Starsync

Watch a video with friends, in sync, with the quality of a real mpv player. Pick from your Jellyfin library, paste any direct video link, or pick a file on your own disk; everyone's play, pause and seek stay together.

Why not Jellyfin SyncPlay: it runs in the browser and its sync is poor. This is a desktop app that drives mpv.

Name: watch-together is a placeholder and the final name is undecided (sync-relay is the working name). It appears in file names and the update manifest until that is settled.

• Desktop only (Linux and Windows), always mpv. No web clients.
• No transcoding. Everyone plays the original file.
• Friends trust model: no accounts, no roles. A blank password means an open server.

How it fits together: docs/HOW-IT-WORKS.md.

Install and update

From the Releases page of this repo:

• Linux: watch-together-<version>-linux-x64.tar.gz. Extract, run watch_together. On start it adds itself to your app launcher (and its icon), so the window shows its own icon on Wayland. Needs mpv's library installed (mpv or libmpv2).
• Windows: watch-together-<version>-windows-x64.zip. Extract, run watch_together.exe. mpv is bundled. New and not yet tried on a real Windows desktop; reports welcome.

Update from the app: when a newer version exists, an update indicator appears in the strip at the top of the window and opens the update card. "Update and restart" downloads, verifies, applies in place and restarts; the install folder must be writable by you. Settings has Check now and an auto-check toggle. From a terminal: ./watch-together-server --upgrade (no display needed, so it works on a server box too), watch_together --upgrade, or the shipped update.sh / update.bat (which run the first). Each updates the whole folder, app and server; restart whichever is running to use the new version. A release with a newer protocol is a required update: everyone on a server must run the same protocol version, and a mismatch is refused with a message telling you to update.

Getting started

Three ways in, all at the top of the Library when you are not in a server. None of them needs Jellyfin or an account.

• Join a friend (most people): your friend sends you an address and, maybe, a password. See Join.
• Host: you have the video and friends watch it with you. See Host from the app.
• Watch solo: just play something from the Library. It plays for you alone, with everything a server has (Jellyfin progress, next episode, skip buttons); nothing is reachable from outside this computer. Invite friends in the strip turns it into a hosted server (the Host card's port and password) and the video carries on from where it was; Stop ends it.

The window

One window. Along the top, the strip: your avatar (click it for your profile: name, picture with a crop editor, colour, and links to the settings), the Library / Schedule / Settings tabs, and the party. Offline the party side has Join a friend and Host; watching solo it says Solo, with Invite friends and Stop; in a server it shows everyone's avatar with a ring for their state (green ready, amber buffering, grey not ready, dimmed reconnecting, dimmed with a moon sitting out; a crown on whoever hosts, a tower on whoever passes the video on). Click the avatars for the member list (force ready, Nudge, Sit out, "Copy invite address"; when you host it lists this computer's network addresses with the port, and friends outside your network need that port forwarded or a server they can all reach, like a VPS). Beside them the server and "ready n/m", the chat button with unread count, and Leave. Empty strip space drags the window.

The player fills the window under the strip. Pick a tab and it shrinks to a mini player in a corner: drag it anywhere and it snaps to the nearest corner; double-click or ⤢ makes it big again, × sits you out (below) and × again closes it; the strip's screen button brings it back. F or double-click on the picture goes fullscreen (the strip hides, the party shows in a corner while paused, the window buttons sit in the player's top bar while its controls are up), Esc leaves it. The controls show the show's cover, title and episode top left (guests get the cover too); before a video first plays, its banner fills the picture until playback starts. One pill above the controls says what playback waits for ("Press Space when you're ready", "Waiting for Bob and Mika · 2/3 ready", "Bob is loading…") or what went wrong, with Retry or Cancel when it helps. Chat and activity open in a panel on the right (chat button or Tab), ready to type; it shows messages, and its "Activity" toggle adds joins, ready, seeks and the like between them. Joins, leaves, chat and other news also pop up bottom left with the person's avatar; click a message to open the chat. Unread chat shows as a count on the chat buttons and, in fullscreen with the controls hidden, as a small chip top right.

Your name and picture change for everyone in the server right away; the colour is yours only.

