I added to my build.gradle.kts:

plugins {
    id("com.google.devtools.ksp")
}

dependencies {
    implementation("io.arrow-kt:arrow-optics:$arrowVersion")
    ksp("io.arrow-kt:arrow-optics-ksp-plugin:$arrowVersion")
}

I have also annotated my data classes:

@optics
data class ConnectorWithSensors(
    val connector: Connector,
    val sensors: List<ItemSelection>,
) {
    companion object
}

@optics
data class ItemSelection(
    val id: String,
    val selected: Boolean,
) : {
    companion object
}

However I cannot access the extensions generated on companion object like

ConnectorWithSensors.sensors

.

my suggestion would be to check: 1. does

./gradlew build

work? if so, the KSP plugin is working 2. if the problem is only in the IDE, this means you need to add the generated sources as explained here. My suggestion is to add the following then to your gradle file

plugins {
   // ...
   idea
}

idea {
   module {
      // Not using += due to <https://github.com/gradle/gradle/issues/8749>
      sourceDirs = sourceDirs + file("build/generated/ksp/main/kotlin") // or tasks["kspKotlin"].destination
      testSourceDirs = testSourceDirs + file("build/generated/ksp/test/kotlin")
      generatedSourceDirs = generatedSourceDirs + file("build/generated/ksp/main/kotlin") + file("build/generated/ksp/test/kotlin")
   }
}

So one problem was (which I didn't mention) that my data classes were

internal

. So ksp complained that "public member exposes internal type" in the generated code. After making the data classes public the project builds, however the extension property

ConnectorWithSensors.sensors

is still red. I will try to import the generated sources as you suggested.