Sudden 401 Authentication Error with @google/genai and Vertex AI - "Request is missing required authentication credential"

[jhutan-acko]

I'm experiencing a sudden 401 authentication error in my production Node.js
application when calling Vertex AI APIs through the @google/genai library.
This was working fine until recently and started failing without any code
changes on our end.

Error Details:
ApiError: {
"error": {
"code": 401,
"message": "Request is missing required authentication credential.
Expected OAuth 2 access token, login cookie or other valid authentication
credential.",
"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"
}
}]
}
}

What I've Verified:

• ✅ Service account key is valid and not expired
• ✅ Service account has roles/aiplatform.user permissions
• ✅ Vertex AI API is enabled in the project
• ✅ Project billing is active
• ✅ Same credentials work with gcloud auth activate-service-account

Environment:

• Library: @google/[email protected] (was working before)
• Node.js: v18.20.3
• Project: XX
• Authentication: Service account JSON key (hardcoded credentials)
• Location: global

Questions:

1. Did Google change authentication requirements for Vertex AI APIs recently?
2. Are there known issues with @google/[email protected] and service account
   authentication?
3. Should I migrate to a different authentication method or library version?

Temporary Fix:
Downgrading to @google/[email protected] resolved the issue, suggesting a
regression in newer versions.

Has anyone else experienced this sudden authentication failure in production?
Any insights on what might have changed would be greatly appreciated.