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Try `project.extensions.create('taskExt', TaskExtension)`  instead.
Also you can also use just interface instead of abstract class.

This call will create an extension in the Project, but I don't want in the Project, but in the Task.

Ah ok. I see also org.gradle.test-retry is using `.add`. I personally don't know if this is a bug or just some limitation. But it looks like a bug, at least from reading ExtensionContainer interface. Feel free to open an issue for team to triage