Use the Extensions Emulator to evaluate extensions

Before using the Extensions emulator with your app, make sure that you understand the overall Firebase Local Emulator Suite workflow, and that you install and configure the Local Emulator Suite and review its CLI commands.

This guide also assumes you are familiar with Firebase Extensions and how to use them in your Firebase apps.

What can I do with the Extensions emulator?

With the Extensions emulator, you can install and manage extensions in a safe local environment and better understand their capabilities while minimizing billing costs. The emulator runs your extension's functions locally, including background event-triggered functions using the emulators for Cloud Firestore, Realtime Database, Cloud Storage for Firebase, Authentication and Pub/Sub, and Eventarc-triggered functions implemented in Cloud Functions v2.

Choose a Firebase project

The Firebase Local Emulator Suite emulates products for a single Firebase project.

To select the project to use, before you start the emulators, in the CLI run firebase use in your working directory. Or, you can pass the --project flag to each emulator command.

Local Emulator Suite supports emulation of real Firebase projects and demo projects.

Project type Features Use with emulators

A real Firebase project is one you created and configured (most likely via the Firebase console).

Real projects have live resources, like database instances, storage buckets, functions, or any other resource you set up for that Firebase project.

When working with real Firebase projects, you can run emulators for any or all of the supported products.

For any products you are not emulating, your apps and code will interact with the live resource (database instance, storage bucket, function, etc.).


A demo Firebase project has no real Firebase configuration and no live resources. These projects are usually accessed via codelabs or other tutorials.

Project IDs for demo projects have the demo- prefix.

When working with demo Firebase projects, your apps and code interact with emulators only. If your app attempts to interact with a resource for which an emulator isn't running, that code will fail.

We recommend you use demo projects wherever possible. Benefits include:

  • Easier setup, since you can run the emulators without ever creating a Firebase project
  • Stronger safety, since if your code accidentally invokes non-emulated (production) resources, there is no chance of data change, usage and billing
  • Better offline support, since there is no need to access the internet to download your SDK configuration.

Install and evaluate an extension

Using the Extensions emulator to evaluate whether an extension meets your needs is straightforward.

Let's assume you're interested in the Trigger Email (firestore-send-email) extension, though the following workflow covers any extension. When run with local emulators, Trigger Email will automatically make use of the Cloud Firestore and Cloud Functions emulators.

To evaluate an extension locally:

  1. Add the extension to the local extensions manifest. An extensions manifest is a list of extension instances and their configurations.

    firebase ext:install --local firebase/firestore-send-email

    Running the above command will prompt you to configure the latest version of firebase/firestore-send-email extension and save the configuration to the manifest, but it won't deploy the configuration to your project. For more about this, see Manage extensions configuration with manifests

  2. Start the Local Emulator Suite as you would normally.

    firebase emulators:start

Now, using the firestore-send-email extension instance listed in your manifest, the Local Emulator Suite will download the source code of that extension to ~/.cache/firebase/extensions. Once soures have been downloaded, the Local Emulator Suite will start and you'll be able to trigger any of the extension's background triggered functions and connect your app to the Local Emulator Suite to test their integration with your app.

You can use Emulator Suite UI to add data to the email documents collection and set up other backend resources, as required by the Trigger Email extension.

Alternatively, for non-interactive testing environments like continuous integration workflows, you can write a test script for evaluating the extension that, among other steps, populates necessary Cloud Firestore data and triggers functions. You would then invoke the Local Emulator Suite to execute your test script:

firebase emulators:exec

How testing with the Extensions emulator differs from production

The Extensions emulator lets you test extensions in a way that closely matches the production experience. However, there are some differences from production behavior.

Cloud IAM

The Firebase Emulator Suite does not attempt to replicate or respect any IAM-related behavior for running. Emulators adhere to the Firebase Security Rules provided, but in situations where IAM would normally be used, for example to set Cloud Functions invoking service account and thus permissions, the emulator is not configurable and will use the globally-available account on your developer machine, similar to running a local script directly.

Triggering type limitation

Currently, the Firebase Local Emulator Suite only supports HTTP request-triggered functions, Eventarc custom event triggers for extensions, and background event-triggered functions for Cloud Firestore, Realtime Database, Cloud Storage for Firebase, Authentication and Pub/Sub. To evaluate extensions that use other types of triggered functions, you need to install your extension in a test Firebase project.

What next?