feat: Add gemini-3-pro-image-preview model support for imageSize parameters (#17019)

- Add model identifier to FLASH_IMAGE_PREVIEW_MODEL_IDENTIFIERS
- Add imageSize parameter support (1K, 2K, 4K) with GeminiImageSize type
- Add tests for imageSize parameter transformation
- Update documentation with new model

docs/my-website/docs/completion/image_generation_chat.md

@@ -224,8 +224,8 @@ asyncio.run(generate_image())
 
 | Provider | Model | 
 |----------|--------|
-| Google AI Studio | `gemini/gemini-2.0-flash-preview-image-generation`, `gemini/gemini-2.5-flash-image-preview` |
-| Vertex AI | `vertex_ai/gemini-2.0-flash-preview-image-generation`, `vertex_ai/gemini-2.5-flash-image-preview` |
+| Google AI Studio | `gemini/gemini-2.0-flash-preview-image-generation`, `gemini/gemini-2.5-flash-image-preview`, `gemini/gemini-3-pro-image-preview` |
+| Vertex AI | `vertex_ai/gemini-2.0-flash-preview-image-generation`, `vertex_ai/gemini-2.5-flash-image-preview`, `vertex_ai/gemini-3-pro-image-preview` |
 
 ## Spec
 

litellm/llms/gemini/image_generation/transformation.py

@@ -25,6 +25,7 @@ FLASH_IMAGE_PREVIEW_MODEL_IDENTIFIERS = (
     "2.0-flash-preview-image",
     "2.0-flash-preview-image-generation",
     "2.5-flash-image-preview",
+    "3-pro-image-preview",
 )
 class GoogleImageGenConfig(BaseImageGenerationConfig):
     DEFAULT_BASE_URL: str = "https://generativelanguage.googleapis.com/v1beta"
@@ -75,7 +76,7 @@ class GoogleImageGenConfig(BaseImageGenerationConfig):
             "1792x1024": "16:9", 
             "1024x1792": "9:16",
             "1280x896": "4:3",
-            "896x1280": "3:4"
+            "896x1280": "3:4",
         }
         return aspect_ratio_map.get(size, "1:1")
 

litellm/types/llms/vertex_ai.py

@@ -183,8 +183,13 @@ GeminiResponseModalities = Literal["TEXT", "IMAGE", "AUDIO", "VIDEO"]
 
 GeminiImageAspectRatio = Literal["1:1", "2:3", "3:2", "3:4", "4:3", "9:16", "16:9", "21:9"]
 
+GeminiImageSize = Literal["1K", "2K", "4K"]
+
+
 class GeminiImageConfig(TypedDict, total=False):
     aspectRatio: GeminiImageAspectRatio
+    imageSize: GeminiImageSize
+
 
 class PrebuiltVoiceConfig(TypedDict):
     voiceName: str

tests/litellm/llms/vertex_ai/gemini/test_transformation.py

@@ -191,4 +191,38 @@ async def test__transform_request_body_image_config_snake_case():
 
     assert "generationConfig" in rb
     assert "image_config" in rb["generationConfig"]
-    assert rb["generationConfig"]["image_config"] == {"aspect_ratio": "16:9"}
+    assert rb["generationConfig"]["image_config"] == {"aspect_ratio": "16:9"}
+
+
+@pytest.mark.asyncio
+async def test__transform_request_body_image_config_with_image_size():
+    """Test imageSize parameter support in imageConfig"""
+    model = "gemini-3-pro-image-preview"
+    messages = [
+        {
+            "role": "user",
+            "content": [
+                {"type": "text", "text": "Generate a 4K image of Tokyo skyline"}
+            ]
+        }
+    ]
+    optional_params = {
+        "imageConfig": {"aspectRatio": "16:9", "imageSize": "4K"},
+        "responseModalities": ["Image"]
+    }
+    litellm_params = {}
+    transform_request_params = {
+        "messages": messages,
+        "model": model,
+        "optional_params": optional_params,
+        "custom_llm_provider": "gemini",
+        "litellm_params": litellm_params,
+        "cached_content": None,
+    }
+
+    rb: RequestBody = transformation._transform_request_body(**transform_request_params)
+
+    assert "generationConfig" in rb
+    assert "imageConfig" in rb["generationConfig"]
+    assert rb["generationConfig"]["imageConfig"]["aspectRatio"] == "16:9"
+    assert rb["generationConfig"]["imageConfig"]["imageSize"] == "4K"

tests/llm_translation/test_gemini.py

@@ -295,6 +295,7 @@ def test_gemini_image_generation():
     [
         "gemini/gemini-2.5-flash-image-preview",
         "gemini/gemini-2.0-flash-preview-image-generation",
+        "gemini/gemini-3-pro-image-preview",
     ],
 )
 def test_gemini_flash_image_preview_models(model_name: str):