diff --git a/buffet/commands/command_manager.cc b/buffet/commands/command_manager.cc
index cbaaeca..d0743de 100644
--- a/buffet/commands/command_manager.cc
+++ b/buffet/commands/command_manager.cc
@@ -134,4 +134,14 @@
   return true;
 }
 
+void CommandManager::AddOnCommandAddedCallback(
+    const CommandQueue::Callback& callback) {
+  command_queue_.AddOnCommandAddedCallback(callback);
+}
+
+void CommandManager::AddOnCommandRemovedCallback(
+    const CommandQueue::Callback& callback) {
+  command_queue_.AddOnCommandRemovedCallback(callback);
+}
+
 }  // namespace buffet
diff --git a/buffet/commands/command_manager.h b/buffet/commands/command_manager.h
index 88ebca7..9a891ba 100644
--- a/buffet/commands/command_manager.h
+++ b/buffet/commands/command_manager.h
@@ -98,6 +98,12 @@
                             CommandDefinition::Visibility visibility,
                             chromeos::ErrorPtr* error);
 
+  // Adds notifications callback for a new command being added to the queue.
+  void AddOnCommandAddedCallback(const CommandQueue::Callback& callback);
+
+  // Adds notifications callback for a command being removed from the queue.
+  void AddOnCommandRemovedCallback(const CommandQueue::Callback& callback);
+
  private:
   CommandDictionary base_dictionary_;  // Base/std command definitions/schemas.
   CommandDictionary dictionary_;  // Command definitions/schemas.
diff --git a/buffet/commands/command_queue.cc b/buffet/commands/command_queue.cc
index 614261e..f4cda60 100644
--- a/buffet/commands/command_queue.cc
+++ b/buffet/commands/command_queue.cc
@@ -15,6 +15,9 @@
 
 void CommandQueue::AddOnCommandAddedCallback(const Callback& callback) {
   on_command_added_.push_back(callback);
+  // Send all pre-existed commands.
+  for (const auto& command : map_)
+    callback.Run(command.second.get());
 }
 
 void CommandQueue::AddOnCommandRemovedCallback(const Callback& callback) {