@U061V0GG2 my main hangup with this implementation is I do not understand why I need to plug the same transformer to :string as well

Why do you need the additional transformer in this example?

i.e. why do need to define :query transformer? Is there something using that?

by default query-params, form-params, header-params and path-params use string transformer: https://github.com/metosin/reitit/blob/master/modules/reitit-core/src/reitit/coercion.cljc#L38

hold over from my implementation where the query params map had its own transformer, too

this is great, thanks for sharing. not sure if you saw the rest of the old thread but turns out I could make it work by adding a :decode/string fn

That's true. This assumption would not have held however if you need something which operates on top of string decoding. I had a use case where I converted milliseconds to seconds. The string decoder gave me back a number. My own decoder divided it by 1000