diff --git a/CHANGELOG.md b/CHANGELOG.md
index 602339e0c..572bf8926 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ User-visible changes worth mentioning.
 
 ## main
 
+- [#1834] Fix default `allow_token_introspection` returning `false` when a custom `application_class` is configured. The default proc compared application objects with `==`, which fails when the authorized client and the introspected token's application are resolved as different classes (e.g. a base `Doorkeeper::Application` vs. a configured subclass) even though they reference the same record. It now compares application ids instead.
 - Please add here
 
 ## 5.9.2
diff --git a/lib/doorkeeper/config.rb b/lib/doorkeeper/config.rb
index b326938e7..de19ebf60 100644
--- a/lib/doorkeeper/config.rb
+++ b/lib/doorkeeper/config.rb
@@ -444,9 +444,9 @@ def configure_secrets_for(type, using:, fallback:)
     option :allow_token_introspection,
            default: (lambda do |token, authorized_client, authorized_token|
              if authorized_token
-               authorized_token.application == token&.application
+               authorized_token.application_id == token&.application_id
              elsif token&.application
-               authorized_client == token.application
+               authorized_client.id == token.application_id
              else
                true
              end
diff --git a/spec/controllers/tokens_controller_spec.rb b/spec/controllers/tokens_controller_spec.rb
index 780bcb22b..00710fdb9 100644
--- a/spec/controllers/tokens_controller_spec.rb
+++ b/spec/controllers/tokens_controller_spec.rb
@@ -371,6 +371,31 @@ def create
       end
     end
 
+    context "when application_class is configured to a custom subclass" do
+      # Regression test for doorkeeper#1833: the default allow_token_introspection
+      # proc compared application objects with `==`, which fails when the
+      # authorized client is resolved as a different (sub)class than the
+      # introspected token's application, even though they reference the same row.
+      let(:custom_application_class) { Class.new(Doorkeeper::Application) }
+
+      before do
+        stub_const("CustomDoorkeeperApplication", custom_application_class)
+        allow(Doorkeeper.config).to receive(:application_model).and_return(CustomDoorkeeperApplication)
+      end
+
+      it "responds with full token introspection when authorized via Client Credentials" do
+        # token_for_introspection.application is a Doorkeeper::Application, while the
+        # authorized client is resolved as CustomDoorkeeperApplication for the same row.
+        expect(token_for_introspection.application).to be_an_instance_of(Doorkeeper::Application)
+
+        request.headers["Authorization"] = basic_auth_header_for_client(client)
+
+        post :introspect, params: { token: token_for_introspection.token }
+
+        expect(json_response).to include("active" => true)
+      end
+    end
+
     context "when token introspection disabled" do
       before do
         Doorkeeper.configure do