VertexAI: Add service account key authentication support

docs/_getting_started/configuration.md

@@ -55,6 +55,7 @@ RubyLLM.configure do |config|
   config.gemini_api_key = ENV['GEMINI_API_KEY']
   config.vertexai_project_id = ENV['GOOGLE_CLOUD_PROJECT'] # Available in v1.7.0+
   config.vertexai_location = ENV['GOOGLE_CLOUD_LOCATION']
+  config.vertexai_service_account_key = ENV['VERTEXAI_SERVICE_ACCOUNT_KEY'] # JSON Key as String from GCP
   config.deepseek_api_key = ENV['DEEPSEEK_API_KEY']
   config.mistral_api_key = ENV['MISTRAL_API_KEY']
   config.perplexity_api_key = ENV['PERPLEXITY_API_KEY']
@@ -96,6 +97,12 @@ end
 
 These headers are optional and only needed for organization-specific billing or project tracking.
 
+### Vertex AI Authentication Configuration
+
+Google Cloud disallows the creation of Vertex AI API keys for Service Accounts, by default. The recommended way to connect is by using a Service Account's JSON key with appropriate IAM roles or by using Application Default Credentials.
+
+RubyLLM supports both methods of authenticating to Vertex AI and will only use a Service Account key if the key is provided in the `config.vertexai_service_account_key` configuration field. Otherwise, it will fallback to ADC.
+
 ## Custom Endpoints
 
 ### OpenAI-Compatible APIs
@@ -389,6 +396,7 @@ RubyLLM.configure do |config|
   config.gemini_api_key = String
   config.vertexai_project_id = String  # GCP project ID
   config.vertexai_location = String     # e.g., 'us-central1'
+  config.vertexai_service_account_key = String # The JSON key as available for GCP Service Accountss
   config.deepseek_api_key = String
   config.mistral_api_key = String
   config.perplexity_api_key = String

lib/ruby_llm/configuration.rb

@@ -16,6 +16,7 @@ class Configuration
                   :gemini_api_base,
                   :vertexai_project_id,
                   :vertexai_location,
+                  :vertexai_service_account_key,
                   :deepseek_api_key,
                   :perplexity_api_key,
                   :bedrock_api_key,

lib/ruby_llm/providers/vertexai.rb

@@ -10,6 +10,11 @@ class VertexAI < Gemini
       include VertexAI::Models
       include VertexAI::Transcription
 
+      SCOPES = [
+        'https://www.googleapis.com/auth/cloud-platform',
+        'https://www.googleapis.com/auth/generative-language.retriever'
+      ]
+
       def initialize(config)
         super
         @authorizer = nil
@@ -36,20 +41,22 @@ def headers
 
       class << self
         def configuration_requirements
-          %i[vertexai_project_id vertexai_location]
+          %i[vertexai_project_id vertexai_location vertexai_service_account_key]
         end
       end
 
       private
 
       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'
-          ]
-        )
+        @authorizer = if @config.vertexai_service_account_key
+          ::Google::Auth::ServiceAccountCredentials.make_creds(
+            json_key_io: StringIO.new(@config.vertexai_service_account_key),
+            scope: SCOPES
+          )
+        else
+          ::Google::Auth.get_application_default(scope: SCOPES)
+        end
       rescue LoadError
         raise Error,
               'The googleauth gem ~> 1.15 is required for Vertex AI. Please add it to your Gemfile: gem "googleauth"'

spec/support/rubyllm_configuration.rb

@@ -14,6 +14,9 @@
       config.bedrock_region = 'us-west-2'
       config.bedrock_secret_key = ENV.fetch('AWS_SECRET_ACCESS_KEY', 'test')
       config.bedrock_session_token = ENV.fetch('AWS_SESSION_TOKEN', nil)
+      config.vertexai_project_id = ENV.fetch('GOOGLE_CLOUD_PROJECT', 'test-project')
+      config.vertexai_location = ENV.fetch('GOOGLE_CLOUD_LOCATION', 'us-central1')
+      config.vertexai_service_account_key = ENV.fetch('VERTEXAI_SERVICE_ACCOUNT_KEY', "{ secret_key: 'test' }")
       config.deepseek_api_key = ENV.fetch('DEEPSEEK_API_KEY', 'test')
       config.gemini_api_key = ENV.fetch('GEMINI_API_KEY', 'test')
       config.gpustack_api_base = ENV.fetch('GPUSTACK_API_BASE', 'http://localhost:11444/v1')