Flutter Firebase Auth: MISSING_CUSTOM_TOKEN

Updated: February 15, 2023 By: Guest Contributor Post a comment

This short article is about an error that often happens when using Firebase Authentication in Flutter.

The Problem

When implementing with Firebase Authentication and Flutter, you may face the MISSING_CUSTOM_TOKEN error.

The response body:

  "error": {
    "code": 400,
    "message": "MISSING_CUSTOM_TOKEN",
    "errors": [
        "message": "MISSING_CUSTOM_TOKEN",
        "domain": "global",
        "reason": "invalid"

The common reasons cause that error are:

  • Wrong API endpoint
  • Invalid API Key

At the time of writing, the API endpoint looks like this:


ACTION: signInWithCustomToken, signUp, signInWithPassword…

Note: The API endpoint structure may change over time. To get the right one, check this official guide from the Firebase website.


1. Make sure you get the right API key by going to Project settings > General > Web API Key.

2. Review your API endpoint.

Sample Firebase Auth Code In Flutter

Install the http plugin, then implement a signUp function as follows:

import 'dart:convert';
import 'package:http/http.dart' as http;

Future<void> signUp(String email, String password) async {
  final url = Uri.parse(

  final response = await http.post(url, body: json.encode({
    'email': email,
    'password': password,
    'returnSecureToken': true


// Now call the signUp function somewhere

That’s it. Further reading:

You can also check out our Flutter category page or Dart category page for the latest tutorials and examples.

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