Remove hardcoded model name and fix breaking change

- Reverted GEMINI_2_5_FLASH_IMAGE_PREVIEW_MODEL constant usage
- Made endpoint selection conditional for gemini-2.5-flash-image-preview only
- Preserved existing Imagen models functionality with :predict endpoint
- Fixed potential breaking change that would affect 6 other Gemini image models

litellm/llms/gemini/image_generation/transformation.py

@@ -86,8 +86,8 @@ class GoogleImageGenConfig(BaseImageGenerationConfig):
         """
         Get the complete url for the request
 
-        Google AI API format: https://generativelanguage.googleapis.com/v1beta/models/{model}:generateContent
-        Note: Gemini image generation models use generateContent, not predict
+        Gemini 2.5 Flash Image Preview: :generateContent
+        Other Imagen models: :predict
         """
         complete_url: str = (
             api_base
@@ -96,7 +96,14 @@ class GoogleImageGenConfig(BaseImageGenerationConfig):
         )
 
         complete_url = complete_url.rstrip("/")
-        complete_url = f"{complete_url}/models/{model}:generateContent"
+
+        # Gemini 2.5 Flash Image Preview uses generateContent endpoint
+        if "2.5-flash-image-preview" in model:
+            complete_url = f"{complete_url}/models/{model}:generateContent"
+        else:
+            # All other Imagen models use predict endpoint
+            complete_url = f"{complete_url}/models/{model}:predict"
+
         return complete_url
 
     def validate_environment(

tests/llm_translation/test_gemini.py

@@ -323,7 +323,7 @@ def test_gemini_2_5_flash_image_preview():
         call_args = mock_post.call_args
         called_url = call_args[0][0] if call_args[0] else call_args.kwargs.get('url', '')
 
-        # Verify it uses generateContent endpoint (not predict)
+        # Verify it uses generateContent endpoint for gemini-2.5-flash-image-preview (not predict)
         assert ":generateContent" in called_url
         assert "gemini-2.5-flash-image-preview" in called_url
 
@@ -333,6 +333,53 @@ def test_gemini_2_5_flash_image_preview():
         assert "parts" in request_data["contents"][0]
 
 
+def test_gemini_imagen_models_use_predict_endpoint():
+    """
+    Test that Imagen models still use :predict endpoint (not broken by gemini-2.5-flash-image-preview fix)
+    """
+    from unittest.mock import patch, MagicMock
+    from litellm.types.utils import ImageResponse, ImageObject
+
+    with patch("litellm.llms.custom_httpx.llm_http_handler.HTTPHandler.post") as mock_post:
+        # Mock successful HTTP response for Imagen
+        mock_http_response = MagicMock()
+        mock_http_response.json.return_value = {
+            "predictions": [
+                {
+                    "bytesBase64Encoded": "test_base64_image_data"
+                }
+            ]
+        }
+        mock_http_response.status_code = 200
+        mock_post.return_value = mock_http_response
+
+        # Test an Imagen model
+        response = litellm.image_generation(
+            model="gemini/imagen-3.0-generate-001",
+            prompt="Generate a simple test image",
+            api_key="test_api_key"
+        )
+
+        # Validate response structure
+        assert response is not None
+        assert hasattr(response, 'data')
+
+        # Validate the correct endpoint was called for Imagen models
+        mock_post.assert_called_once()
+        call_args = mock_post.call_args
+        called_url = call_args[0][0] if call_args[0] else call_args.kwargs.get('url', '')
+
+        # Verify Imagen models use predict endpoint (not generateContent)
+        assert ":predict" in called_url
+        assert "imagen-3.0-generate-001" in called_url
+        assert ":generateContent" not in called_url
+
+        # Verify request format is Imagen format (not Gemini)
+        request_data = call_args.kwargs.get('json', {})
+        assert "instances" in request_data
+        assert "parameters" in request_data
+
+
 def test_gemini_thinking():
     litellm._turn_on_debug()
     from litellm.types.utils import Message, CallTypes