Subpair

English | 한국어 | 日本語

A free, open-source alternative to Language Reactor, for Netflix and your own Jellyfin library. Subpair shows two subtitle lines at once, the language you are learning and your own, with line replay, auto-pause, a transcript panel and clickable words.

• Free and MIT-licensed: no account, no tracking.
• Your own Jellyfin library, not only Netflix.
• Translation when a show has no subtitles in your language, through a small helper on your computer: Claude, any OpenAI-compatible model, a local model or offline ezTrans. Translations are saved and reused, the next episode is translated ahead, and on Jellyfin a whole season at once.
• Japanese word colors and one-click lookup with Yomitan.
• English, Korean and Japanese interface.

Website: wonjoonseol.github.io/subpair

How it works

flowchart LR
  N[Netflix] -->|subtitle tracks| E[Subpair extension]
  J[Your Jellyfin server] -->|subtitle tracks| E
  E -. optional: lines to translate .-> B[Bridge on your computer]
  B --> T[ezTrans, Claude Code, an AI API or a local model]

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The extension reads the subtitle tracks of what you are playing and shows two of them. When your language has no track, the optional bridge, a small Node.js program on your computer, translates the main line and saves the result.

Install

From the browser stores: Chrome Web Store. The Firefox Add-ons listing is still in review; until it is approved, Firefox users can install a release:

From a release: download the zip for your browser from the latest release.

1. Chrome, Edge or Brave: unzip subpair-chrome-<version>.zip, open chrome://extensions (Edge: edge://extensions), switch on Developer mode, click Load unpacked and pick the unzipped folder.
2. Firefox: open about:debugging#/runtime/this-firefox, click Load Temporary Add-on and pick subpair-firefox-<version>.zip (the file is not signed yet, so it stays until Firefox restarts).

From source (Chrome 111+, Edge, Brave; Firefox 128+):

1. Download or clone this repository.
2. Chrome or Edge: open chrome://extensions (Edge: edge://extensions), switch on Developer mode, click Load unpacked and pick this folder. Firefox: run npm run build:firefox, open about:debugging#/runtime/this-firefox, click Load Temporary Add-on and pick dist/firefox/manifest.json (temporary until Firefox restarts).

First run

1. The settings page opens by itself: choose the main line (the language you are learning) and the second line (yours; your browser's language is pre-selected), then Save settings.
2. Netflix: reload the Netflix tab once after installing, then play a title.
3. Jellyfin: open your server's web client and click the Subpair button in the browser toolbar once (pin it from the extensions menu if it is hidden). Unless the address is localhost or 127.0.0.1, the browser asks you to allow that server first. You can also add the address under Jellyfin servers in the settings. Then play an item with text subtitles.

Key	Action
A / R / D	previous / repeat / next line
Q	auto-pause at the end of every line
click / Ctrl+click / Alt+click	copy a word / its dictionary form / the whole line
Alt+Shift+S, or the toolbar button	turn Subpair on or off (the sites' own subtitles come back)

The gear button on the video changes languages, translator, timing, size, position and background for the video you are watching, and can hide the main line to show only the second one. Keys can be changed in the settings.

Translation (optional)

When a title has no subtitle in your second-line language, the bridge (the translation helper in bridge/, a small Node.js program) translates the main line. The step-by-step guide for everyone, starting with ezTrans on Windows, is the Translation page. In short:

1. Install Node.js 21 or newer. The bridge uses only Node's own modules, so it needs no npm install.
2. Start it: double-click start-bridge.cmd on Windows, or run npm run bridge (the same as node bridge/server.mjs). It listens on http://127.0.0.1:8765, the extension's default bridge address.
3. Set up a translator (Bridge setup), check http://127.0.0.1:8765/health, and pick the translator in the Subpair settings or the gear menu.

Problems: docs/troubleshooting.md.

Suggested: Yomitan

Turn on Yomitan's clipboard monitor and every word you click in Subpair is looked up, or hold Shift over a word for Yomitan's pop-up.

Platforms

	Windows	macOS	Linux
Extension	yes	yes	yes
Bridge with Claude Code, an AI API or a local model	yes	yes	yes
ezTrans	yes	no	no

Only Windows is tested by the author.

Privacy

Nothing leaves your computer unless you turn on translation: then subtitle lines go to the bridge on 127.0.0.1 and on to the translator you chose. Subpair runs only on the Jellyfin servers you enable. Full privacy policy.

