How do I set default values for path parameters?

In the below example, how can I set a default value for the path parameter item-id?

(POST "/:id" [item-id]
  :path-params [item-id :- Int]
  :body [body Body]
  :query-params [{item-name :- Str nil}]
                 :summary "Create or update a item."
                 (ok ...))

You should match the path-parameter name to the string placeholder. Path-params don't need defaults - if there is no path-parameter present, the route doesn't match. Here's a working example:

(require '[compojure.api.sweet :refer :all])
(require '[ring.util.http-response :refer :all])
(require '[schema.core :as s])
(require '[muuntaja.core :as m])

(def app
  (api
    (POST "/:item-id" []
      :path-params [item-id :- s/Int]
      :query-params [{item-name :- s/Str nil}]
      :summary "Create or update a item."
      (ok {:item-id item-id
           :item-name item-name}))))

(->> {:request-method :post
      :uri "/123"
      :query-params {"item-name" "kikka"}}
     (app)
     :body
     (m/decode m/instance "application/json"))
; => {:item-name "kikka", :item-id 123}