fix(gemini): use JSON instead of form-data for image edit requests (#18012)

* fix(gemini): use JSON instead of form-data for image edit requests

Gemini's image edit API expects JSON body, not multipart/form-data.
The handler was sending form-encoded data which caused 400 errors:
"Invalid JSON payload received. Unexpected token."

Changes:
- Add use_multipart_form_data() method to BaseImageEditConfig (default True)
- Modify image_edit_handler to use json= when use_multipart_form_data() is False
- Override use_multipart_form_data() in GeminiImageEditConfig to return False

* test(gemini): add test for use_multipart_form_data

litellm/llms/base_llm/image_edit/transformation.py

@@ -109,6 +109,15 @@ class BaseImageEditConfig(ABC):
     ) -> ImageResponse:
         pass
 
+    def use_multipart_form_data(self) -> bool:
+        """
+        Return True if the provider uses multipart/form-data for image edit requests.
+        Return False if the provider uses JSON requests.
+
+        Default is True for backwards compatibility with OpenAI-style providers.
+        """
+        return True
+
     def get_error_class(
         self, error_message: str, status_code: int, headers: Union[dict, httpx.Headers]
     ) -> BaseLLMException:

litellm/llms/custom_httpx/llm_http_handler.py

@@ -3768,13 +3768,24 @@ class BaseLLMHTTPHandler:
         )
 
         try:
-            response = sync_httpx_client.post(
-                url=api_base,
-                headers=headers,
-                data=data,
-                files=files,
-                timeout=timeout,
-            )
+            # Check if provider uses multipart/form-data or JSON
+            if image_edit_provider_config.use_multipart_form_data():
+                # Use form-data (OpenAI style)
+                response = sync_httpx_client.post(
+                    url=api_base,
+                    headers=headers,
+                    data=data,
+                    files=files,
+                    timeout=timeout,
+                )
+            else:
+                # Use JSON (Gemini style)
+                response = sync_httpx_client.post(
+                    url=api_base,
+                    headers=headers,
+                    json=data,
+                    timeout=timeout,
+                )
 
         except Exception as e:
             raise self._handle_error(
@@ -3853,13 +3864,24 @@ class BaseLLMHTTPHandler:
         )
 
         try:
-            response = await async_httpx_client.post(
-                url=api_base,
-                headers=headers,
-                data=data,
-                files=files,
-                timeout=timeout,
-            )
+            # Check if provider uses multipart/form-data or JSON
+            if image_edit_provider_config.use_multipart_form_data():
+                # Use form-data (OpenAI style)
+                response = await async_httpx_client.post(
+                    url=api_base,
+                    headers=headers,
+                    data=data,
+                    files=files,
+                    timeout=timeout,
+                )
+            else:
+                # Use JSON (Gemini style)
+                response = await async_httpx_client.post(
+                    url=api_base,
+                    headers=headers,
+                    json=data,
+                    timeout=timeout,
+                )
 
         except Exception as e:
             raise self._handle_error(

litellm/llms/gemini/image_edit/transformation.py

@@ -63,6 +63,10 @@ class GeminiImageEditConfig(BaseImageEditConfig):
         headers["Content-Type"] = "application/json"
         return headers
 
+    def use_multipart_form_data(self) -> bool:
+        """Gemini uses JSON requests, not multipart/form-data."""
+        return False
+
     def get_complete_url(
         self,
         model: str,

tests/test_litellm/llms/gemini/image_edit/test_gemini_image_edit_transformation.py

@@ -147,3 +147,14 @@ class TestGeminiImageEditTransformation:
                 headers={},
             )
 
+    def test_use_multipart_form_data_returns_false(self) -> None:
+        """
+        Gemini uses JSON requests, not multipart/form-data.
+        This is critical because httpx sends data differently:
+        - data=dict sends form-encoded
+        - json=dict sends JSON
+
+        Without this, Gemini returns: "Invalid JSON payload received. Unexpected token."
+        """
+        assert self.config.use_multipart_form_data() is False
+