Bridge setup

The bridge keeps every translated line and every finished title, so nothing is paid for twice and a title you have watched loads at once.

sequenceDiagram
  participant E as Subpair (browser)
  participant B as Bridge (127.0.0.1)
  participant T as Translator
  E->>B: lines of the main subtitle
  B->>B: skip lines it already has
  B->>T: batches of new lines, with a few earlier lines as context
  T-->>B: translations
  B-->>E: second line
  B->>B: save the finished title

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Settings files

The bridge reads, from lowest to highest priority:

1. bridge/config.defaults.json: every setting with its default (part of the code; do not edit).
2. bridge/config.json: your own changes. Start from bridge/config.example.json. It can hold API keys, so it is ignored by git.
3. Environment variables: BRIDGE_PORT, BRIDGE_STORE, ANTHROPIC_API_KEY, OPENAI_API_KEY.

A setting name that does not exist in the defaults file is an error, so a typo never silently does nothing. Restart the bridge after changing a setting or an environment variable.

Translators

Translator	Key	Setting up
ezTrans	none	Windows only, Japanese to Korean. See ezTrans.
Claude (claude -p)	none	Install Claude Code and run claude once to log in, as the same user that runs the bridge. Translations count against your Claude plan.
Anthropic API	ANTHROPIC_API_KEY or api.key	The default model is claude-haiku-5-5.
OpenAI-compatible (OpenAI, Gemini, DeepSeek, Ollama, LM Studio...)	OPENAI_API_KEY or openai.key	Set openai.baseUrl and openai.model. Servers on your computer or local network usually need no key.

The claude command is started without a shell. On Windows install Claude Code with its native installer (an .exe on the PATH), or set claude.command to the full path of claude.exe.

Example bridge/config.json for Gemini through its OpenAI-compatible endpoint:

{
  "openai": {
    "baseUrl": "https://generativelanguage.googleapis.com/v1beta/openai",
    "model": "<a Gemini model name>"
  }
}

and for a local Ollama:

{ "openai": { "baseUrl": "http://127.0.0.1:11434/v1", "model": "<the model you pulled>" } }

Set a key for the terminal or service that starts the bridge:

System	This session only	Permanently
Windows PowerShell	$env:OPENAI_API_KEY = "<key>"; npm run bridge	setx OPENAI_API_KEY "<key>", then open a new terminal
macOS, Linux (bash, zsh)	OPENAI_API_KEY="<key>" npm run bridge	add export OPENAI_API_KEY="<key>" to ~/.zshrc or ~/.bashrc

Services started by launchd (macOS) or systemd (Linux) do not read your shell profile: put the key in bridge/config.json or in the service definition.

ezTrans

ezTrans XP is a Windows Japanese-to-Korean translator. It is not included: install it yourself. The bridge reaches it through LunaTranslator's 32-bit helper, which ships with Subpair in bridge/vendor/lunatranslator (GPL-3.0, from the official LunaTranslator release; source and version in that folder's README).

The bridge finds the ezTrans XP folder by itself, taking the first of these that holds the engine (J2KEngine.dll):

1. eztrans.dir in bridge/config.json;
2. the folder ezTrans XP recorded for the current user (registry HKEY_CURRENT_USER\Software\ChangShin\ezTrans, value FilePath);
3. ChangShinSoft\ezTrans XP under Program Files (x86), then under Program Files.

It prints the folder it uses when it starts. To name the folder yourself:

{ "eztrans": { "dir": "D:/Apps/ezTrans XP" } }

To use your own copy of the helper instead, set eztrans.helper to its full path (and eztrans.cwd if it needs another working folder). On macOS and Linux ezTrans is not offered; use another translator.

Models

The Subpair settings have a model box per translator. Empty means the bridge's default, shown in grey while the bridge runs. To change the default for everyone using this bridge, set it in bridge/config.json, for example:

{ "claude": { "model": "sonnet" } }

A model name may use letters, digits and . : / - _. Each model's translations are saved separately, so switching models never mixes them.

How lines are sent

• ezTrans translates line by line, a few lines ahead of the video.
• The language models get batches of up to 40 lines as a numbered list, plus the 6 lines before the batch as context. A whole title is split into batches, lines already translated (or repeated) are skipped, and 2 batches run at a time (3 for the Anthropic API). A batch that comes back wrong is split in half and tried again; a single line that still fails is left out instead of failing the title.

Per translator you can change batch, context, concurrency and timeoutMs in bridge/config.json.

Cost

Rough cost of one anime episode of about 300 lines with the Anthropic API and Claude Haiku 5.5 (claude-haiku-5-5, the default; $0.10 per million input tokens and $0.50 per million output tokens for prompts under 100K tokens, as of October 2026, see Anthropic's pricing):

	Per episode	12-episode season
Requests	about 8 (batches of 40 lines)	about 96
Input	about 1,500 tokens per request
Output	about 1,200 to 2,000 tokens per request
Cost	about $0.006 to $0.01	about $0.07 to $0.12 (under $0.30 with retries)

These are estimates, not measurements: about one token per Japanese or Korean character, a 250-token instruction and 6 context lines per request, and api.effort set to low. Lines already translated are never paid for again. The Claude translator (claude -p) uses your Claude plan instead of per-token billing; ezTrans and local models cost nothing per line.

