fxtranslate

fxtranslate

Translate with Firefox Translations models from Node — a command-line tool and a library, powered by the same lightweight, CPU-only models Firefox ships for on-device translation. It discovers, downloads, and caches the models for you; no API keys, no network round-trips to a translation service, nothing leaves your machine after the model download.

The engine is the Rust fxtranslate crate compiled to WebAssembly, so translations match the native Rust CLI. The command-line interface is byte-for-byte compatible with the Rust fxtranslate-cli: same subcommands, flags, help text, and exit codes. Prefer a native binary or a Rust dependency? See Related packages.

Command-line tool

$ npm install -g fxtranslate

# List supported languages (or filter to one); --all shows the raw model pairs:
$ fxtranslate list
$ fxtranslate list es
$ fxtranslate list --all

# Translate a phrase. The model for the pair is discovered, downloaded, and
# cached on first use, then reused from disk on subsequent runs.
$ fxtranslate translate en es "The weather is nice today."
El clima es agradable hoy.

$ fxtranslate translate en de "Knowledge is power."
Wissen ist Macht.

$ fxtranslate translate es en "Buenos días, ¿cómo estás?"
Good morning, how are you?

# Neither side is English? It pivots through English automatically.
$ fxtranslate translate es fr "Buenos días."
Bonjour.

Every Firefox Translations model translates to or from English, so each direction is its own model (en → es and es → en are separate downloads). A pair where neither side is English — like es → fr — is served by pivoting: it runs es → en then en → fr automatically, so the full cartesian product of languages works. That's also why list shows languages by default rather than raw model pairs.

With no text argument, translate reads from stdin — one translation per line when piped, or an interactive prompt on a terminal:

# Pipe mode: one line in, one translation out.
$ echo "The library opens at nine in the morning." | fxtranslate translate en fr
La bibliothèque ouvre à neuf heures du matin.

# Interactive prompt (Ctrl-D to quit).
$ fxtranslate translate en es
Interactive en→es. Type a sentence and press Enter; Ctrl-D to quit.
en→es» ...

Status lines (model resolution, download progress) are written to stderr, so piped stdout carries only the translations.

Library

$ npm install fxtranslate

import { Translator } from "fxtranslate";
import { readFileSync } from "node:fs";

// You supply the model triple: the marian `.bin` and the source/target
// SentencePiece vocabularies (the same file for most pairs; only CJK pairs
// differ). Pass the source vocab twice for a shared-vocab pair.
const model = readFileSync("en-es/model.bin");
const vocab = readFileSync("en-es/vocab.spm");

const translator = new Translator(model, vocab, vocab);
console.log(translator.translate("The weather is nice today."));
// El clima es agradable hoy.

// translate_long segments multi-sentence / long input and translates each part
// within the model's context window, instead of truncating.
console.log(translator.translate_long(longArticleText));

The library also re-exports the pure discovery/routing helpers the CLI is built on — resolveRoute, catalog, modelPairs, parseRecords, segmentSentences, and verifyAndDecompress — for building your own model-management flow. Types ship in the box (fxtranslate is authored with JSDoc and a hand-maintained .d.ts).

The models

fxtranslate uses the models Firefox ships. They're discovered through Mozilla's Remote Settings and downloaded from Firefox's CDN, then cached under the platform-native cache directory — ~/Library/Caches/fxtranslate/models on macOS, $XDG_CACHE_HOME/fxtranslate/models on Linux, %LOCALAPPDATA%\fxtranslate\models on Windows — one subdirectory per language pair. Override the CLI's location with --cache-dir <DIR>.

That hosting is provisioned for Firefox, not for third-party traffic, and neither the endpoints nor the availability of any particular model are a stable contract for downstream projects. This package is a convenient way to try the engine; if you're shipping a product on top of it, mirror the model files you depend on, serve them from infrastructure you control, and load them with the Translator constructor.

Related packages

The same engine is published across ecosystems on one shared version, so you can pick the artifact that fits:

• fxtranslate (crates.io) — the Rust engine library. Native SIMD int8 kernel, optional built-in model management.
• fxtranslate-cli (crates.io) — the native Rust CLI. Same interface as this package's fxtranslate binary, installed with cargo install fxtranslate-cli.
• fxtranslate (PyPI) — the Python library (native compiled engine, batteries-included model management), installed with pip install fxtranslate.

Contributing

Building the wasm core, the conformance strategy that keeps this package byte-identical to the Rust CLI, and how to run the parity harness are documented in DEVELOPMENT.md.

License

MPL-2.0. The engine is a Rust port of the Firefox Translations inference engine.