diff --git a/lib/ruby_llm/providers/vertexai.rb b/lib/ruby_llm/providers/vertexai.rb
index 268774eec..8cf7bc057 100644
--- a/lib/ruby_llm/providers/vertexai.rb
+++ b/lib/ruby_llm/providers/vertexai.rb
@@ -41,7 +41,7 @@ def configuration_requirements
       def initialize_authorizer
         require 'googleauth'
         @authorizer = ::Google::Auth.get_application_default(
-          scope: [
+          [
             'https://www.googleapis.com/auth/cloud-platform',
             'https://www.googleapis.com/auth/generative-language.retriever'
           ]
diff --git a/spec/ruby_llm/providers/vertexai/auth_spec.rb b/spec/ruby_llm/providers/vertexai/auth_spec.rb
new file mode 100644
index 000000000..bcb5325f0
--- /dev/null
+++ b/spec/ruby_llm/providers/vertexai/auth_spec.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe RubyLLM::Providers::VertexAI do
+  include_context 'with configured RubyLLM'
+
+  describe '#initialize_authorizer' do
+    let(:provider) do
+      described_class.new(RubyLLM.config)
+    end
+
+    let(:mock_credentials) do
+      instance_double(Google::Auth::GCECredentials, apply: { 'Authorization' => 'Bearer test-token' })
+    end
+
+    before do
+      provider.instance_variable_set(:@authorizer, nil)
+    end
+
+    it 'passes scope as a positional Array argument to get_application_default' do
+      # This test verifies the fix for the TypeError that occurs when running on GCE.
+      # Google::Auth.get_application_default expects scope as a positional argument,
+      # not a keyword argument. Passing `scope: [...]` causes Ruby to interpret it
+      # as a Hash, which triggers: TypeError: Expected Array or String, got Hash
+      expected_scopes = [
+        'https://www.googleapis.com/auth/cloud-platform',
+        'https://www.googleapis.com/auth/generative-language.retriever'
+      ]
+
+      expect(Google::Auth).to receive(:get_application_default)
+        .with(expected_scopes)
+        .and_return(mock_credentials)
+
+      provider.headers
+    end
+  end
+end