diff --git a/lib/http_client_options.rb b/lib/http_client_options.rb
index 23b1e48..1c15ac1 100644
--- a/lib/http_client_options.rb
+++ b/lib/http_client_options.rb
@@ -8,7 +8,7 @@ class HTTPClientOptions
     attr_reader :proxy, :send_platform_info, :enable_ssl_verification, :cert_path, :open_timeout,
                 :read_timeout, :write_timeout, :ssl_timeout
 
-    def initialize(proxy, cert_path, enable_ssl_verification: true, open_timeout: nil, # rubocop:disable Metrics/ParameterLists
+    def initialize(proxy = {}, cert_path = nil, enable_ssl_verification: true, open_timeout: nil, # rubocop:disable Metrics/ParameterLists
                    read_timeout: nil, write_timeout: nil, ssl_timeout: nil)
       @proxy = proxy
       @enable_ssl_verification = enable_ssl_verification
diff --git a/spec/http_client_options_spec.rb b/spec/http_client_options_spec.rb
new file mode 100644
index 0000000..81dfbf8
--- /dev/null
+++ b/spec/http_client_options_spec.rb
@@ -0,0 +1,66 @@
+# Copyright 2024 DeepL SE (https://www.deepl.com)
+# Use of this source code is governed by an MIT
+# license that can be found in the LICENSE file.
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe DeepL::HTTPClientOptions do
+  describe '#initialize' do
+    context 'when initialized without arguments' do
+      it 'sets default values' do
+        options = DeepL::HTTPClientOptions.new
+        expect(options.proxy).to eq({})
+        expect(options.cert_path).to be_nil
+        expect(options.enable_ssl_verification).to be true
+        expect(options.open_timeout).to be_nil
+        expect(options.read_timeout).to be_nil
+        expect(options.write_timeout).to be_nil
+        expect(options.ssl_timeout).to be_nil
+      end
+    end
+
+    context 'when initialized with all parameters' do
+      it 'sets all attributes correctly' do
+        proxy = { 'proxy_addr' => 'proxy.example.com', 'proxy_port' => 8080 }
+        cert_path = '/path/to/cert.pem'
+        options = DeepL::HTTPClientOptions.new(
+          proxy,
+          cert_path,
+          enable_ssl_verification: false,
+          open_timeout: 5,
+          read_timeout: 10,
+          write_timeout: 15,
+          ssl_timeout: 20
+        )
+        expect(options.proxy).to eq(proxy)
+        expect(options.proxy['proxy_addr']).to eq('proxy.example.com')
+        expect(options.proxy['proxy_port']).to eq(8080)
+        
+        expect(options.cert_path).to eq(cert_path)
+        expect(options.enable_ssl_verification).to be false
+        expect(options.open_timeout).to eq(5)
+        expect(options.read_timeout).to eq(10)
+        expect(options.write_timeout).to eq(15)
+        expect(options.ssl_timeout).to eq(20)
+      end
+    end
+
+    context 'when initialized with invalid keyword arguments' do
+      it 'raises an ArgumentError' do
+        expect {
+          DeepL::HTTPClientOptions.new({}, nil, invalid_option: true)
+        }.to raise_error(ArgumentError)
+      end
+    end
+
+    context 'attribute readers' do
+      it 'does not allow attributes to be modified after initialization' do
+        options = DeepL::HTTPClientOptions.new
+        expect {
+          options.proxy = { 'proxy_addr' => 'new.proxy.com' }
+        }.to raise_error(NoMethodError)
+      end
+    end
+  end
+end