Introducing Firenook

Firenook is a drop-in replacement for the Firebase Local Emulator Suite. It reads your firebase.json, listens on the same ports and keeps your data between runs. 0.2.0 is its first release for public use. This post explains why it exists, what it does today, what it does not do yet, and how to try it on your project in three steps.

A day with the emulator

If you build on Firebase, the emulator suite is part of your day. You start it in the morning, point your app and your tests at it, and leave it running. It does its job: it behaves like production. But the day has costs you stop noticing.

Before the first start, you install Java, because the official Firestore emulator runs on it. Then the data lives in the Java heap, and the heap grows. In our benchmark of a real application, two hours of its traffic took the official emulator from 15.42 GiB of memory to 22.57 GiB.

When you stop it, the data is gone unless you exported it. Keeping it means --export-on-exit at every stop and --import at every start. On that application’s data, 211,205 Firestore documents and 6.7 GB in Storage, the export took 54 to 58 seconds and the next start 30.7 to 31.7 seconds. A crash loses everything since the last export.

Writes slow down under load, too. With the application running, its listeners open and its functions firing, the official emulator acknowledged a write in 208 ms at the median.

None of this is a bug. An emulator that keeps its data in memory needs the memory to hold it and an export to keep it. Firenook is what we built to remove those costs without changing anything else in your project.

What Firenook is

One npm package. Installing it adds a small launcher, and npm picks the native engine for your machine from its optional dependencies: macOS on Apple silicon or Intel, Linux x64 or arm64 with glibc 2.28 or newer, or Windows x64. Nothing else is downloaded, at install or at start: no Java, no firebase-tools, no emulator archives. Node runs your Functions, as it does in production, so the emulators need Node 22, 24 or 26.

The same project. Firenook reads firebase.json and .firebaserc the way the Firebase CLI does, and serves each emulator on the port you gave it: Firestore on 8080, Auth on 9099, Storage on 9199, Functions on 5001, Pub/Sub on 8085, Cloud Tasks on 9499 and Eventarc on 9299 by default. Your client and Admin SDKs connect unchanged, connectFirestoreEmulator and FIRESTORE_EMULATOR_HOST included. It imports and exports the official format, so your data moves both ways.

A native engine. The services are written in Rust, and Firestore keeps its data on disk, not in a heap. Add --keep-data and Firestore, Auth and Storage come back with all your data at the next start, with no export and no import. Every acknowledged write is durable within a second; how Firenook writes 40× faster walks through how, and how an ordered index answers orderBy, ranges and cursors.

A console. The Emulator UI’s port serves Firenook’s console, at http://127.0.0.1:4000/console. It has a page for every service the suite runs: Firestore and Storage to browse and edit, Auth users with an inbox of the sign-in codes and email links Auth would have sent, and every function call with what triggered it, its outcome, its duration and its logs. Pub/Sub, Cloud Tasks and Eventarc have pages too, where the official UI has only its log.

A terminal. In a terminal, emulators:start draws its own screen: the start-up, a ready summary, and a line for every function call, with the stack trace when one fails. Press o to open the console, r to reload your functions and q to stop, which saves your data first.

A new project in one screen. npx firenook init in an empty folder asks every question on one screen, with an answer already chosen for each: the project id, the emulators, TypeScript or JavaScript Functions, and your coding agent. Then it installs, builds and offers to start the emulators in the same terminal. --build compiles your TypeScript as you save.

An MCP server. firenook mcp gives a coding agent 54 tools over the running suite, from Firestore queries to the line of your Security Rules that denied a request. Give your coding agent the emulator shows a session.

What we measured

We ran one real application against each emulator in turn, on the same server, with the same data and two hours of the same traffic. Then we ran each emulator alone for starts, stops, 26 query shapes and a store of 2.7 million documents. The official emulator was firebase-tools 15.22.0 on Java 26 with its default settings, and Firenook ran with its defaults.

Firenook was faster on 25 of the 26 query shapes and tied on the last, 500 document ids with no fields. Queries that order or range-filter a collection are served from an ordered index: on 2.7 million documents they take 3 to 6 ms, where the official emulator ran out of memory on Java’s defaults and needed a 24 GiB heap and 8 to 28 seconds. Within the Storage cycle, downloads are still a little slower on Firenook: 2.30 ms against 2.12 ms at the median.

Every figure, the method and its limits are on the benchmarks page. It is one server and one application, so measure your own workload before you rely on the numbers.

What it does not do yet

0.2.0 is the first release we ask the public to use. Commit your lockfile and upgrade on purpose, as you would any tool your tests depend on. Before a release is published, it passes conformance replays against recorded behaviour of the official emulator and a packed install on all five platforms.

Some things are not there yet:

• Realtime Database, Hosting, App Hosting and Data Connect are not emulated. A firebase.json that configures them still starts, with a warning, and the rest of the suite runs.
• There is no functions:shell. firenook functions:invoke calls any function on the running suite instead, background triggers included.
• There is no deploy or login command. Keep firebase-tools for deploying.
• Linux with musl (Alpine) and Windows on Arm are not qualified.

The compatibility page records what each service covers and what it does not.

Try it in three steps

Install it in your project, next to firebase-tools:

npm install --save-dev firenook

Check your project. This reads firebase.json the way the Firebase CLI does and changes nothing:

$ npx firenook init --adopt --dry-run
Firenook init --adopt: ~/demo-shop/firebase.json
  add     give Claude Code the Firenook MCP server: ~/demo-shop/.mcp.json
Dry run: nothing was written.

It names any service Firenook skips and any addition it would make, such as a missing emulators entry or the MCP server for the coding agent the project uses. Without --dry-run, it applies the additions, and edits your files as text, so their formatting stays.

Swap the command. The flags, the ports and the export format stay the same, so your app and your data need no change:

"scripts": {
  "emulators": "firebase emulators:start --import ./data --export-on-exit",
  "test:integration": "firebase emulators:exec \"npm test\""
  "emulators": "firenook emulators:start --import ./data --export-on-exit",
  "test:integration": "firenook emulators:exec \"npm test\""
}

Once that works, you can stop exporting. Replace --export-on-exit with --keep-data: the first start imports ./data into .firenook/state, every later start picks up where you stopped, and ./data is never written. Add .firenook/ to your .gitignore.

Going back takes the same steps in reverse. While Firenook runs, export with npx firenook emulators:export ./export, stop it, and start the official CLI on that export.

Open source and independent

Firenook is open source under the Apache License 2.0, and the code is at github.com/firenook-dev/firenook. It is an independent project, not affiliated with or endorsed by Google. Firebase, Cloud Firestore and the other product names are trademarks of Google LLC.

Every change to Firenook is checked against recorded behaviour of the official emulator, and the differences it keeps on purpose are written down. If you find one that is not, or anything else that breaks on your project, open an issue.

To start, switch an existing project or set up a new one.