feat(vertex): add Imagen image generation model support

Add support for Imagen 3.0 and 4.0 image generation models in Vertex AI:

- Add 5 Imagen model definitions (4.0, 4.0-ultra, 4.0-fast, 3.0, 3.0-fast)
- Implement :predict action routing for Imagen models
- Convert Imagen request/response format to match Gemini structure like gemini-3-pro-image
- Transform prompts to Imagen's instances/parameters format
- Convert base64 image responses to Gemini-compatible inline data

internal/registry/model_definitions.go

@@ -287,6 +287,67 @@ func GetGeminiVertexModels() []*ModelInfo {
 			SupportedGenerationMethods: []string{"generateContent", "countTokens", "createCachedContent", "batchGenerateContent"},
 			Thinking:                   &ThinkingSupport{Min: 128, Max: 32768, ZeroAllowed: false, DynamicAllowed: true, Levels: []string{"low", "high"}},
 		},
+		// Imagen image generation models - use :predict action
+		{
+			ID:                         "imagen-4.0-generate-001",
+			Object:                     "model",
+			Created:                    1750000000,
+			OwnedBy:                    "google",
+			Type:                       "gemini",
+			Name:                       "models/imagen-4.0-generate-001",
+			Version:                    "4.0",
+			DisplayName:                "Imagen 4.0 Generate",
+			Description:                "Imagen 4.0 image generation model",
+			SupportedGenerationMethods: []string{"predict"},
+		},
+		{
+			ID:                         "imagen-4.0-ultra-generate-001",
+			Object:                     "model",
+			Created:                    1750000000,
+			OwnedBy:                    "google",
+			Type:                       "gemini",
+			Name:                       "models/imagen-4.0-ultra-generate-001",
+			Version:                    "4.0",
+			DisplayName:                "Imagen 4.0 Ultra Generate",
+			Description:                "Imagen 4.0 Ultra high-quality image generation model",
+			SupportedGenerationMethods: []string{"predict"},
+		},
+		{
+			ID:                         "imagen-3.0-generate-002",
+			Object:                     "model",
+			Created:                    1740000000,
+			OwnedBy:                    "google",
+			Type:                       "gemini",
+			Name:                       "models/imagen-3.0-generate-002",
+			Version:                    "3.0",
+			DisplayName:                "Imagen 3.0 Generate",
+			Description:                "Imagen 3.0 image generation model",
+			SupportedGenerationMethods: []string{"predict"},
+		},
+		{
+			ID:                         "imagen-3.0-fast-generate-001",
+			Object:                     "model",
+			Created:                    1740000000,
+			OwnedBy:                    "google",
+			Type:                       "gemini",
+			Name:                       "models/imagen-3.0-fast-generate-001",
+			Version:                    "3.0",
+			DisplayName:                "Imagen 3.0 Fast Generate",
+			Description:                "Imagen 3.0 fast image generation model",
+			SupportedGenerationMethods: []string{"predict"},
+		},
+		{
+			ID:                         "imagen-4.0-fast-generate-001",
+			Object:                     "model",
+			Created:                    1750000000,
+			OwnedBy:                    "google",
+			Type:                       "gemini",
+			Name:                       "models/imagen-4.0-fast-generate-001",
+			Version:                    "4.0",
+			DisplayName:                "Imagen 4.0 Fast Generate",
+			Description:                "Imagen 4.0 fast image generation model",
+			SupportedGenerationMethods: []string{"predict"},
+		},
 	}
 }
 

internal/runtime/executor/gemini_vertex_executor.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"net/http"
 	"strings"
+	"time"
 
 	vertexauth "github.com/router-for-me/CLIProxyAPI/v6/internal/auth/vertex"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
@@ -31,6 +32,143 @@ const (
 	vertexAPIVersion = "v1"
 )
 
