I'm running into a very interesting coercion issue and I'm wondering if anyone here can help me see what I'm doing wrong or if there's a workaround. Here's my basic router:

(def handler
  (ring/ring-handler
    (ring/router
      ["/api"
       {:coercion   spec-coercion/coercion
        :middleware [parameters/parameters-middleware           ;; query-params & form-params
                     coercion/coerce-request-middleware         ;; coercing request parameters
                 ]}
       ["/foo"
        {:post {:parameters {:consumes "application/x-www-form-urlencoded"
                             :form     {:ids [int?]}}
                :handler    (fn [{{{:keys [ids]} :form} :parameters}]
                              (ok (str ids)))}}]
       ])))

(comment
  (require '[ring.mock.request :as mock])
  ;I expect this to fail
  (handler (mock/request :post "/api/foo" {:ids 1}))
  ;Why does this fail?
  (handler (mock/request :post "/api/foo" {:ids [1]}))
  ;These two pass
  (handler (mock/request :post "/api/foo" {:ids [1 3]}))
  (handler (mock/request :post "/api/foo" {:ids [1 2 3]}))
  )

I manage to reproduce your error, it seems odd indeed. Which is interesting is that when you use swagger support, the way it handles x-www-form-urlencoded input type is by separating each value in a newline, therefore I cannot be sure if I express in a swagger interface your second case correctly. I manage to workaround the problem with the following spec: (s/or :int int? :ints (s/coll-of int?)) but your first two cases will be handled equally.