diff --git a/lib/bundler/gem_bytes/actions.rb b/lib/bundler/gem_bytes/actions.rb
index a1ce0d9..aca5c56 100644
--- a/lib/bundler/gem_bytes/actions.rb
+++ b/lib/bundler/gem_bytes/actions.rb
@@ -1,9 +1,37 @@
 # frozen_string_literal: true
 
+require 'bundler/gem_bytes'
+
 module Bundler
   module GemBytes
     # The API for GemBytes templates
     # @api public
-    module Actions; end
+    module Actions
+      # Adds (or updates) a dependency in the project's gemspec file
+      #
+      # @example
+      #   add_dependency(:development, 'rspec', '~> 3.13')
+      #   add_dependency(:runtime, 'activesupport', '>= 6.0.0')
+      #
+      # @param dependency_type [Symbol] the type of dependency to add (either :development or :runtime)
+      # @param gem_name [String] the name of the gem to add
+      # @param version_constraint [String] the version constraint for the gem
+      # @param force [Boolean] whether to overwrite the existing dependency
+      # @param gemspec [String] the path to the gemspec file
+      #
+      # @return [void]
+      #
+      # @api public
+      #
+      def add_dependency(dependency_type, gem_name, version_constraint, force: false, gemspec: Dir['*.gemspec'].first)
+        puts 'Staring'
+        source = File.read(gemspec)
+        updated_source = Bundler::GemBytes::Gemspec::UpsertDependency.new(
+          dependency_type, gem_name, version_constraint, force: force
+        ).call(source)
+        File.write(gemspec, updated_source)
+        puts 'Ending'
+      end
+    end
   end
 end
diff --git a/spec/lib/bundler/gem_bytes/actions_spec.rb b/spec/lib/bundler/gem_bytes/actions_spec.rb
new file mode 100644
index 0000000..39c7ff8
--- /dev/null
+++ b/spec/lib/bundler/gem_bytes/actions_spec.rb
@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+RSpec.describe Bundler::GemBytes::Actions do
+  let(:including_class) do
+    Class.new do
+      include Bundler::GemBytes::Actions
+    end
+  end
+
+  let(:instance) { including_class.new }
+
+  # More detailed tests of add_dependency are in the tests for UpsertDependency
+  # class.
+  #
+  # These tests just make sure that calls to add_dependency are correctly delegated
+  # to the UpsertDependency class.
+  #
+  describe '.add_dependency' do
+    subject { instance.add_dependency(dependency_type, gem_name, version_constraint, force:) }
+
+    let(:dependency_type) { :development }
+    let(:gem_name) { 'rspec' }
+    let(:version_constraint) { '~> 3.13' }
+    let(:force) { false }
+
+    it 'adds the dependency to the gemspec' do
+      # Make a temporary directory to work in
+      Dir.mktmpdir do |temp_dir|
+        Dir.chdir(temp_dir) do
+          # Create a new gemspec file
+          gemspec_file = 'my_gem.gemspec'
+          File.write(gemspec_file, <<~GEMSPEC)
+            Gem::Specification.new do |spec|
+              spec.name = 'my_gem'
+              spec.version = '0.1.0'
+            end
+          GEMSPEC
+
+          # Add the dependency
+          instance.add_dependency(dependency_type, gem_name, version_constraint, force: force)
+
+          # Read the gemspec file
+          gemspec_content = File.read(gemspec_file)
+
+          # Check that the dependency was added
+          expect(gemspec_content).to include("spec.add_development_dependency 'rspec', '~> 3.13'")
+        end
+      end
+    end
+  end
+end