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I attached a repro project. This is the stacktrace printed from the Retrofit error (I included the keep rules for the Kodein issue):

java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
        at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:365)
        at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:4)
        at retrofit2.Retrofit$1.invoke(Retrofit.java:7)
        at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
        at $Proxy1.getAllBreeds(Unknown Source)
        at com.example.api.DogBreedApiImpl.getAllBreeds(DogBreedApi.kt:5)
        at com.example.kodein7.MainActivity$onCreate$1.invokeSuspend(MainActivity.kt:5)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:3)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:18)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7660)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

R8 fullmode will remove all attributes for classes that are not kept. Attributes and annotations can only be read by reflection and for full mode we require an explicit keep rule to keep the signatures (and other attributes).

As you point to, adding keep rules for the types in kodein will force R8 to keep the signatures.

For retrofit, you will have to keep the api classes. Interestingly, you also have to keep the kotlin.coroutines.Continuation as well. Adding the below rules fixes the issue in the sample for me (note, you should refine the keep rule for the api to only keep what is needed).

-keep class com.example.api.** { *; }
-keep class kotlin.coroutines.Continuation

I will make a pull request on retrofit to add kotlin.coroutines.Continuation to the rules we consume.

Will R8 strip the signature even if I have -keepattributes Signature set in my rules?

If so, is there a rule that could be used to keep the signature just for the api classes?