google.genai.Client suddenly hits 401 error using Gemini API Key

    client = genai.Client(api_key=os.environ.get("GEMINI_API_KEY"))
    model_id = "gemini-2.0-flash"
    google_search_tool = Tool(
        google_search = GoogleSearch()
    )
    response = await client.models.generate_content(
        model=model_id,
        contents=[{"role": "user", "parts": [{"text": query}]}], 
        config=GenerateContentConfig(
            tools=[google_search_tool],
            response_modalities=["TEXT"],
        )
    )

I has been working past few days but today, suddenly, boom!

Error: ClientError("401 UNAUTHENTICATED. {'error': {'code': 401, 'message': 'API keys are not supported by this API. Expected OAuth2 access token or other authentication credentials that assert a principal. See https://cloud.google.com/docs/authentication', 'status': 'UNAUTHENTICATED', 'details': [{'@type': 'type.googleapis.com/google.rpc.ErrorInfo', 'reason': 'CREDENTIALS_MISSING', 'domain': 'googleapis.com', 'metadata': {'method': 'google.cloud.aiplatform.v1beta1.PredictionService.GenerateContent', 'service': 'aiplatform.googleapis.com'}}]}}")

Remove GOOGLE_GENAI_USE_VERTEXAI=true solves the problem.