Introduce support_system_message? method (#824)

* Introduce #support_system_message? in LLM adapter

Definition of whether the adapter supports system message stays in adapter. That makes an assistant class unchangeable when new adapter would be introduced.

* Add specs

* Cleanup

* fix linter

---------

Co-authored-by: Karol Bajko <[email protected]>

Author: andreibondarev

lib/langchain/assistant.rb

@@ -48,17 +48,13 @@ def initialize(
       self.messages = messages
       @tools = tools
       self.tool_choice = tool_choice
-      @instructions = instructions
+      self.instructions = instructions
       @block = block
       @state = :ready
 
       @total_prompt_tokens = 0
       @total_completion_tokens = 0
       @total_tokens = 0
-
-      # The first message in the messages array should be the system instructions
-      # For Google Gemini, and Anthropic system instructions are added to the `system:` param in the `chat` method
-      initialize_instructions
     end
 
     # Add a user message to the messages array
@@ -184,12 +180,9 @@ def clear_messages!
     def instructions=(new_instructions)
       @instructions = new_instructions
 
-      # This only needs to be done that support Message#@role="system"
-      if !llm.is_a?(Langchain::LLM::GoogleGemini) &&
-          !llm.is_a?(Langchain::LLM::GoogleVertexAI) &&
-          !llm.is_a?(Langchain::LLM::Anthropic)
+      if @llm_adapter.support_system_message?
         # Find message with role: "system" in messages and delete it from the messages array
-        replace_system_message!(content: new_instructions)
+        replace_system_message!(content: new_instructions) if @instructions
       end
     end
 
@@ -323,12 +316,6 @@ def execute_tools
       :failed
     end
 
-    def initialize_instructions
-      if llm.is_a?(Langchain::LLM::OpenAI) || llm.is_a?(Langchain::LLM::MistralAI)
-        self.instructions = @instructions if @instructions
-      end
-    end
-
     # Call to the LLM#chat() method
     #
     # @return [Langchain::LLM::BaseResponse] The LLM response object

lib/langchain/assistant/llm/adapters/anthropic.rb

@@ -70,6 +70,10 @@ def tool_role
             Messages::AnthropicMessage::TOOL_ROLE
           end
 
+          def support_system_message?
+            Messages::AnthropicMessage::ROLES.include?("system")
+          end
+
           private
 
           def build_tool_choice(choice)

lib/langchain/assistant/llm/adapters/base.rb

@@ -36,6 +36,13 @@ def build_message(role:, content: nil, image_url: nil, tool_calls: [], tool_call
             raise NotImplementedError, "Subclasses must implement build_message"
           end
 
+          # Does this adapter accept messages with role="system"?
+          #
+          # @return [Boolean] Whether the adapter supports system messages
+          def support_system_message?
+            raise NotImplementedError, "Subclasses must implement support_system_message?"
+          end
+
           # Role name used to return the tool output
           #
           # @return [String] The tool role

lib/langchain/assistant/llm/adapters/google_gemini.rb

@@ -70,6 +70,10 @@ def tool_role
             Messages::GoogleGeminiMessage::TOOL_ROLE
           end
 
+          def support_system_message?
+            Messages::GoogleGeminiMessage::ROLES.include?("system")
+          end
+
           private
 
           def build_tool_config(choice)

lib/langchain/assistant/llm/adapters/mistral_ai.rb

@@ -72,6 +72,10 @@ def tool_role
             Messages::MistralAIMessage::TOOL_ROLE
           end
 
+          def support_system_message?
+            Messages::MistralAIMessage::ROLES.include?("system")
+          end
+
           private
 
           def build_tool_choice(choice)

lib/langchain/assistant/llm/adapters/ollama.rb

@@ -68,6 +68,10 @@ def tool_role
             Messages::OllamaMessage::TOOL_ROLE
           end
 
+          def support_system_message?
+            Messages::OllamaMessage::ROLES.include?("system")
+          end
+
           private
 
           def build_tools(tools)

lib/langchain/assistant/llm/adapters/openai.rb

@@ -72,6 +72,10 @@ def tool_role
             Messages::OpenAIMessage::TOOL_ROLE
           end
 
+          def support_system_message?
+            Messages::OpenAIMessage::ROLES.include?("system")
+          end
+
           private
 
           def build_tool_choice(choice)

spec/langchain/assistant/assistant_spec.rb

@@ -472,6 +472,7 @@
     describe "#instructions=" do
       it "resets instructions" do
         subject.instructions = "New instructions"
+        expect(subject.messages.first.role).to eq("system")
         expect(subject.messages.first.content).to eq("New instructions")
         expect(subject.instructions).to eq("New instructions")
       end
@@ -829,6 +830,7 @@
     describe "#instructions=" do
       it "resets instructions" do
         subject.instructions = "New instructions"
+        expect(subject.messages.first.role).to eq("system")
         expect(subject.messages.first.content).to eq("New instructions")
         expect(subject.instructions).to eq("New instructions")
       end

spec/langchain/assistant/llm/adapters/.keep

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe Langchain::Assistant::LLM::Adapters::Anthropic do
+  let(:adapter) { described_class.new }
+
+  describe "#support_system_message?" do
+    it "returns true" do
+      expect(adapter.support_system_message?).to eq(false)
+    end
+  end
+
+  describe "#tool_role" do
+    it "returns 'tool'" do
+      expect(adapter.tool_role).to eq("tool_result")
+    end
+  end
+end