+// isImagenModel checks if the model name is an Imagen image generation model.
+// Imagen models use the :predict action instead of :generateContent.
+func isImagenModel(model string) bool {
+	lowerModel := strings.ToLower(model)
+	return strings.Contains(lowerModel, "imagen")
+}
+
+// getVertexAction returns the appropriate action for the given model.
+// Imagen models use "predict", while Gemini models use "generateContent".
+func getVertexAction(model string, isStream bool) string {
+	if isImagenModel(model) {
+		return "predict"
+	}
+	if isStream {
+		return "streamGenerateContent"
+	}
+	return "generateContent"
+}
+
+// convertImagenToGeminiResponse converts Imagen API response to Gemini format
+// so it can be processed by the standard translation pipeline.
+// This ensures Imagen models return responses in the same format as gemini-3-pro-image-preview.
+func convertImagenToGeminiResponse(data []byte, model string) []byte {
+	predictions := gjson.GetBytes(data, "predictions")
+	if !predictions.Exists() || !predictions.IsArray() {
+		return data
+	}
+
+	// Build Gemini-compatible response with inlineData
+	parts := make([]map[string]any, 0)
+	for _, pred := range predictions.Array() {
+		imageData := pred.Get("bytesBase64Encoded").String()
+		mimeType := pred.Get("mimeType").String()
+		if mimeType == "" {
+			mimeType = "image/png"
+		}
+		if imageData != "" {
+			parts = append(parts, map[string]any{
+				"inlineData": map[string]any{
+					"mimeType": mimeType,
+					"data":     imageData,
+				},
+			})
+		}
+	}
+
+	// Generate unique response ID using timestamp
+	responseId := fmt.Sprintf("imagen-%d", time.Now().UnixNano())
+
+	response := map[string]any{
+		"candidates": []map[string]any{{
+			"content": map[string]any{
+				"parts": parts,
+				"role":  "model",
+			},
+			"finishReason": "STOP",
+		}},
+		"responseId":   responseId,
+		"modelVersion": model,
+		// Imagen API doesn't return token counts, set to 0 for tracking purposes
+		"usageMetadata": map[string]any{
+			"promptTokenCount":     0,
+			"candidatesTokenCount": 0,
+			"totalTokenCount":      0,
+		},
+	}
+
+	result, err := json.Marshal(response)
+	if err != nil {
+		return data
+	}
+	return result
+}
+
+// convertToImagenRequest converts a Gemini-style request to Imagen API format.
+// Imagen API uses a different structure: instances[].prompt instead of contents[].
+func convertToImagenRequest(payload []byte) ([]byte, error) {
+	// Extract prompt from Gemini-style contents
+	prompt := ""
+
+	// Try to get prompt from contents[0].parts[0].text
+	contentsText := gjson.GetBytes(payload, "contents.0.parts.0.text")
+	if contentsText.Exists() {
+		prompt = contentsText.String()
+	}
+
+	// If no contents, try messages format (OpenAI-compatible)
+	if prompt == "" {
+		messagesText := gjson.GetBytes(payload, "messages.#.content")
+		if messagesText.Exists() && messagesText.IsArray() {
+			for _, msg := range messagesText.Array() {
+				if msg.String() != "" {
+					prompt = msg.String()
+					break
+				}
+			}
+		}
+	}
+
+	// If still no prompt, try direct prompt field
+	if prompt == "" {
+		directPrompt := gjson.GetBytes(payload, "prompt")
+		if directPrompt.Exists() {
+			prompt = directPrompt.String()
+		}
+	}
+
+	if prompt == "" {
+		return nil, fmt.Errorf("imagen: no prompt found in request")
+	}
+
+	// Build Imagen API request
+	imagenReq := map[string]any{
+		"instances": []map[string]any{
+			{
+				"prompt": prompt,
+			},
+		},
+		"parameters": map[string]any{
+			"sampleCount": 1,
+		},
+	}
+
+	// Extract optional parameters
+	if aspectRatio := gjson.GetBytes(payload, "aspectRatio"); aspectRatio.Exists() {
+		imagenReq["parameters"].(map[string]any)["aspectRatio"] = aspectRatio.String()
+	}
+	if sampleCount := gjson.GetBytes(payload, "sampleCount"); sampleCount.Exists() {
+		imagenReq["parameters"].(map[string]any)["sampleCount"] = int(sampleCount.Int())
+	}
+	if negativePrompt := gjson.GetBytes(payload, "negativePrompt"); negativePrompt.Exists() {
+		imagenReq["instances"].([]map[string]any)[0]["negativePrompt"] = negativePrompt.String()
+	}
+
+	return json.Marshal(imagenReq)
+}
+
 // GeminiVertexExecutor sends requests to Vertex AI Gemini endpoints using service account credentials.
 type GeminiVertexExecutor struct {
 	cfg *config.Config
@@ -160,26 +298,38 @@ func (e *GeminiVertexExecutor) executeWithServiceAccount(ctx context.Context, au
 	reporter := newUsageReporter(ctx, e.Identifier(), baseModel, auth)
 	defer reporter.trackFailure(ctx, &err)
 
-	from := opts.SourceFormat
+	var body []byte
-	to := sdktranslator.FromString("gemini")
 
-	originalPayload := bytes.Clone(req.Payload)
+	// Handle Imagen models with special request format
-	if len(opts.OriginalRequest) > 0 {
+	if isImagenModel(baseModel) {
-		originalPayload = bytes.Clone(opts.OriginalRequest)
+		imagenBody, errImagen := convertToImagenRequest(req.Payload)
-	}
+		if errImagen != nil {
-	originalTranslated := sdktranslator.TranslateRequest(from, to, baseModel, originalPayload, false)
+			return resp, errImagen
-	body := sdktranslator.TranslateRequest(from, to, baseModel, bytes.Clone(req.Payload), false)
+		}
+		body = imagenBody
+	} else {
+		// Standard Gemini translation flow
+		from := opts.SourceFormat
+		to := sdktranslator.FromString("gemini")
 
