fix(gemini): Preserve non-ASCII characters in function call arguments (#16550)

Fixes #16533

Before this fix, non-ASCII characters (Japanese, Spanish, Chinese, etc.)
in function call arguments were being escaped as Unicode sequences.

Example:
- Before: "やあ" → "\u3084\u3042"
- After: "やあ" → "やあ" (preserved)

Changes:
- Add ensure_ascii=False to json.dumps() in _transform_parts()
- Add test for Japanese and Spanish Unicode character preservation

This is not a breaking change as both formats are equivalent in JSON.
The fix improves readability and aligns with OpenAI's behavior.

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -1022,7 +1022,7 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
             if "functionCall" in part:
                 _function_chunk = ChatCompletionToolCallFunctionChunk(
                     name=part["functionCall"]["name"],
-                    arguments=json.dumps(part["functionCall"]["args"]),
+                    arguments=json.dumps(part["functionCall"]["args"], ensure_ascii=False),
                 )
                 if is_function_call is True:
                     function = _function_chunk

tests/llm_translation/test_gemini.py

@@ -1135,3 +1135,72 @@ def test_gemini_embedding():
     )
     print("response: ", response)
     assert response is not None
+
+
+def test_gemini_function_args_preserve_unicode():
+    """
+    Test for Issue #16533: Gemini function call arguments should preserve non-ASCII characters
+    https://github.com/BerriAI/litellm/issues/16533
+
+    Before fix: "や" becomes "\u3084"
+    After fix: "や" stays as "や"
+    """
+    from litellm.llms.vertex_ai.gemini.vertex_and_google_ai_studio_gemini import VertexGeminiConfig
+
+    # Test Japanese characters
+    parts = [
+        {
+            "functionCall": {
+                "name": "send_message",
+                "args": {
+                    "message": "やあ",  # Japanese "hello"
+                    "recipient": "たけし"  # Japanese name
+                }
+            }
+        }
+    ]
+
+    function, tools, _ = VertexGeminiConfig._transform_parts(
+        parts=parts,
+        cumulative_tool_call_idx=0,
+        is_function_call=False
+    )
+
+    arguments_str = tools[0]['function']['arguments']
+    parsed_args = json.loads(arguments_str)
+
+    # Verify characters are preserved
+    assert parsed_args["message"] == "やあ", "Japanese characters should be preserved"
+    assert parsed_args["recipient"] == "たけし", "Japanese characters should be preserved"
+
+    # Verify no Unicode escape sequences in raw string
+    assert "\\u" not in arguments_str, "Should not contain Unicode escape sequences"
+    assert "やあ" in arguments_str, "Original Japanese characters should be in the string"
+    assert "たけし" in arguments_str, "Original Japanese characters should be in the string"
+
+    # Test Spanish characters
+    parts_spanish = [
+        {
+            "functionCall": {
+                "name": "send_message",
+                "args": {
+                    "message": "¡Hola! ¿Cómo estás?",
+                    "recipient": "José"
+                }
+            }
+        }
+    ]
+
+    function, tools, _ = VertexGeminiConfig._transform_parts(
+        parts=parts_spanish,
+        cumulative_tool_call_idx=0,
+        is_function_call=False
+    )
+
+    arguments_str = tools[0]['function']['arguments']
+    parsed_args = json.loads(arguments_str)
+
+    assert parsed_args["message"] == "¡Hola! ¿Cómo estás?"
+    assert parsed_args["recipient"] == "José"
+    assert "\\u" not in arguments_str
+    assert "José" in arguments_str