Given the following structure of plugins used in a project, how would I get access to the extension provided by the helm plugin from our own convention plugin?

gradle module
\-- convention plugin
    \-- in-house published plugin
        \-- helm plugin

See thread for more detail.

I suppose it's because of how the helm plugin is applied by our in-house plugin:

No, that should be fine. And if that were the issue, you would not have any accessor at all. But you have the accessor, just not typed.

implementation

vs.

api

should not be important I think, because this distinction is when you compile something against the in-house plugin. But here you use the in-house plugin, so the runtime classpath is the relevant thing. Why your accessor is untyped I don't know just from this snippet. I've seen such a situation exactly once before. In https://github.com/gradle/gradle/issues/27979 where a plugin was added to init script classpath and applied to root project. Then in the root project you had the accessor for the extension, but as the plugin was only present in the init script classloader which is not a parent of the root project classloader, the extension was untyped like in your case. But why it happens in your exact case is hard to tell without an MCVE.

Thanks for the details here. I'll do some digging. As far as I can tell, the plugin is not added through an init script. But perhaps there's some other classloader isolation issue.