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It's complaining about the project building your convention plugin, not about the convention plugin itself. So you need to add a toolchain setting to

buildSrc/build.gradle.kts

. but don't use 17 there without reason or you limit the version with which you can execute Gradle to 17+. So unless some dependency requires something higher, use the minimum supported version of your Gradle version there, so most likely 8.

Looks like it goes away in Gradle 7.5, confirmed that for myself. (Gradle 7.4.1 is the version you get when you install Gradle via snap on Ubuntu 22.04.) Made another attempt today to search for more info about this issue, and I found this: https://github.com/gradle/gradle/issues/18935

Ah, right, forgot that issue.

you get when you install Gradle via snap

I never install Gradle anywhere, as any sane project should contain the wrapper files that should be used to run the build. 🙂

It was a project I created via

gradle init

, so it installed the wrapper files for 7.4.1. (But those wrapper files are checked in, and they've since been upgraded to 8.0.2; there's a plugin I use that's incompatible with 8.1.1.)