Extract RubyLLM::Prompt from Agent's private prompt rendering

lib/ruby_llm/agent.rb

@@ -138,14 +138,8 @@ def sync_instructions!(chat_or_id, **kwargs)
       end
 
       def render_prompt(name, chat:, inputs:, locals:)
-        path = prompt_path_for(name)
-        unless File.exist?(path)
-          raise RubyLLM::PromptNotFoundError,
-                "Prompt file not found for #{self}: #{path}. Create the file or use inline instructions."
-        end
-
         resolved_locals = resolve_prompt_locals(locals, runtime: runtime_context(chat:, inputs:), chat:, inputs:)
-        ERB.new(File.read(path)).result_with_hash(resolved_locals)
+        Prompt.new("#{prompt_agent_path}/#{name}").render(**resolved_locals)
       end
 
       private
@@ -304,12 +298,6 @@ def runtime_context(chat:, inputs:)
         end
       end
 
-      def prompt_path_for(name)
-        filename = name.to_s
-        filename += '.txt.erb' unless filename.end_with?('.txt.erb')
-        prompt_root.join(prompt_agent_path, filename)
-      end
-
       def prompt_agent_path
         class_name = name || 'agent'
         class_name.gsub('::', '/')
@@ -319,14 +307,6 @@ def prompt_agent_path
                   .downcase
       end
 
-      def prompt_root
-        if defined?(Rails) && Rails.respond_to?(:root) && Rails.root
-          Rails.root.join('app/prompts')
-        else
-          Pathname.new(Dir.pwd).join('app/prompts')
-        end
-      end
-
       def resolved_chat_model
         return @resolved_chat_model if defined?(@resolved_chat_model)
 

lib/ruby_llm/prompt.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module RubyLLM
+  # Renders ERB prompt templates from the prompts directory.
+  class Prompt
+    attr_reader :name, :path
+
+    def initialize(name)
+      @name = name.to_s
+      @path = self.class.root.join(
+        @name.end_with?('.txt.erb') ? @name : "#{@name}.txt.erb"
+      )
+    end
+
+    def render(**locals)
+      raise PromptNotFoundError, "Prompt file not found: #{@path}" unless File.exist?(@path)
+
+      ERB.new(File.read(@path)).result_with_hash(locals)
+    end
+
+    def self.render(name, **locals)
+      new(name).render(**locals)
+    end
+
+    def self.root
+      if defined?(Rails) && Rails.respond_to?(:root) && Rails.root
+        Rails.root.join('app/prompts')
+      else
+        Pathname.new(Dir.pwd).join('app/prompts')
+      end
+    end
+  end
+end

spec/ruby_llm/prompt_spec.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'tmpdir'
+
+RSpec.describe RubyLLM::Prompt do
+  let(:tmpdir) { Dir.mktmpdir }
+  let(:prompt_dir) { Pathname.new(tmpdir).join('app/prompts') }
+
+  before do
+    prompt_dir.mkpath
+    allow(described_class).to receive(:root).and_return(prompt_dir)
+  end
+
+  after do
+    FileUtils.rm_rf(tmpdir)
+  end
+
+  def create_prompt(name, content)
+    path = prompt_dir.join("#{name}.txt.erb")
+    path.dirname.mkpath
+    path.write(content)
+  end
+
+  describe '.render' do
+    it 'renders a prompt with locals' do
+      create_prompt('friend', 'Hello, <%= name %>!')
+      expect(described_class.render('friend', name: 'Andrey')).to eq('Hello, Andrey!')
+    end
+
+    it 'renders a nested prompt path' do
+      create_prompt('work_assistant/instructions', 'You assist <%= user %>.')
+      expect(described_class.render('work_assistant/instructions', user: 'Bob')).to eq('You assist Bob.')
+    end
+
+    it 'renders without locals' do
+      create_prompt('simple', 'Just a static prompt.')
+      expect(described_class.render('simple')).to eq('Just a static prompt.')
+    end
+
+    it 'raises PromptNotFoundError for missing prompts' do
+      expect { described_class.render('nonexistent') }.to raise_error(RubyLLM::PromptNotFoundError)
+    end
+  end
+
+  describe '#render' do
+    it 'renders the prompt with locals' do
+      create_prompt('greeting', 'Hi <%= name %>, welcome!')
+      prompt = described_class.new('greeting')
+      expect(prompt.render(name: 'Andrey')).to eq('Hi Andrey, welcome!')
+    end
+
+    it 'exposes name and path' do
+      prompt = described_class.new('greeting')
+      expect(prompt.name).to eq('greeting')
+      expect(prompt.path).to eq(prompt_dir.join('greeting.txt.erb'))
+    end
+  end
+end