fix(vertex_ai): use BadRequestError with exception chaining in Gemini transformation

Replace generic Exception raises with litellm.BadRequestError in
_gemini_convert_messages_with_history() for two error paths:

1. When file_id and file_data are both None — raises BadRequestError
2. When _process_gemini_media throws:
   - ImageFetchError — re-raised as BadRequestError preserving original message
   - BadRequestError — re-raised as-is
   - Any other exception — wrapped as BadRequestError with original error details

All wrapping raise statements use `raise ... from e` to preserve the
original exception chain for server-side debugging.

Added tests/test_litellm/llms/vertex_ai/gemini/test_gemini_transformation_exception_handling.py
covering all error paths.

Fixes BerriAI/litellm#24193

Co-authored-by: Kris Xia <xiajiayi0506@gmail.com>

litellm/llms/vertex_ai/gemini/transformation.py

@@ -3,6 +3,7 @@ Transformation logic from OpenAI format to Gemini format.
 
 Why separate file? Make it easy to see how transformation works
 """
+
 import json
 import os
 from typing import TYPE_CHECKING, Dict, List, Literal, Optional, Tuple, Union, cast
@@ -225,7 +226,11 @@ def _process_gemini_media(
             return _apply_gemini_3_metadata(
                 part, model, media_resolution_enum, video_metadata
             )
-        raise Exception("Invalid image received - {}".format(image_url))
+        raise litellm.BadRequestError(
+            message=f"Invalid image received - {image_url}. Supported formats are http://, https://, gs://, or base64 data.",
+            model=model,
+            llm_provider="vertex_ai",
+        )
     except Exception as e:
         raise e
 
@@ -381,10 +386,11 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
                             video_metadata = file_element["file"].get("video_metadata")
                             passed_file = file_id or file_data
                             if passed_file is None:
-                                raise Exception(
-                                    "Unknown file type. Please pass in a file_id or file_data"
+                                raise litellm.BadRequestError(
+                                    message="Unknown file type. Please pass in a file_id or file_data",
+                                    model=model,
+                                    llm_provider="vertex_ai",
                                 )
-
                             # Convert detail to media_resolution_enum
                             media_resolution_enum = (
                                 _convert_detail_to_media_resolution_enum(detail)
@@ -399,12 +405,23 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
                                     video_metadata=video_metadata,
                                 )
                                 _parts.append(_part)
-                            except Exception:
-                                raise Exception(
-                                    "Unable to determine mime type for file_id: {}, set this explicitly using message[{}].content[{}].file.format".format(
-                                        file_id, msg_i, element_idx
-                                    )
-                                )
+                            except Exception as e:
+                                if isinstance(e, litellm.ImageFetchError):
+                                    # ImageFetchError (e.g., 403/404 on URL) — wrap with model/provider context
+                                    raise litellm.BadRequestError(
+                                        message=str(e),
+                                        model=model,
+                                        llm_provider="vertex_ai",
+                                    ) from e
+                                elif isinstance(e, litellm.BadRequestError):
+                                    # Other BadRequestError (e.g., unsupported format) — preserve original message
+                                    raise
+                                else:
+                                    raise litellm.BadRequestError(
+                                        message=f"Unable to determine mime type for file_id: {file_id}, set this explicitly using message[{msg_i}].content[{element_idx}].file.format. Original error: {str(e)}",
+                                        model=model,
+                                        llm_provider="vertex_ai",
+                                    ) from e
                     user_content.extend(_parts)
                 elif _message_content is not None and isinstance(_message_content, str):
                     _part = PartType(text=_message_content)

tests/test_litellm/llms/vertex_ai/gemini/test_gemini_transformation_exception_handling.py

@@ -0,0 +1,73 @@
+import pytest
+from typing import List, cast
+from unittest.mock import patch
+
+import litellm
+from litellm.llms.vertex_ai.gemini.transformation import (
+    _gemini_convert_messages_with_history,
+)
+from litellm.types.llms.openai import AllMessageValues
+
+
+def test_missing_file_id_and_file_data_raises_bad_request_error():
+    """When file element has neither file_id nor file_data, a BadRequestError is raised."""
+    messages = cast(List[AllMessageValues], [{"role": "user", "content": [{"type": "file", "file": {}}]}])
+    with pytest.raises(litellm.BadRequestError) as exc_info:
+        _gemini_convert_messages_with_history(messages, model="gemini-1.5-pro")
+    assert "Unknown file type" in str(exc_info.value)
+
+
+def test_image_fetch_error_raises_bad_request_error():
+    """ImageFetchError from _process_gemini_media is re-raised as BadRequestError."""
+    messages = cast(
+        List[AllMessageValues],
+        [{"role": "user", "content": [{"type": "file", "file": {"file_id": "some_id"}}]}],
+    )
+    with patch(
+        "litellm.llms.vertex_ai.gemini.transformation._process_gemini_media",
+        side_effect=litellm.ImageFetchError(
+            message="403 Forbidden",
+            model="gemini-1.5-pro",
+            llm_provider="vertex_ai",
+        ),
+    ):
+        with pytest.raises(litellm.BadRequestError) as exc_info:
+            _gemini_convert_messages_with_history(messages, model="gemini-1.5-pro")
+    assert "403 Forbidden" in str(exc_info.value)
+
+
+def test_generic_exception_raises_bad_request_error_with_mime_message():
+    """Generic exception from _process_gemini_media is wrapped as BadRequestError with MIME message."""
+    messages = cast(
+        List[AllMessageValues],
+        [{"role": "user", "content": [{"type": "file", "file": {"file_id": "some_id"}}]}],
+    )
+    with patch(
+        "litellm.llms.vertex_ai.gemini.transformation._process_gemini_media",
+        side_effect=ValueError("cannot determine mime type"),
+    ):
+        with pytest.raises(litellm.BadRequestError) as exc_info:
+            _gemini_convert_messages_with_history(messages, model="gemini-1.5-pro")
+    assert "Unable to determine mime type" in str(exc_info.value)
+    assert "cannot determine mime type" in str(exc_info.value)
+
+
+def test_bad_request_error_from_process_gemini_media_is_re_raised_as_is():
+    """BadRequestError from _process_gemini_media is re-raised verbatim (not replaced by generic message)."""
+    original_message = "Invalid image received - https://example.com/img.png. Supported formats are..."
+    messages = cast(
+        List[AllMessageValues],
+        [{"role": "user", "content": [{"type": "file", "file": {"file_id": "some_id"}}]}],
+    )
+    with patch(
+        "litellm.llms.vertex_ai.gemini.transformation._process_gemini_media",
+        side_effect=litellm.BadRequestError(
+            message=original_message,
+            model="gemini-1.5-pro",
+            llm_provider="vertex_ai",
+        ),
+    ):
+        with pytest.raises(litellm.BadRequestError) as exc_info:
+            _gemini_convert_messages_with_history(messages, model="gemini-1.5-pro")
+    assert original_message in str(exc_info.value)
+    assert "Unable to determine mime type" not in str(exc_info.value)