Is there a formal definition of what :body should be? Should it be a Reader or a String?

Depends if request/response. There is a spec file in the ring repo that formalizes this

https://github.com/ring-clojure/ring/blob/master/SPEC

Hmm InputStream .. I have a middleware that turns the body into a map - maybe that is not correct from a spec perspective

i guess body is tricky anyway because once you consume the InputStream, you can’t re-read it again

Depends if you control whatever consumes it. For instance turning an inputstream representing json data to a map is ok depending on the context

specially if it is the last in a chain of middleware