add this line
}

This solution ensures compatibility between your Kotlin compiler and the serialization library.

Solution 2: Use Compatible Serialization Version

If you prefer to stay with Kotlin 1.9.0, use an older version of kotlinx-serialization that’s compatible with it.

Update your dependency:

dependencies {
    // Use version 1.6.3 instead of 1.9.0
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
}

According to Stack Overflow, 1.6.x versions are compatible with Kotlin 1.9.x:

For the JSON library, 1.6.x versions go with the 1.9.x Kotlin versions (docs). The very latest is 1.6.3, which you might as well use.

This approach allows you to maintain your current Kotlin version while still using serialization functionality.

Solution 3: Verify Plugin Configuration

Ensure all Kotlin-related plugins and dependencies are properly configured and version-aligned.

Check your build.gradle.kts structure:

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.jetbrains.kotlin.android)
    kotlin("plugin.serialization") version "2.2.0" // Ensure version matches Kotlin
}

android {
    // ... existing configuration ...
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.9.0")
}

Verify your libs.versions.toml:

[versions]
kotlin = "2.2.0"
kotlinx-serialization = "1.9.0"

[libraries]
kotlinx-serialization-json = { group = "org.jetbrains.kotlinx", name = "kotlinx-serialization-json", version.ref = "kotlinx-serialization" }

[plugins]
kotlin-android = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }

This ensures all Kotlin-related components use consistent versions.

Additional Troubleshooting Steps

If the above solutions don’t resolve the issue, try these additional steps:

1. Clean and rebuild your project:

./gradlew clean build --rerun-tasks

2. Check for conflicting dependencies:

// In your build.gradle.kts
configurations.all {
    resolutionStrategy {
        force("org.jetbrains.kotlin:kotlin-stdlib:2.2.0")
    }
}

3. Update Gradle wrapper:

Ensure you’re using a compatible Gradle version (8.0 or later) for Kotlin 2.2.0.

4. Clear Gradle cache:

./gradlew --stop
rm -rf ~/.gradle/caches/

5. Check for transitive dependencies:

Use ./gradlew :app:dependencies to see the dependency tree and identify any conflicting versions.

Best Practices for Version Management

Version Compatibility Matrix:

Regular Updates:

• Monitor Kotlin releases for compatibility announcements
• Use version catalogs to manage dependencies consistently
• Consider using the Android Tools for version management

Multi-module Projects:

For complex projects, ensure all modules use consistent Kotlin and serialization versions. As noted in GitHub discussions, version mismatches across modules can cause similar issues.

Sources

1. Kotlin Documentation - Serialization
2. GitHub - Kotlinx Serialization Releases
3. Stack Overflow - Kotlinx Serialization Compatibility
4. Stack Overflow - Version Compatibility Guide
5. GitHub - Kotlinx Serialization Changelog

Conclusion

To resolve the “Class ‘kotlin.Unit’ was compiled with an incompatible version of Kotlin” error:

1. Update to Kotlin 2.2.0 - This is the recommended solution as it fully supports kotlinx-serialization 1.9.0
2. Use kotlinx-serialization 1.6.3 - If you prefer to stay with Kotlin 1.9.x
3. Ensure plugin consistency - Verify all Kotlin-related plugins use compatible versions
4. Clean and rebuild - After making version changes, perform a clean rebuild
5. Check dependency conflicts - Use Gradle dependency tree analysis to identify version mismatches

The key takeaway is maintaining version compatibility across all Kotlin-related dependencies and plugins. Starting with Kotlin 2.2.0 and kotlinx-serialization 1.9.0 provides the most current and feature-rich solution, while using the older serialization version offers a stable path if you’re not ready to upgrade your Kotlin version.