diff --git a/app/controllers/discourse_ai/ai_helper/assistant_controller.rb b/app/controllers/discourse_ai/ai_helper/assistant_controller.rb
index c32ea5d1..eea254ec 100644
--- a/app/controllers/discourse_ai/ai_helper/assistant_controller.rb
+++ b/app/controllers/discourse_ai/ai_helper/assistant_controller.rb
@@ -23,8 +23,11 @@ module DiscourseAi
         prompt = CompletionPrompt.find_by(id: params[:mode])
 
         raise Discourse::InvalidParameters.new(:mode) if !prompt || !prompt.enabled?
-        if prompt.prompt_type == "custom_prompt" && params[:custom_prompt].blank?
-          raise Discourse::InvalidParameters.new(:custom_prompt)
+
+        if prompt.id == CompletionPrompt::CUSTOM_PROMPT
+          raise Discourse::InvalidParameters.new(:custom_prompt) if params[:custom_prompt].blank?
+
+          prompt.custom_instruction = params[:custom_prompt]
         end
 
         hijack do
diff --git a/app/models/completion_prompt.rb b/app/models/completion_prompt.rb
index 9167ffef..c4f4bf61 100644
--- a/app/models/completion_prompt.rb
+++ b/app/models/completion_prompt.rb
@@ -20,10 +20,23 @@ class CompletionPrompt < ActiveRecord::Base
     where(enabled: true).find_by(name: name)
   end
 
+  attr_accessor :custom_instruction
+
   def messages_with_input(input)
     return unless input
 
-    messages_hash.merge(input: "<input>#{input}</input>")
+    user_input =
+      if id == CUSTOM_PROMPT && custom_instruction.present?
+        "#{custom_instruction}:\n#{input}"
+      else
+        input
+      end
+
+    messages_hash.merge(input: <<~TEXT)
+    <input>
+    #{user_input}
+    </input>
+    TEXT
   end
 
   private
diff --git a/lib/completions/endpoints/canned_response.rb b/lib/completions/endpoints/canned_response.rb
index 2bdf7226..83300fcc 100644
--- a/lib/completions/endpoints/canned_response.rb
+++ b/lib/completions/endpoints/canned_response.rb
@@ -13,11 +13,13 @@ module DiscourseAi
         def initialize(responses)
           @responses = responses
           @completions = 0
+          @prompt = nil
         end
 
-        attr_reader :responses, :completions
+        attr_reader :responses, :completions, :prompt
 
-        def perform_completion!(_prompt, _user, _model_params)
+        def perform_completion!(prompt, _user, _model_params)
+          @prompt = prompt
           response = responses[completions]
           if response.nil?
             raise CANNED_RESPONSE_ERROR,
diff --git a/spec/models/completion_prompt_spec.rb b/spec/models/completion_prompt_spec.rb
index 1eb356df..6f159f9e 100644
--- a/spec/models/completion_prompt_spec.rb
+++ b/spec/models/completion_prompt_spec.rb
@@ -18,4 +18,44 @@ RSpec.describe CompletionPrompt do
       end
     end
   end
+
+  describe "messages_with_input" do
+    let(:user_input) { "A user wrote this." }
+
+    context "when the record has the custom_prompt type" do
+      let(:custom_prompt) { described_class.find(described_class::CUSTOM_PROMPT) }
+
+      it "wraps the user input with <input> XML tags and adds a custom instruction if given" do
+        expected = <<~TEXT
+        <input>
+        Translate to Turkish:
+        #{user_input}
+        </input>
+        TEXT
+
+        custom_prompt.custom_instruction = "Translate to Turkish"
+
+        prompt = custom_prompt.messages_with_input(user_input)
+
+        expect(prompt[:input]).to eq(expected)
+      end
+    end
+
+    context "when the records don't have the custom_prompt type" do
+      let(:title_prompt) { described_class.find(described_class::GENERATE_TITLES) }
+
+      it "wraps user input with <input> XML tags" do
+        expected = <<~TEXT
+        <input>
+        #{user_input}
+        </input>
+        TEXT
+        title_prompt.custom_instruction = "Translate to Turkish"
+
+        prompt = title_prompt.messages_with_input(user_input)
+
+        expect(prompt[:input]).to eq(expected)
+      end
+    end
+  end
 end
diff --git a/spec/requests/ai_helper/assistant_controller_spec.rb b/spec/requests/ai_helper/assistant_controller_spec.rb
index 0f46a202..42ea139e 100644
--- a/spec/requests/ai_helper/assistant_controller_spec.rb
+++ b/spec/requests/ai_helper/assistant_controller_spec.rb
@@ -79,6 +79,33 @@ RSpec.describe DiscourseAi::AiHelper::AssistantController do
           expect(response.parsed_body["diff"]).to eq(expected_diff)
         end
       end
+
+      it "uses custom instruction when using custom_prompt mode" do
+        translated_text = "Un usuario escribio esto"
+        expected_diff =
+          "<div class=\"inline-diff\"><p><ins>Un </ins><ins>usuario </ins><ins>escribio </ins><ins>esto</ins><del>A </del><del>user </del><del>wrote </del><del>this</del></p></div>"
+
+        expected_input = <<~TEXT
+        <input>
+        Translate to Spanish:
+        A user wrote this
+        </input>
+        TEXT
+
+        DiscourseAi::Completions::Llm.with_prepared_responses([translated_text]) do |spy|
+          post "/discourse-ai/ai-helper/suggest",
+               params: {
+                 mode: CompletionPrompt::CUSTOM_PROMPT,
+                 text: "A user wrote this",
+                 custom_prompt: "Translate to Spanish",
+               }
+
+          expect(response.status).to eq(200)
+          expect(response.parsed_body["suggestions"].first).to eq(translated_text)
+          expect(response.parsed_body["diff"]).to eq(expected_diff)
+          expect(spy.prompt.last[:content]).to eq(expected_input)
+        end
+      end
     end
   end
 end