diff --git a/.rubocop.yml b/.rubocop.yml
index 5be3d3c..0d3cce5 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 3.1
+  TargetRubyVersion: 3.2
   Exclude:
     - 'exe/**/*'
     - 'vendor/**/*'
diff --git a/Gemfile b/Gemfile
index 1281f97..18975cb 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,5 +6,6 @@ source "https://rubygems.org"
 gemspec
 
 gem "rake", "~> 13.0"
+gem "rbs", "~> 3.4"
 gem "rspec", "~> 3.0"
 gem "rubocop", "~> 1.21"
diff --git a/Gemfile.lock b/Gemfile.lock
index 4b8134c..de4830f 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -16,17 +16,20 @@ GEM
     json (2.13.2)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
+    logger (1.7.0)
     openai (0.22.1)
       connection_pool
     parallel (1.27.0)
     parser (3.3.9.0)
       ast (~> 2.4.1)
       racc
-    prism (1.4.0)
+    prism (1.5.1)
     racc (1.8.1)
     rainbow (3.1.1)
     rake (13.3.0)
-    regexp_parser (2.11.2)
+    rbs (3.9.5)
+      logger
+    regexp_parser (2.11.3)
     rspec (3.13.1)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
@@ -67,8 +70,9 @@ PLATFORMS
 DEPENDENCIES
   ai_git_commit!
   rake (~> 13.0)
+  rbs (~> 3.4)
   rspec (~> 3.0)
   rubocop (~> 1.21)
 
 BUNDLED WITH
-   2.6.9
+   2.7.2
diff --git a/ai_git_commit.gemspec b/ai_git_commit.gemspec
index a9666d7..3816e0e 100644
--- a/ai_git_commit.gemspec
+++ b/ai_git_commit.gemspec
@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   DESCRIPTION
   spec.homepage = "https://github.com/drkmen/ai_git_commit"
   spec.license = "MIT"
-  spec.required_ruby_version = ">= 3.1.0"
+  spec.required_ruby_version = ">= 3.2.0"
 
   spec.metadata["allowed_push_host"] = "https://rubygems.org"
 
diff --git a/sig/ai_git_commit.rbs b/sig/ai_git_commit.rbs
index 16cf48b..d068cdc 100644
--- a/sig/ai_git_commit.rbs
+++ b/sig/ai_git_commit.rbs
@@ -1,4 +1,3 @@
 module AiGitCommit
   VERSION: String
-  # See the writing guide of rbs: https://github.com/ruby/rbs#guides
 end
diff --git a/sig/ai_git_commit/cli.rbs b/sig/ai_git_commit/cli.rbs
new file mode 100644
index 0000000..7240659
--- /dev/null
+++ b/sig/ai_git_commit/cli.rbs
@@ -0,0 +1,14 @@
+module AiGitCommit
+  class CLI < Thor
+    HOOK_PATH: String
+
+    def install: () -> void
+
+    private
+
+    def create_hook: () -> void
+    def copy_initializer: () -> void
+    def script: () -> ::String
+    def hook_file_exists?: () -> bool
+  end
+end
diff --git a/sig/ai_git_commit/config.rbs b/sig/ai_git_commit/config.rbs
new file mode 100644
index 0000000..8a40bb4
--- /dev/null
+++ b/sig/ai_git_commit/config.rbs
@@ -0,0 +1,15 @@
+module AiGitCommit
+  class Config
+    attr_accessor openai_api_key: String
+    attr_accessor model: String
+    attr_accessor program_language: String
+    attr_accessor max_tokens: Integer
+    attr_accessor temperature: Float
+    attr_accessor system_role_message: String
+
+    def initialize: () -> void
+  end
+
+  def self.config: () -> Config
+  def self.configure: () { (Config) -> void } -> void
+end
diff --git a/sig/ai_git_commit/generator.rbs b/sig/ai_git_commit/generator.rbs
new file mode 100644
index 0000000..74ddaa8
--- /dev/null
+++ b/sig/ai_git_commit/generator.rbs
@@ -0,0 +1,33 @@
+module AiGitCommit
+  class Generator
+    self.@openai: OpenAI::Client
+    self.@config: Config
+
+    def self.commit_message: () -> String
+
+    private
+
+    def self.fetch_message: (String diff) -> String
+    def self.completion_payload: (String diff) -> {
+      model: String,
+      messages: ::Array[
+        {
+          role: String,
+          content: String
+        } | {
+          role: String,
+          content: String
+        }
+      ],
+      max_tokens: Integer,
+      temperature: Float
+    }
+
+    def self.user_role_message: (String diff) -> String
+    def self.openai: () -> OpenAI::Client
+    def self.config: () -> Config
+    def self.openai_api_key: () -> String?
+    def self.staged_diff: () -> String
+  end
+end
+