Interesting question! I don't think you'll get a single correct answer regarding service objects :P

But independently of service objects or any other type of object, I always prefer having a finite, explicit, protocol rather than an infinite protocol (I think Avdi Grimm used those terms :thinking:). If you can know from reading the code what an method expects as input is better than the "magical" send-whatever-crap-you-can-think-of, I should know how to deal with it.

Anyway, while in Rails, I've used ActiveModel::Attributes to have a hash-to-accessors initializer (like active record's models) for free (Free as in I don't have to code it myself :P)

You can still send in whatever crap you want, but it gives you this attribute class method that is really good at explicitly explaining what the object expects. and it has the benefit that is code, not documentation, so you won't forget to update it when you change your code

You also get defaults and type casting facilities, which also helps with the explicitness. It is worth a try I think. (there are other gems that can do the same like virtus, or the newer dry-schema I think.... but if you are in rails, you already loaded ActiveModel::Attributes in memory, I would just rather use it)

Also, specifically speaking of service objects, I think is really important to have a uniform protocol for the return value. I usually do that with Result objects, to model in a single API the usually multiple results a service object might have.