Rename Agent to AgentStep with agent macro

Phase 3 of the Mars v2 refactor.

- Rename Mars::Agent → Mars::AgentStep with class-level `agent` macro
  that wraps a RubyLLM::Agent subclass
- AgentStep#run creates a new agent instance and delegates via .ask
- Remove old Agent with its manual Chat setup (before_run/after_run,
  system_prompt, tools, schema instance methods)
- Rename rendering graph module accordingly
- Bump ruby_llm dependency to ~> 1.12

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: santib

lib/mars/agent.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module MARS
+  class AgentStep < Runnable
+    class << self
+      def agent(klass = nil)
+        klass ? @agent_class = klass : @agent_class
+      end
+    end
+
+    def run(input)
+      self.class.agent.new.ask(input).content
+    end
+  end
+end

lib/mars/agent_step.rb

@@ -4,7 +4,7 @@ module MARS
   module Rendering
     module Graph
       def self.include_extensions
-        MARS::Agent.include(Agent)
+        MARS::AgentStep.include(AgentStep)
         MARS::Gate.include(Gate)
         MARS::Workflows::Sequential.include(SequentialWorkflow)
         MARS::Workflows::Parallel.include(ParallelWorkflow)

lib/mars/rendering/graph.rb

@@ -3,7 +3,7 @@
 module MARS
   module Rendering
     module Graph
-      module Agent
+      module AgentStep
         include Base
 
         def to_graph(builder, parent_id: nil, value: nil)
@@ -12,10 +12,6 @@ def to_graph(builder, parent_id: nil, value: nil)
 
           [node_id]
         end
-
-        def name
-          self.class.name
-        end
       end
     end
   end

lib/mars/rendering/graph/agent.rb lib/mars/rendering/graph/agent_step.rblib/mars/rendering/graph/agent.rb renamed to lib/mars/rendering/graph/agent_step.rb

@@ -36,7 +36,7 @@ Gem::Specification.new do |spec|
 
   # Uncomment to register a new dependency of your gem
   spec.add_dependency "async", "~> 2.34"
-  spec.add_dependency "ruby_llm", "~> 1.9"
+  spec.add_dependency "ruby_llm", "~> 1.12"
   spec.add_dependency "zeitwerk", "~> 2.7"
 
   # For more information and examples about making a new gem, check out our

mars.gemspec

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+RSpec.describe MARS::AgentStep do
+  describe ".agent" do
+    it "stores and retrieves the agent class" do
+      agent_class = Class.new
+      step_class = Class.new(described_class) do
+        agent agent_class
+      end
+
+      expect(step_class.agent).to eq(agent_class)
+    end
+
+    it "returns nil when no agent class is set" do
+      step_class = Class.new(described_class)
+      expect(step_class.agent).to be_nil
+    end
+  end
+
+  describe "#run" do
+    let(:mock_agent_instance) do
+      instance_double("RubyLLM::Agent").tap do |mock|
+        allow(mock).to receive(:ask).and_return(instance_double("RubyLLM::Message", content: "agent response"))
+      end
+    end
+
+    let(:mock_agent_class) do
+      instance_double("Class").tap do |mock|
+        allow(mock).to receive(:new).and_return(mock_agent_instance)
+      end
+    end
+
+    let(:step_class) do
+      klass = mock_agent_class
+      Class.new(described_class) do
+        agent klass
+      end
+    end
+
+    it "creates a new agent instance and calls ask" do
+      step = step_class.new
+      result = step.run("hello")
+
+      expect(result).to eq("agent response")
+      expect(mock_agent_class).to have_received(:new)
+      expect(mock_agent_instance).to have_received(:ask).with("hello")
+    end
+  end
+
+  describe "inheritance" do
+    it "inherits from MARS::Runnable" do
+      expect(described_class.ancestors).to include(MARS::Runnable)
+    end
+
+    it "has access to name, formatter, and hooks from Runnable" do
+      step = described_class.new(name: "my_agent")
+      expect(step.name).to eq("my_agent")
+      expect(step.formatter).to be_a(MARS::Formatter)
+    end
+  end
+end