I feel I'm missing something. Can I use :malformed? to check a json POST with spec? And if so, is there some way to only parse the body once and make it available to :post! ?

I'd rather use :processible instead of malformed. Every decision method can return an updated context, or just a map which will be merged with the context. E.f.

(fn [ctx] 
  (let [[err value] (-> ctx :request :body (coerce some-schema))
    (if value 
       {:entity value}
       [false {:error err}])))

Ohhhhh? So [false {:error err}] is a vector of the return code and the map to merge into the context?

I was trying to return just a map and that caused a "bad request".

Which makes sense for malformed? because a map would be true. IOW: yes, it's malformed.

Exactly.

From the docs >Liberator merges a map that is returned from a decision function with the current context. This way we can easily store the value for the choice and retrieve it later in the handle-ok function: (get ctx :choice).

This page should cover it in detail http://clojure-liberator.github.io/liberator/doc/execution-model.html

Thanks!! I was working down the tutorial so I hadn't seen that yet, and didn't catch what was happening in the "putting it all together" section.