diff --git a/lib/langchain/assistants/assistant.rb b/lib/langchain/assistants/assistant.rb
index 7b1d76425..9f689a688 100644
--- a/lib/langchain/assistants/assistant.rb
+++ b/lib/langchain/assistants/assistant.rb
@@ -27,6 +27,7 @@ class Assistant
     # @params add_message_callback [Proc] A callback function (Proc or lambda) that is called when any message is added to the conversation
     def initialize(
       llm:,
+      llm_adapter: nil,
       tools: [],
       instructions: nil,
       tool_choice: "auto",
@@ -39,7 +40,7 @@ def initialize(
       end
 
       @llm = llm
-      @llm_adapter = LLM::Adapter.build(llm)
+      @llm_adapter = llm_adapter || LLM::Adapter.build(llm)
 
       # TODO: Validate that it is, indeed, a Proc or lambda
       if !add_message_callback.nil? && !add_message_callback.respond_to?(:call)
diff --git a/lib/langchain/assistants/llm/adapters/_base.rb b/lib/langchain/assistants/llm/adapters/_base.rb
index 4cdcad18c..8b7d3f551 100644
--- a/lib/langchain/assistants/llm/adapters/_base.rb
+++ b/lib/langchain/assistants/llm/adapters/_base.rb
@@ -20,7 +20,11 @@ def tool_role
           end
 
           def support_system_message?
-            raise NotImplementedError, "Subclasses must implement set_instructions"
+            raise NotImplementedError, "Subclasses must implement support_system_message?"
+          end
+
+          def allowed_tool_choices
+            raise NotImplementedError, "Subclasses must implement allowed_tool_choices"
           end
         end
       end
diff --git a/spec/langchain/assistants/assistant_spec.rb b/spec/langchain/assistants/assistant_spec.rb
index ac4595ea2..8bca60f7c 100644
--- a/spec/langchain/assistants/assistant_spec.rb
+++ b/spec/langchain/assistants/assistant_spec.rb
@@ -1360,4 +1360,72 @@
       end
     end
   end
+
+  context "when llm is custom" do
+    let(:llm) { Langchain::LLM::Base.new }
+    let(:llm_adapter_class) do
+      Class.new(Langchain::Assistant::LLM::Adapters::Base) do
+        def allowed_tool_choices
+          ["auto"]
+        end
+
+        def available_tool_names(tools)
+          []
+        end
+
+        def support_system_message?
+          true
+        end
+
+        def build_message(role:, content: nil, image_url: nil, tool_calls: [], tool_call_id: nil)
+          message_class = Class.new(Langchain::Messages::Base) do
+            def initialize(role:, content: nil, image_url: nil, tool_calls: [], tool_call_id: nil)
+              @role = role
+              @content = content.to_s
+              @image_url = image_url
+              @tool_calls = tool_calls
+              @tool_call_id = tool_call_id
+            end
+          end
+
+          message_class.new(
+            role: role,
+            content: content,
+            image_url: image_url,
+            tool_calls: tool_calls,
+            tool_call_id: tool_call_id
+          )
+        end
+      end
+    end
+    let(:llm_adapter) { llm_adapter_class.new }
+    let(:calculator) { Langchain::Tool::Calculator.new }
+    let(:instructions) { "You are an expert assistant" }
+
+    subject {
+      described_class.new(
+        llm: llm,
+        llm_adapter: llm_adapter,
+        tools: [calculator],
+        instructions: instructions
+      )
+    }
+
+    describe ".new" do
+      it "initiates an assistant without error" do
+        expect { subject }.not_to raise_error
+      end
+    end
+
+    describe "#messages" do
+      it "returns list of messages" do
+        expect(subject.messages).to contain_exactly(
+          an_object_having_attributes(
+            role: "system",
+            content: "You are an expert assistant"
+          )
+        )
+      end
+    end
+  end
 end