Saved translations

A finished title is saved in the bridge's data folder: %LOCALAPPDATA%\subpair on Windows, ~/Library/Application Support/subpair on macOS, $XDG_DATA_HOME/subpair or ~/.local/share/subpair on Linux (storeDir or BRIDGE_STORE change it).

On Jellyfin, Subpair also adds a finished translation to the item as an external subtitle track (POST /Videos/{itemId}/Subtitles; Jellyfin saves it next to the video as <video>.<language>.srt), so every Jellyfin app shows it. It does so only when the item has no subtitle in that language, and the account needs permission to manage subtitles. Episodes translated ahead (the next episode, a whole season) are added the same way. Translate this title again replaces the track Subpair added before (deleting it needs an administrator); a track Subpair did not make is never replaced or deleted.

Run the bridge at startup

Windows: double-click scripts\windows\autostart-on.cmd. It puts a shortcut in your Startup folder that runs scripts\windows\run-bridge.ps1 hidden at every sign-in (no administrator rights needed), and starts it now. autostart-off.cmd removes the shortcut and stops that bridge.

run-bridge.ps1 restarts the bridge when it stops (waiting 5 s, doubling up to 60 s, giving up after 8 quick failures in a row), allows only one copy per port, stops a bridge left running on that port, and appends its output to a log (default %LOCALAPPDATA%\subpair\bridge.log). Its options: -Port, -Log, -Repo, -Node, -MaxQuickFailures.

To keep an always-on copy next to a development bridge, give it another port, for example 8777, and set the extension's Bridge address to it. As a scheduled task that starts at logon and checks every 5 minutes (PowerShell, from the repository folder):

$script   = (Resolve-Path 'scripts\windows\run-bridge.ps1').Path
$action   = New-ScheduledTaskAction -Execute 'powershell.exe' -Argument "-WindowStyle Hidden -NoProfile -ExecutionPolicy Bypass -File `"$script`" -Port 8777"
$triggers = @(
  New-ScheduledTaskTrigger -AtLogOn -User "$env:USERDOMAIN\$env:USERNAME"
  New-ScheduledTaskTrigger -Once -At (Get-Date).Date -RepetitionInterval (New-TimeSpan -Minutes 5)
)
$settings = New-ScheduledTaskSettingsSet -ExecutionTimeLimit (New-TimeSpan -Seconds 0) -MultipleInstances IgnoreNew -AllowStartIfOnBatteries -DontStopIfGoingOnBatteries
Register-ScheduledTask -TaskName 'Subpair bridge' -Action $action -Trigger $triggers -Settings $settings
Start-ScheduledTask -TaskName 'Subpair bridge'

Check it with Invoke-RestMethod http://127.0.0.1:8777/health. To stop it for good: Unregister-ScheduledTask -TaskName 'Subpair bridge', then end the node process running bridge\server.mjs.

macOS (not tested): save as ~/Library/LaunchAgents/com.subpair.bridge.plist, with your paths (which node shows Node's):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key><string>com.subpair.bridge</string>
  <key>ProgramArguments</key>
  <array><string>/opt/homebrew/bin/node</string><string>/path/to/subpair/bridge/server.mjs</string></array>
  <key>RunAtLoad</key><true/>
  <key>KeepAlive</key><true/>
  <key>StandardOutPath</key><string>/tmp/subpair-bridge.log</string>
  <key>StandardErrorPath</key><string>/tmp/subpair-bridge.log</string>
</dict>
</plist>

Then launchctl bootstrap gui/$(id -u) ~/Library/LaunchAgents/com.subpair.bridge.plist.

Linux (not tested): save as ~/.config/systemd/user/subpair-bridge.service:

[Unit]
Description=Subpair translation bridge

[Service]
ExecStart=/usr/bin/node /path/to/subpair/bridge/server.mjs
Restart=on-failure

[Install]
WantedBy=default.target

Then systemctl --user enable --now subpair-bridge.

Allow only Subpair

By default any browser extension on your computer may call the bridge, and the bridge says so when it starts. To allow only Subpair, add its origin to bridge/config.json and restart the bridge:

{ "allowedOrigins": ["chrome-extension://<the ID shown on chrome://extensions>"] }

For Firefox use moz-extension://<the Internal UUID shown on about:debugging#/runtime/this-firefox>. The ID changes when the extension is loaded from another folder; the bridge then answers "forbidden origin" until you update it.

Contributing and license

See CONTRIBUTING.md. MIT License. vendor/kuromoji is Apache-2.0 and bridge/vendor/lunatranslator (ezTrans helper) is GPL-3.0. Not affiliated with Netflix, Jellyfin or Language Reactor.

Try my other projects

Like this? I make other small open source tools too: Android apps, a Claude chat game, Claude add-ons and more. See them all at wonjoonseol.github.io.