diff --git a/lib/ruby_llm/active_record/chat_methods.rb b/lib/ruby_llm/active_record/chat_methods.rb
index 41930548c..88b8d0194 100644
--- a/lib/ruby_llm/active_record/chat_methods.rb
+++ b/lib/ruby_llm/active_record/chat_methods.rb
@@ -79,7 +79,8 @@ def to_llm
         model_record = model_association
         @chat ||= (context || RubyLLM).chat(
           model: model_record.model_id,
-          provider: model_record.provider.to_sym
+          provider: model_record.provider.to_sym,
+          assume_model_exists: assume_model_exists || false
         )
         @chat.reset_messages!
 
diff --git a/spec/ruby_llm/active_record/acts_as_model_spec.rb b/spec/ruby_llm/active_record/acts_as_model_spec.rb
index 56fb01570..4ef603965 100644
--- a/spec/ruby_llm/active_record/acts_as_model_spec.rb
+++ b/spec/ruby_llm/active_record/acts_as_model_spec.rb
@@ -255,7 +255,8 @@ def messages
         # Mock the chat creation to verify parameters
         expect(RubyLLM).to receive(:chat).with( # rubocop:disable RSpec/MessageSpies,RSpec/StubbedMock
           model: 'test-gpt',
-          provider: :openai
+          provider: :openai,
+          assume_model_exists: false
         ).and_return(
           instance_double(RubyLLM::Chat, reset_messages!: nil, add_message: nil,
                                          instance_variable_get: {}, on_new_message: nil, on_end_message: nil,
@@ -272,7 +273,8 @@ def messages
 
         expect(RubyLLM).to receive(:chat).with( # rubocop:disable RSpec/MessageSpies,RSpec/StubbedMock
           model: 'test-claude',
-          provider: :anthropic
+          provider: :anthropic,
+          assume_model_exists: false
         ).and_return(
           instance_double(RubyLLM::Chat, reset_messages!: nil, add_message: nil,
                                          instance_variable_get: {}, on_new_message: nil, on_end_message: nil,
diff --git a/spec/ruby_llm/active_record/acts_as_spec.rb b/spec/ruby_llm/active_record/acts_as_spec.rb
index a36c00b49..8612ce0c8 100644
--- a/spec/ruby_llm/active_record/acts_as_spec.rb
+++ b/spec/ruby_llm/active_record/acts_as_spec.rb
@@ -500,6 +500,14 @@ class ToolCall < ActiveRecord::Base # rubocop:disable RSpec/LeakyConstantDeclara
         expect(llm_tool_call.name).to eq('calculator')
         expect(llm_tool_call.arguments).to eq({ 'expression' => '2 + 2' })
       end
+
+      it 'correctly preserves custom model' do
+        custom_model = 'my-custom-model'
+        bot_chat = Assistants::BotChat.create!(model: custom_model, assume_model_exists: true, provider: 'openrouter')
+        bot_chat.save!
+        llm_chat = bot_chat.to_llm
+        expect(llm_chat.model.id).to eq(custom_model)
+      end
     end
   end