Fix anthropic system messages

lib/langchain/assistant/messages/anthropic_message.rb

@@ -7,7 +7,8 @@ class AnthropicMessage < Base
         ROLES = [
           "assistant",
           "user",
-          "tool_result"
+          "tool_result",
+          "system"
         ].freeze
 
         TOOL_ROLE = "tool_result"
@@ -42,6 +43,8 @@ def initialize(
         def to_hash
           if assistant?
             assistant_hash
+          elsif system?
+            system_hash
           elsif tool?
             tool_hash
           elsif user?
@@ -64,6 +67,16 @@ def assistant_hash
           }
         end
 
+        # Convert the message to an Anthropic API-compatible hash
+        #
+        # @return [Hash] The message as an Anthropic API-compatible hash, with the role as "system"
+        def system_hash
+          {
+            role: "system",
+            content: build_content_array
+          }
+        end
+
         # Convert the message to an Anthropic API-compatible hash
         #
         # @return [Hash] The message as an Anthropic API-compatible hash, with the role as "user"
@@ -125,9 +138,11 @@ def tool?
           role == "tool_result"
         end
 
-        # Anthropic does not implement system prompts
+        # Check if the message is a system message
+        #
+        # @return [Boolean] true/false whether this message is a system message
         def system?
-          false
+          role == "system"
         end
 
         # Check if the message came from an LLM

spec/lib/langchain/assistant/assistant_spec.rb

@@ -1223,7 +1223,10 @@
         before do
           allow(subject.llm).to receive(:chat)
             .with(
-              messages: [{role: "user", content: [{text: "Please calculate 2+2", type: "text"}]}],
+              messages: [
+                {role: "system", content: [{text: instructions, type: "text"}]},
+                {role: "user", content: [{text: "Please calculate 2+2", type: "text"}]}
+              ],
               tools: calculator.class.function_schemas.to_anthropic_format,
               tool_choice: {disable_parallel_tool_use: false, type: "auto"},
               system: instructions
@@ -1268,6 +1271,7 @@
           allow(subject.llm).to receive(:chat)
             .with(
               messages: [
+                {role: "system", content: [{text: instructions, type: "text"}]},
                 {role: "user", content: [{text: "Please calculate 2+2", type: "text"}]},
                 {role: "assistant", content: [
                   {

spec/lib/langchain/assistant/llm/adapters/anthropic_spec.rb

@@ -22,7 +22,7 @@
 
   describe "#support_system_message?" do
     it "returns true" do
-      expect(subject.support_system_message?).to eq(false)
+      expect(subject.support_system_message?).to eq(true)
     end
   end
 

spec/lib/langchain/assistant/messages/anthropic_message_spec.rb

@@ -96,6 +96,32 @@
       end
     end
 
+    context "when role is system" do
+      let(:role) { "system" }
+
+      it "returns system_hash" do
+        message = described_class.new(role: role, content: "You are a helpful assistant.")
+        expect(message).to receive(:system_hash).and_call_original
+        expect(message.to_hash).to eq(
+          role: role,
+          content: [
+            {
+              type: "text",
+              text: "You are a helpful assistant."
+            }
+          ]
+        )
+      end
+
+      it "returns system_hash without content" do
+        message = described_class.new(role: role, content: nil)
+        expect(message.to_hash).to eq(
+          role: role,
+          content: []
+        )
+      end
+    end
+
     context "when role is tool_result" do
       let(:message) { described_class.new(role: "tool_result", content: "4.0", tool_call_id: "toolu_014eSx9oBA5DMe8gZqaqcJ3H") }
 
@@ -153,4 +179,16 @@
       end
     end
   end
+
+  describe "#system?" do
+    it "returns true when role is system" do
+      message = described_class.new(role: "system", content: "You are a helpful assistant.")
+      expect(message.system?).to be true
+    end
+
+    it "returns false when role is not system" do
+      message = described_class.new(role: "user", content: "Hello")
+      expect(message.system?).to be false
+    end
+  end
 end