-	body, err = thinking.ApplyThinking(body, req.Model, from.String(), to.String())
+		originalPayload := bytes.Clone(req.Payload)
-	if err != nil {
+		if len(opts.OriginalRequest) > 0 {
-		return resp, err
+			originalPayload = bytes.Clone(opts.OriginalRequest)
+		}
+		originalTranslated := sdktranslator.TranslateRequest(from, to, baseModel, originalPayload, false)
+		body = sdktranslator.TranslateRequest(from, to, baseModel, bytes.Clone(req.Payload), false)
+
+		body, err = thinking.ApplyThinking(body, req.Model, from.String(), to.String())
+		if err != nil {
+			return resp, err
+		}
+
+		body = fixGeminiImageAspectRatio(baseModel, body)
+		body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated)
+		body, _ = sjson.SetBytes(body, "model", baseModel)
 	}
 
-	body = fixGeminiImageAspectRatio(baseModel, body)
+	action := getVertexAction(baseModel, false)
-	body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated)
-	body, _ = sjson.SetBytes(body, "model", baseModel)
-
-	action := "generateContent"
 	if req.Metadata != nil {
 		if a, _ := req.Metadata["action"].(string); a == "countTokens" {
 			action = "countTokens"
@@ -249,6 +399,16 @@ func (e *GeminiVertexExecutor) executeWithServiceAccount(ctx context.Context, au
 	}
 	appendAPIResponseChunk(ctx, e.cfg, data)
 	reporter.publish(ctx, parseGeminiUsage(data))
+
+	// For Imagen models, convert response to Gemini format before translation
+	// This ensures Imagen responses use the same format as gemini-3-pro-image-preview
+	if isImagenModel(baseModel) {
+		data = convertImagenToGeminiResponse(data, baseModel)
+	}
+
+	// Standard Gemini translation (works for both Gemini and converted Imagen responses)
+	from := opts.SourceFormat
+	to := sdktranslator.FromString("gemini")
 	var param any
 	out := sdktranslator.TranslateNonStream(ctx, to, from, req.Model, bytes.Clone(opts.OriginalRequest), body, data, &param)
 	resp = cliproxyexecutor.Response{Payload: []byte(out)}
@@ -281,7 +441,7 @@ func (e *GeminiVertexExecutor) executeWithAPIKey(ctx context.Context, auth *clip
 	body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated)
 	body, _ = sjson.SetBytes(body, "model", baseModel)
 
-	action := "generateContent"
+	action := getVertexAction(baseModel, false)
 	if req.Metadata != nil {
 		if a, _ := req.Metadata["action"].(string); a == "countTokens" {
 			action = "countTokens"
@@ -384,12 +544,16 @@ func (e *GeminiVertexExecutor) executeStreamWithServiceAccount(ctx context.Conte
 	body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated)
 	body, _ = sjson.SetBytes(body, "model", baseModel)
 
+	action := getVertexAction(baseModel, true)
 	baseURL := vertexBaseURL(location)
-	url := fmt.Sprintf("%s/%s/projects/%s/locations/%s/publishers/google/models/%s:%s", baseURL, vertexAPIVersion, projectID, location, baseModel, "streamGenerateContent")
+	url := fmt.Sprintf("%s/%s/projects/%s/locations/%s/publishers/google/models/%s:%s", baseURL, vertexAPIVersion, projectID, location, baseModel, action)
-	if opts.Alt == "" {
+	// Imagen models don't support streaming, skip SSE params
-		url = url + "?alt=sse"
+	if !isImagenModel(baseModel) {
-	} else {
+		if opts.Alt == "" {
-		url = url + fmt.Sprintf("?$alt=%s", opts.Alt)
+			url = url + "?alt=sse"
+		} else {
+			url = url + fmt.Sprintf("?$alt=%s", opts.Alt)
+		}
 	}
 	body, _ = sjson.DeleteBytes(body, "session_id")
 
@@ -503,15 +667,19 @@ func (e *GeminiVertexExecutor) executeStreamWithAPIKey(ctx context.Context, auth
 	body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated)
 	body, _ = sjson.SetBytes(body, "model", baseModel)
 
+	action := getVertexAction(baseModel, true)
 	// For API key auth, use simpler URL format without project/location
 	if baseURL == "" {
 		baseURL = "https://generativelanguage.googleapis.com"
 	}
-	url := fmt.Sprintf("%s/%s/publishers/google/models/%s:%s", baseURL, vertexAPIVersion, baseModel, "streamGenerateContent")
+	url := fmt.Sprintf("%s/%s/publishers/google/models/%s:%s", baseURL, vertexAPIVersion, baseModel, action)
-	if opts.Alt == "" {
+	// Imagen models don't support streaming, skip SSE params
-		url = url + "?alt=sse"
+	if !isImagenModel(baseModel) {
-	} else {
+		if opts.Alt == "" {
-		url = url + fmt.Sprintf("?$alt=%s", opts.Alt)
+			url = url + "?alt=sse"
+		} else {
+			url = url + fmt.Sprintf("?$alt=%s", opts.Alt)
+		}
 	}
 	body, _ = sjson.DeleteBytes(body, "session_id")