Sitting out. Step away without leaving: Sit out (your profile, your row in the member list, or × on the mini player) unloads the video on your end, and nobody waits for you any more; you stay in chat. Anyone can sit anyone out, the host too ("Alice sat you out" tells you who). The player then shows the cover, title and where the others are; Join back (or Space) loads the video there and readies you. Settings → Party & network → "When idle, sit out instead of just unready" does it after 10 idle minutes.

Nudge. Nudge in the member list pokes someone, sitting out or not: only they hear it (a sound, a pop-up, and the taskbar entry asks for attention when the window is in the background).

Host from the app

Host in the strip: set a port and an optional password (blank = anyone can join), then Host. The app starts a server inside itself and joins it; you are the host (the crown). Nothing is up yet: pick what to watch in the Library tab (below) and put it up. Leaving or closing the app stops the server. Friends need to reach your machine: open the TCP port on your router (or use a LAN or VPN address). Friends open nothing. Ports you hosted on before are one click away in the Host card.

If you put up a local file, the server streams it to everyone from your disk. Nothing is uploaded or stored anywhere.

Library

What to watch, from three sources (the switch at the top). Drop a video, a folder or a link anywhere on the window, or press Ctrl+V outside a text field, and a small card asks how to open it (put it up, add the folder to Files, or fill Link).

• Jellyfin: your linked servers (see below); with none linked it points to Files and Link and offers Jellyfin as optional. Library chips beside the switch, then one Continue row (what you were watching and what comes next, one card per series, the one you watched most recently first; series not touched for two months drop out) and Latest in each library (one card per series: "S1E1" or "3 new"; Show all opens the library newest first). Re-uploads (added long after they aired) are hidden from Latest; the row's ⋮ menu turns that off. Search (/), library grids (sort, unwatched, favourites) and series pages sit behind them. Click a card to select it: its details open on the right with "Put it up · S1E3", mark watched / unwatched, remove from Continue and the episode list (click an episode to choose it). Double-click a card or an episode to put it up at once.
• Link: paste an http(s) link: a video file, or a web page with a video (YouTube, X, and whatever else yt-dlp knows). The preview shows what it is (a page's title, uploader, length and thumbnail, else the file name); in a server, pick how the others get it (everyone fetches it, the server fetches it, or through you). Your last 12 links are listed under it.
• Files: video folders you add on this computer, grouped by series (click a series to list its episodes), or any file through Browse. The others stream it from you.

Put it up and the player opens. Not in a server, the buttons say Play and it plays for you alone (Watch solo).

Dedicated server

For an always-on box (no display needed): the release tarball also contains watch-together-server, a small standalone server without the app. Run it, then everyone (you included) uses Join.

./watch-together-server --port 8080 --secret <password>

Flag	Default	Meaning
--port, -p	8080	TCP port, all IPv4 interfaces
--secret	empty	password; empty = open server (a warning is printed)
--name	server	name reported by /health
--media	none	local file to serve as the starting video
--advertise-host	none	host put in file URLs instead of the one each viewer dialled
--[no-]yt-dlp	on	read web pages (YouTube, X, …) with yt-dlp when the server fetches a link
--yt-dlp-dir	next to the binary	writable folder for the server's yt-dlp (systemd: the unit's StateDirectory)
--yt-dlp-cookies	none	cookies.txt or cookie-extension JSON export for that yt-dlp (sites that refuse guests); copied into the yt-dlp folder at start, removed from it when not given. An account login: keep it private, use a throwaway account for YouTube
--config	none	YAML or JSON file with the same settings (port, secret, name, mediaPath, advertiseHost, ytDlp, ytDlpDir, ytDlpCookies); replaces the flags
--upgrade		update the install folder to the latest release (Install and update), then exit
--check-update		print whether a newer release exists, then exit

It prints READY port=... name=... once listening and stops cleanly on Ctrl+C or SIGTERM. A dedicated server has no host. To run it as a service, unpack the tarball to /opt/watch-together and use deploy/watch-together.service (copy it to /etc/systemd/system/, edit the flags, systemctl enable --now watch-together).

