diff --git a/spec/lib/captain/reply_suggestion_service_spec.rb b/spec/lib/captain/reply_suggestion_service_spec.rb
new file mode 100644
index 000000000..c5ce45ea2
--- /dev/null
+++ b/spec/lib/captain/reply_suggestion_service_spec.rb
@@ -0,0 +1,75 @@
+require 'rails_helper'
+
+RSpec.describe Captain::ReplySuggestionService do
+  let(:account) { create(:account) }
+  let(:inbox) { create(:inbox, account: account) }
+  let(:conversation) { create(:conversation, account: account, inbox: inbox) }
+  let(:event) do
+    {
+      'name' => 'reply_suggestion',
+      'data' => {
+        'conversation_display_id' => conversation.display_id
+      }
+    }
+  end
+  let(:service) { described_class.new(account: account, event: event) }
+  let(:mock_chat) { instance_double(RubyLLM::Chat) }
+  let(:mock_context) { instance_double(RubyLLM::Context, chat: mock_chat) }
+  let(:mock_response) { instance_double(RubyLLM::Message, content: 'Suggested reply', input_tokens: 100, output_tokens: 50) }
+
+  before do
+    create(:installation_config, name: 'CAPTAIN_OPEN_AI_API_KEY', value: 'test-key')
+    allow(Llm::Config).to receive(:with_api_key).and_yield(mock_context)
+    allow(mock_chat).to receive(:with_instructions)
+    allow(mock_chat).to receive(:add_message)
+    allow(mock_chat).to receive(:ask).and_return(mock_response)
+  end
+
+  describe '#reply_suggestion_message' do
+    let(:message1) { create(:message, conversation: conversation, message_type: :incoming, content: 'Hello') }
+    let(:message2) { create(:message, conversation: conversation, message_type: :outgoing, content: 'Hi there') }
+
+    before do
+      message1
+      message2
+    end
+
+    it 'uses conversation_messages to build message history' do
+      expect(service).to receive(:conversation_messages).and_call_original
+      service.reply_suggestion_message
+    end
+
+    it 'concatenates system prompt with conversation history' do
+      allow(service).to receive(:prompt_from_file).with('reply').and_return('Help with reply')
+
+      expect(service).to receive(:make_api_call) do |args|
+        expected_messages = [
+          { role: 'system', content: 'Help with reply' },
+          { role: 'user', content: 'Hello' },
+          { role: 'assistant', content: 'Hi there' }
+        ]
+
+        expect(args[:messages]).to eq(expected_messages)
+        { message: 'Suggested reply' }
+      end
+
+      service.reply_suggestion_message
+    end
+
+    it 'passes correct model to API' do
+      expect(service).to receive(:make_api_call).with(
+        hash_including(model: Captain::BaseEditorService::GPT_MODEL)
+      ).and_call_original
+
+      service.reply_suggestion_message
+    end
+
+    it 'returns formatted response' do
+      result = service.reply_suggestion_message
+
+      expect(result[:message]).to eq('Suggested reply')
+      expect(result[:usage]['prompt_tokens']).to eq(100)
+      expect(result[:usage]['completion_tokens']).to eq(50)
+    end
+  end
+end