'An error occurred when trying to authenticate to the FCM servers' on Firebase Cloud Functions

Question

I'm trying to send a message to a topic with FCM in a Firebase Cloud Function triggered when a Firestore document (a message) is created. Subscribing to the topic (also done with Functions) and triggering the send function works fine, but the actual send fails with:

Error: An error occurred when trying to authenticate to the FCM servers. Make sure the credential used to authenticate this SDK has the proper permissions. See https://firebase.google.com/docs/admin/setup for setup instructions.

and some raw HTML containing <H1>PROJECT_NOT_PERMITTED</H1> and <H1>PROJECT_NOT_PERMITTED</H1> .

Here is my code (index.ts):

import * as admin from 'firebase-admin';

admin.initializeApp({
    credential: admin.credential.applicationDefault(),
});

export * from './messages';

and (messages.ts):

import * as admin from 'firebase-admin';
import * as functions from 'firebase-functions';

export const publishMessage = functions
  .firestore.document('/messages/{messageId}').onCreate(
    (snapshot, context) => {
      const data = snapshot.data();
      const message = {
        notification: {
          title: `${data.sentBy} sent a message`,
          body: data.message,
        },
      };

      return admin.messaging().sendToTopic('messages', message);
    },
  );

According to https://firebase.google.com/docs/cloud-messaging/auth-server#provide-credentials-using-adc this should work. I have also tried doing it without any parameters (https://firebase.google.com/docs/admin/setup#initialize-without-parameters) but it fails all the same. What am I missing?

Answer 1

Turns out that instead of this

      const message = {
        notification: {
          title: `${data.sentBy} sent a message`,
          body: data.message,
        },
      };

      return admin.messaging().sendToTopic('messages', message);

I needed this:

      const message = {
        notification: {
          title: `${data.sentBy} sent a message`,
          body: data.message,
        },
        topic: 'messages',
      };

      return admin.messaging().send('messages', message);