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If the configured toolchain is compatible with the JVM used to run Gradle, then compilation can happen in-process, and you can debug your annotation processor when attaching to the Gradle process.
But as soon as you configure a toolchain that's *not* compatible, then Gradle will have to launch a separate daemon for compilation using the specified toolchain.

To debug your compilation, I'd suggest either:
• using a `javax.tool.JavaCompiler` (possibly through compile-testing) in a test, and then debug the test process: <https://docs.gradle.org/current/userguide/java_testing.html#sec:debugging_java_tests>
• run Gradle with `--debug`, find the "Compiler arguments" line in the output, then run the equivalent `javac` command (with Gradle out of the equation), passing the `-agent` argument as an additional `-J` argument (`javac -J-agent:…`), and transforming other JVM arguments to prefix them with `-J` the same way (you could also of course pass an `-agent` argument to the Gradle compilation task's `jvmArgs` and force Gradle to fork a compiler daemon)

Ah, I might just leave it unset then. A little more consistency in the build environment is nice-to-have, but not necessary here.