Firebase callable function only returns 'internal' and nothing else

"internal" is what the client sees when the function throws something that isn't an HttpsError. The real error is in the function logs: Firebase console → Functions → Logs, or firebase functions:log --only createInvite.

Once you find it, throw errors you want the client to see like this:

import { HttpsError } from "firebase-functions/v2/https";
throw new HttpsError("invalid-argument", "Email is missing");

Also make sure the region matches. If the function is deployed to europe-west1 and the client uses getFunctions(app) without a region, it calls us-central1 and you can get confusing errors. Use getFunctions(app, "europe-west1").