Which is the best way to debug cache misses in the build cache if using Gradle scans is not an option?

set org.gradle.caching.debug property to true and examine the logs

Even with build scans this sometimes provides better or additional insights

Speculation time: I've noticed that Gradle has a service that can be used to detect cache misses. I think it's specifically for adding a tag to a Build Scan, but I wonder if it could help in concert with the debug build cache logs https://github.com/gradle/gradle/blob/ebfcf97806a2c04b870652374bcbee2c915dac93/build-logic-commons/gradle-plugin/src/main/kotlin/gradlebuild/AbstractBuildScanInfoCollectingService.kt#L28-L34

That service is only part of the Gradle build itself, not something you can use in your own build. Not it is nothing more than s simple build service that also is an operation completion listener. You could always create such a service yourself already.

Have you more info about that operation completion listener? Is it part of the revamp about of FlowScope APIs?

No, that precates flowscope api