I have (schema/optional-key :start) org.joda.time.DateTime in the :query field for a GET and it works great.

However, the same thing in the :body field (for a POST) will say I have a malformed request.

Is there something different in how the coercion is made?

Ah, I think I understand... when in the body, the decoding is done using the format (ie transit) while in the query it's decoded using some kind of default.