Command-line Isolated Projects flags are not reflected in KSP source wiring #3188

KSP's enableProjectIsolationCompatibleCodepath() reads Gradle/system properties directly, so it misses Gradle's --isolated-projects command-line flag. It also reports isolation enabled when org.gradle.isolated-projects=true is overridden by --no-isolated-projects.

This is present in KSP 2.3.11 and current main. The compatible source-registration path is selected by this function in both KspSubplugin and KspAATask.

A small Groovy build demonstrates the detection error:

import com.google.devtools.ksp.gradle.utils.KgpUtilsKt

plugins {
    id 'org.jetbrains.kotlin.jvm' version '2.3.20' apply false
    id 'com.google.devtools.ksp' version '2.3.11' apply false
}

def isolated = KgpUtilsKt.enableProjectIsolationCompatibleCodepath(project)
tasks.register('checkIsolation') {
    inputs.property('isolated', isolated)
    doLast { println('KSP isolation: ' + inputs.properties.isolated) }
}

With Gradle 9.7.1:

• ./gradlew checkIsolation --isolated-projects prints Gradle's isolation banner, but KSP isolation: false.
• Put org.gradle.isolated-projects=true in gradle.properties; ./gradlew checkIsolation --no-isolated-projects prints KSP isolation: true.
• Explicit ksp.project.isolation.enabled=true remains a useful opt-in/workaround and should continue to work with Gradle isolation disabled.

I have reproduced both failures in focused TestKit tests against current main. BuildFeatures.isolatedProjects.active supplies Gradle's effective feature state, including CLI precedence. It is available since Gradle 8.5, so a version-gated injected helper can use it while retaining the existing property fallback on older Gradle versions.

I am preparing that small fix with CLI/cache-reuse, explicit opt-in, older-Gradle, and actual generated-source compilation coverage. This investigation and the tests are substantially AI-assisted with Codex.