Gmail API with service account- Precondition check failed

I’m trying to connect with the google API’s for the first time, and when I attempt to make a request to the gmail API I’m getting a Precondition check failed error. I am using a service account authorization, not Oauth2 user consent. Things I’ve tried:

1. Authorized “domain wide delegation” for the service account.
2. Ensured the APP is trusted in the G suite account.
3. Have also tried just returning the projectId from the API - and that works. So it is authorizing and connecting. It’s when I try to make a request to gmail specifically that’s giving me issue.
4. Service account role is “owner”

This is adapted from a sample, but the sample did not use service account auth so I wasn’t able to use the sample directly.

const path = require('path');
const {google} = require('googleapis');

const gmail = google.gmail('v1');

async function runSample() {
  // Obtain user credentials to use for the request
  const auth = new google.auth.GoogleAuth({
    keyFile: path.resolve(__dirname, 'google-key.json'),
    scopes: ['https://www.googleapis.com/auth/gmail.readonly'],
  });
  google.options({auth});

  const res = await gmail.users.messages.list({userId: 'me'}); // have tried with my gsuite email address as well
  console.log(res.data);
  return res.data;
}

if (module === require.main) {
  runSample().catch(console.error);
}
module.exports = runSample;

returning Error: Precondition check failed.

[vettloffah]

I figured out how to solve the issue.

I would like to sbumit a pull request for this issue to add a sample to the gmail sample library. It would look something like this (can work on refactoring to match code style in other samples if you give me the go-ahead on submitting a PR):

const path = require('path');
const {google} = require('googleapis');

async getMessageList(userId, qty) {

  const JWT = google.auth.JWT;
  const authClient = new JWT({
    keyFile: path.resolve(__dirname, 'google-key.json'),
    scopes: ['https://www.googleapis.com/auth/gmail.readonly'],
    subject: '[e-mail]' // google admin email address to impersonate
  });

  await authClient.authorize(); // once authorized, can do whatever you want

  const gmail = google.gmail({
    auth: authClient,
    version: 'v1'
  });

  const response = await gmail.users.messages.list({
    includeSpamTrash: false,
    maxResults: qty,
    q: "",
    userId: userId
  });

  // the data object includes a "messages" array of message data
  return response.data;

}

[sqrrrl]

The Gmail API isn’t intended to be used with service accounts (other than domain-wide delegation use cases.) You need to be acting as a real user – either using oauth credentials obtained with user consent, or in the case of a Gsuite domain, using a service account delegating/impersonating a real user.

I’ll file a bug to make the error message clearer, but it’s likely that the failed precondition is that the service account isn’t a valid gmail user.