Add Vertex AI support

internal/config/config.go

@@ -48,6 +48,8 @@ type ProviderEntry struct {
 	APIKey     string `yaml:"api_key"`
 	Endpoint   string `yaml:"endpoint"`
 	APIVersion string `yaml:"api_version"`
+	Project    string `yaml:"project"`  // For Vertex AI
+	Location   string `yaml:"location"` // For Vertex AI
 }
 
 // ModelEntry maps a model name to a provider entry.
@@ -78,6 +80,12 @@ type AzureAnthropicConfig struct {
 	Model    string `yaml:"model"`
 }
 
+// VertexAIConfig contains Vertex AI-specific settings used internally by the Google provider.
+type VertexAIConfig struct {
+	Project  string `yaml:"project"`
+	Location string `yaml:"location"`
+}
+
 // Load reads and parses a YAML configuration file, expanding ${VAR} env references.
 func Load(path string) (*Config, error) {
 	data, err := os.ReadFile(path)

internal/providers/google/google.go

@@ -19,7 +19,7 @@ type Provider struct {
 	client *genai.Client
 }
 
-// New constructs a Provider using the provided configuration.
+// New constructs a Provider using the Google AI API with API key authentication.
 func New(cfg config.ProviderConfig) *Provider {
 	var client *genai.Client
 	if cfg.APIKey != "" {
@@ -38,13 +38,36 @@ func New(cfg config.ProviderConfig) *Provider {
 	}
 }
 
+// NewVertexAI constructs a Provider targeting Vertex AI.
+// Vertex AI uses the same genai SDK but with GCP project/location configuration
+// and Application Default Credentials (ADC) or service account authentication.
+func NewVertexAI(vertexCfg config.VertexAIConfig) *Provider {
+	var client *genai.Client
+	if vertexCfg.Project != "" && vertexCfg.Location != "" {
+		var err error
+		client, err = genai.NewClient(context.Background(), &genai.ClientConfig{
+			Project:  vertexCfg.Project,
+			Location: vertexCfg.Location,
+			Backend:  genai.BackendVertexAI,
+		})
+		if err != nil {
+			// Log error but don't fail construction - will fail on Generate
+			fmt.Printf("warning: failed to create vertex ai client: %v\n", err)
+		}
+	}
+	return &Provider{
+		cfg: config.ProviderConfig{
+			// Vertex AI doesn't use API key, but set empty for consistency
+			APIKey: "",
+		},
+		client: client,
+	}
+}
+
 func (p *Provider) Name() string { return Name }
 
 // Generate routes the request to Gemini and returns a ProviderResult.
 func (p *Provider) Generate(ctx context.Context, messages []api.Message, req *api.ResponseRequest) (*api.ProviderResult, error) {
-	if p.cfg.APIKey == "" {
-		return nil, fmt.Errorf("google api key missing")
-	}
 	if p.client == nil {
 		return nil, fmt.Errorf("google client not initialized")
 	}
@@ -96,10 +119,6 @@ func (p *Provider) GenerateStream(ctx context.Context, messages []api.Message, r
 		defer close(deltaChan)
 		defer close(errChan)
 
-		if p.cfg.APIKey == "" {
-			errChan <- fmt.Errorf("google api key missing")
-			return
-		}
 		if p.client == nil {
 			errChan <- fmt.Errorf("google client not initialized")
 			return

internal/providers/providers.go

@@ -60,7 +60,8 @@ func NewRegistry(entries map[string]config.ProviderEntry, models []config.ModelE
 }
 
 func buildProvider(entry config.ProviderEntry) (Provider, error) {
-	if entry.APIKey == "" {
+	// Vertex AI doesn't require APIKey, so check for it separately
+	if entry.Type != "vertexai" && entry.APIKey == "" {
 		return nil, nil
 	}
 
@@ -97,6 +98,14 @@ func buildProvider(entry config.ProviderEntry) (Provider, error) {
 			APIKey:   entry.APIKey,
 			Endpoint: entry.Endpoint,
 		}), nil
+	case "vertexai":
+		if entry.Project == "" || entry.Location == "" {
+			return nil, fmt.Errorf("project and location are required for vertexai")
+		}
+		return googleprovider.NewVertexAI(config.VertexAIConfig{
+			Project:  entry.Project,
+			Location: entry.Location,
+		}), nil
 	default:
 		return nil, fmt.Errorf("unknown provider type %q", entry.Type)
 	}