Vertex Agent authentication issue with Google Video Intelligence API

[Paulbarnes00001]

HI

I'm building an agent on Vertex. It has a tool that is setup to call the Google Video Intelligence API using OpenAPI.

I get this error after giving the agent a GS uri in the chat:

The API returns :

{

"error": {

"message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",

"status": "UNAUTHENTICATED",

"code": 401

}

}

I'm using a service account and authenticating with a Service Agent Token. I've set the 

$GOOGLE_APPLICATION_CREDENTIALS environment variable locally and initiated with cloud on my client.

But I am unable to overcome this issue. I get the same error when I run in postman. And, I can't get any of the other authentication types to work. 

I have run out of my limited GCP knowledge and Gemini has exhausted all of it's answers too.

Would be grateful for some help to overcome this.

Thanks

Paul