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I assume that build cache is only related to task. And plugins code that happens in apply is about the configuration cache.

AFAIK, it's the same guidance as for regular plugins:
• For CC, don't use `task.project` , etc... <https://docs.gradle.org/current/userguide/configuration_cache.html>
• For BC, don't use absolute paths, etc...

Is "$projectDir/...." considered absolute?

I mean at the end it is absolute but it is relative to the project

If your task input is annotated with PathSensitivity.Relative you're fine

haha, no idea what it means. Let me dig in documentation first.

<https://docs.gradle.org/current/userguide/build_cache_concepts.html#relocatability|https://docs.gradle.org/current/userguide/build_cache_concepts.html#relocatability>

The most problematic is not "writing" but testing BC and CC :)