Batch API - "Request contains an invalid argument" - Despite Correct File Uri

Gemini API
1

I am using Batch API which sends audio files for running some identification in that audio file. But I am getting

Request contains an invalid argument

My request looks like this,

request_item = {
                "request": { 
                    "contents": [
                        {
                            "role": "user",
                            "parts": [
                                {"text": TRANSCRIPTION_SYSTEM_PROMPT + "\n" + TRANSCRIPTION_USER_PROMPT},
                                {
                                    "file_data": {
                                        "file_uri": google_file.uri,
                                        "mime_type": mime_type
                                    }
                                }
                            ]
                        }
                    ],
                    "generation_config": {
                        "response_mime_type": "application/json",
                        "response_schema": TRANSCRIPT_SCHEMA,
                        "temperature": 0.0
                    }
                }
            }

The file is uploaded via File API. I have tried system prompt with role system and also tried after removing them. But still I am getting the error constantly. I have tried running Gemini-2.5-flash and also Gemini-3-flash-preview. What is the fix for this?