OAuth Fails To Authenticate With Non Owner User

[user1342]

To summarise the Canary Chrome extension reads HTML content, sends it off to the Google Vertex API, and has it analysed by the Gemini model. To do this I'm using Google OAuth 2.0, of which I've had my OAuth screen accepted for public use.

This works perfectly when I authenticate through the same email that is connected to my Google cloud account, however, when I authenticate with any other email address, it authenticates and I get a token back, however when using the API I then get a 403 error back indicating that the user does not have access to the API/ endpoint.

I'm assuming that there is something I need to do here in addition to what I've already done, maybe in setting up a service account or something similar. However, I can't find any documentation or support on what I'm missing.

If anyone has any support or advice for tackling this error it would be appreciated. Advice from Google support so far has not solved the issue: https://www.googlecloudcommunity.com/gc/Cloud-Hub/OAuth-Fails-To-Authenticate-With-Non-Owner-User/td-p/734545

Code here: https://github.com/CartographerLabs/Canary/blob/main/scripts/background.js
Chrome extension here: https://chromewebstore.google.com/detail/canary/ckbdhndglkicbpcnebjlglajljogaclj