Join

Join in the strip. Servers you got into are listed first (up to 6), with live status (online and how many are watching, or offline, plus "needs update" when the server runs another protocol): click one to rejoin, x to forget it. Or enter the host's address, port, and the password if there is one, then Enter or Join. Your name and picture come from your profile (your avatar at the top left).

Settings

The Settings tab (Ctrl+,) has a page per topic, each setting with a line of help: Profile; Party & network (the Host card's default port and password, recent servers with remove); Player (below); Jellyfin & integrations (linked Jellyfin servers, below, the AnimeFlow connection: "connected as " with what you may do, or why not, and playing the next episode automatically); Notifications & sounds (what pops up; the sound volume and sound set, and every sound with a preview, its own set and a switch); Updates; Keyboard shortcuts (read-only); About (the version).

Player settings live in the player: ⚙ at the right end of its controls (both designs) opens them, and Settings → Player has the same ones (plus the custom shader files). Player design: Standard or uosc (below); progress line when hidden (on by default: a 2 px line at the bottom of the video while the controls are hidden); upscaling shader (see below); hardware decoding (auto, off or custom; the env var SYNC_RELAY_HWDEC overrides it); in the player also the subtitle and audio track. Every change applies right away.

Anywhere in a server (unless you are typing): Space = ready / unready, Tab = chat & activity (Tab never moves between buttons). Keys while the player fills the window: Left/Right = seek 10 s, Up/Down = volume, Shift+Right = skip 1:30, PgUp/PgDn = previous / next chapter (as in mpv), F = fullscreen, Esc = close the chat, then fullscreen, then shrink the player to mini; from the mini it asks to leave, Ctrl+R = reload video, < / > or Shift+P / Shift+N = previous / next Jellyfin episode, Ctrl+1..6 = Anime4K mode A, B, C, A+A, B+B, C+A, Ctrl+0 = shaders off. Every seek, skip and chapter jump moves everyone.

Controls: the seek bar marks chapter starts with a diamond; hovering it shows the time and the chapter title. The chapters button lists them (current one checked; click to jump). "1:30" skips ahead 1:30. Audio and Subtitles buttons (with the track count) list tracks as "Japanese · E-AC3 2.0 · title"; subtitles have Off. Volume (hover the speaker, scroll on it, or Up/Down) goes to 150 % like mpv: past the tick at 100 % the bar turns amber and mpv amplifies.

The uosc player design (⚙ → Player design, Standard by default, switches right away): video controls modeled on mpv's uosc theme. A thick timeline across the bottom shows the played part, the cached part ahead (hatched), chapter starts as diamonds and the time; hovering it shows the chapter title and the time there. With the controls hidden it stays as a thin line (unless "Progress line when hidden" is off). Above it flat buttons: ≡ menu, Subtitles and Audio (with the track count), the shader badge (the active Anime4K mode, e.g. "A HQ"), ready, chapters, 1:30, previous / next episode, change video, volume, ⚙ and fullscreen; the title top left, chat and back-to-library top right. Keys and skip buttons work the same.

Skipping: during the opening a "Skip OP" button floats in the corner (it jumps to the end of the opening), during the ending "Skip ED" (to the end of the ending only; what comes after is yours to skip). Any skip button (also "Skip 1:30") shows for 5 seconds when it comes up, then only while the controls are up (move the mouse). The opening and ending come from Jellyfin's intro / outro segments when your Jellyfin has them (needs a segment plugin, e.g. Intro Skipper), else from chapter names (OP, Opening, Intro / ED, Ending, Credits, Outro). Without an opening or any chapters, on anything longer than 20 minutes, "Skip 1:30" floats there during the first 6 minutes.

Ports

What	Default	Notes
Sync WebSocket and /health	TCP 8080	plain ws://, no TLS
Local file streaming	same port	path /m/..., needs a session token

Only the host needs the port reachable.

Jellyfin

Settings > Jellyfin & integrations > Add server: type the address, then sign in with Quick Connect (the app shows a code; approve it in Jellyfin under Profile > Quick Connect) or your username and password. Several servers can be linked; each shows its status, and a star picks the default. Sign out revokes the app's sign-in on the server. If a sign-in expires the server shows "Reconnect" instead of disappearing.

