Answer HTML from the show action by default

docs/UPGRADING.md

@@ -398,6 +398,27 @@ end
 
 Note the block: a bare `inverse_of nil` reads the option rather than setting it.
 
+## The show action answers HTML by default
+
+`show` used to declare its JSON format before its HTML one, so a request carrying no
+`Accept` header — `curl`, a bare `fetch()`, most HTTP client libraries — arrived as
+`*/*`, matched every registered type, and got JSON back. Every other action answered
+HTML for the same request.
+
+It now declares HTML first. A client that explicitly sends `Accept: application/json`,
+or requests the `.json` extension, is unaffected.
+
+If you have a script relying on the unheadered JSON response, send the header or use
+`.json`:
+
+```bash
+curl -H 'Accept: application/json' https://example.com/admin/player/1
+curl https://example.com/admin/player/1.json
+```
+
+Note that the JSON branch renders the whole record, including columns the model config
+hides from the show view.
+
 ## Dropped support
 
 These are intentionally gone — migrate off them before upgrading:

lib/rails_admin_next/config/actions/show.rb

@@ -20,9 +20,11 @@ class Show < RailsAdminNext::Config::Actions::Base
 
         register_instance_option :controller do
           proc do
+            # HTML first: an Accept-less request arrives as */*, which matches every
+            # registered type, so Rails falls back to whichever is declared first.
             respond_to do |format|
-              format.json { render json: @object }
               format.html { render @action.template_name }
+              format.json { render json: @object }
             end
           end
         end

spec/integration/actions/show_spec.rb

@@ -47,6 +47,15 @@
     end
   end
 
+  context "with no explicit format" do
+    it "responds with HTML" do
+      page.driver.options[:headers] = {"HTTP_ACCEPT" => "*/*"}
+      visit show_path(model_name: "team", id: team.id)
+
+      expect(Mime::Type.parse(response_headers["Content-Type"]).first).to be_html
+    end
+  end
+
   context "when compact_show_view is enabled" do
     it "hides nil fields in show view by default" do
       visit show_path(model_name: "team", id: team.id)