Failing spec: login view hardcodes /admin/auth/oidc form action

app/views/active_admin/devise/sessions/new.html.erb

@@ -5,7 +5,7 @@
     </h2>
 
     <%= button_to ActiveAdmin::Oidc.config.login_button_label,
-                  "/admin/auth/oidc",
+                  "#{OmniAuth.config.path_prefix}/oidc",
                   method: :post,
                   class: "activeadmin-oidc-login-button w-full",
                   form_class: 'formtastic',
@@ -15,7 +15,7 @@
   <div id="login">
     <h2><%= active_admin_application.site_title(self) %></h2>
 
-    <%= form_tag "/admin/auth/oidc",
+    <%= form_tag "#{OmniAuth.config.path_prefix}/oidc",
                  method: :post,
                  class: "activeadmin-oidc-login-form formtastic",
                  data: { turbo: false } do %>

spec/requests/login_path_helper_spec.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+# Regression spec for MEDIUM #4 — login view must derive the OmniAuth
+# callback path from `OmniAuth.config.path_prefix`, not hardcode it.
+#
+# `app/views/active_admin/devise/sessions/new.html.erb` used to hardcode
+# `"/admin/auth/oidc"` in the form action. Hosts that customise
+# `Devise.omniauth_path_prefix` (mount Devise at a non-`/admin` path,
+# or use a different sub-prefix for SSO) ended up with a button POSTing
+# to a dead URL — the gem's strategy is registered at the configured
+# prefix, not the hardcoded one.
+#
+# We can't use Devise's `omniauth_authorize_path` helper here because
+# the OmniAuth middleware lives at the Rack level (global path prefix),
+# while Devise route helpers resolve through the engine and get
+# re-prefixed by the engine mount — producing e.g. `/admin/admin/auth/oidc`
+# when Devise is engine-mounted. `OmniAuth.config.path_prefix` is the
+# single source of truth for where the middleware actually listens.
+RSpec.describe "Login view OmniAuth path", type: :request do
+  before do
+    ActiveAdmin::Oidc.configure do |c|
+      c.issuer    = "https://idp.example.com"
+      c.client_id = "client-abc"
+      c.on_login  = ->(*) { true }
+    end
+
+    # Force routes to load NOW. Otherwise Rails 8 lazy loading defers
+    # `devise_for` until the first request — at which point the stub
+    # below is active, Devise's "OmniAuth.config.path_prefix matches
+    # Devise.omniauth_path_prefix" guard sees the sentinel, and raises.
+    # `execute_unless_loaded` is Rails 8+; fall back for 7.x.
+    reloader = Rails.application.routes_reloader
+    if reloader.respond_to?(:execute_unless_loaded)
+      reloader.execute_unless_loaded
+    else
+      Rails.application.reload_routes!
+    end
+  end
+
+  it "renders the form action from OmniAuth.config.path_prefix (no hardcoded literal)" do
+    # Stub the OmniAuth path prefix to a sentinel value the hardcoded
+    # string could never match. If the view actually reads the prefix,
+    # the rendered form action will be `<sentinel>/oidc`; if it
+    # hardcodes the path, the literal "/admin/auth/oidc" stays.
+    sentinel = "/sentinel-omniauth-prefix"
+    allow(OmniAuth.config).to receive(:path_prefix).and_return(sentinel)
+
+    get "/admin/login"
+
+    expect(response.body).to include(%(action="#{sentinel}/oidc")),
+      "form action ignores Devise.omniauth_path_prefix — hosts that " \
+      "customise the prefix get a button POSTing to a dead URL"
+  end
+end