Well, that's just metaprogramming.

Also, java annotation processors are strictly "append-only", they can't change code written as is. They may subclass it, or build new classes that make use of the annotations.

In my experience LLMs are more than happy with annotations, especially from the widely used ones (spring/jakarta ee, lombok though it's not an annotation processor, etc).

If you think about it, an annotation like @Path("/endpoint") is very informative for both humans and LLMs alike, being local to what it attaches to. Within an agent, the code serving the endpoint will be immediately visible in the context returned by a simple string search, no need to do another round of "find wherever this codebase registers routes".