The Library's Jellyfin source is a library browser: Continue watching (with time left), Next up and the latest per library on the start page; library grids with sort (name, date added, release date, rating, random) and Unwatched / Favorites filters; series pages with seasons, episode progress and a "Next up" button; details with the file's tracks; search. Clicking a movie or episode picks it. On wide windows a side pane shows details of whatever is under the mouse.

Keys in the browser: arrows move, Enter picks or opens, I = details, Esc / Backspace / Alt+Left = back (Esc first clears a search), / or Ctrl+F = search.

Putting up a movie or episode sends only the item to the server (which Jellyfin server and which item), never a link or your sign-in. Pasting a Jellyfin link (web page or stream link) does the same when the app can reach that Jellyfin; otherwise it is shared as a plain link.

Watching:

• Everyone with their own account on that Jellyfin server plays the file directly with their own sign-in. Jellyfin tracks progress per account: your watched state, Continue watching and resume point follow what you saw (reported on start, every 10 s, on pause, resume, seek and track change, and when it stops).
• Friends without an account watch through the server: whoever put the item up passes it on automatically, with their own sign-in, which never leaves their computer (only the current item, only to people in the session). They show "via host" in the member list (click the avatars). When the app hosts, or a dedicated server has its own login (--jellyfin <file>, see docs/ARCHITECTURE.md, Jellyfin relay), the server fetches it itself instead.
• If nobody can pass it on (the person who put it up left), the app says "No Jellyfin account here for ..." with a button to link one; it plays as soon as you are signed in.

Episodes: the ⏮ / ⏭ buttons next to the controls (or < / >, Shift+P / Shift+N) put up the previous or next episode for everyone. From the ending (when it is known, see Skipping above; else in the last two minutes) a "Next episode" button floats in the corner. When an episode ends, a card shows what is next with Play next; with "Play the next episode automatically" (Settings, on by default) it counts down 10 s first, and Cancel stops it on your screen (seeking back stops it for everyone). Anyone with an account can put the next one up; friends watching via the host cannot.

Audio and subtitles: the Audio and Subtitles buttons in the controls pick tracks. Your choice is remembered per series (and as a default) and the closest match (language, then title, then codec and channels) is selected on the next episode. This is per person; nobody else is affected.

"Change video" in the player's controls opens the Library tab (the player goes mini): the library starts on the series you are watching with the current season, else where you left it.

Anyone can put up a file from their own computer, also on a dedicated server: it streams from your computer through the server to everyone (the server keeps a few minutes in memory, so seeks and skipping the opening are quick). Keep the app open while it plays. For a link you choose how the others get it:

• Everyone fetches it (default): each app plays the link itself.
• Server fetches it: the server downloads it once for everyone, for links that are slow or blocked for some. The server must be able to reach it, and it must have a password (an open server refuses: anyone could make it fetch addresses only it can reach).
• Through me: your computer downloads it and passes it on, for links only you can open.

Web pages work in all three: the app and the dedicated server each download yt-dlp by themselves (checked against the release's checksums, updated once a day; Settings → Player shows its version and has "Update now"), so there is nothing to install. Sites that refuse guests (Bilibili answers "HTTP Error 412", YouTube may ask to sign in) need cookies: Settings → Player → yt-dlp → Cookies reads them from a browser, or from a cookies.txt or a cookie extension's JSON export (picked or dropped; every site in it is kept). They stay on this computer and are used by its yt-dlp only (your own playback, "through me", and a server you host). Cookies log yt-dlp in as you on those sites: keep the file private, and for YouTube consider a throwaway account (Google may flag accounts used with yt-dlp). YouTube works in all three too, and so do pages and links that only offer streaming playlists (HLS .m3u8, DASH .mpd, the HLS of X videos). The last two never show the link to anyone; they can't pass on live streams or DRM-protected video (use "Everyone fetches it" for live). Your upload has to beat the video's bitrate: a 25-minute 2 GB episode is about 11 Mbit/s.

Video quality and shaders

Each viewer can run mpv GLSL shaders on their own picture; nobody else is affected. The app ships Anime4K v4.0.1 (MIT) with the presets of its mpv guide: Mode A (1080p sources), B (720p), C (480p), A+A, B+B, C+A, each as HQ (strong GPUs, GTX 1080 and up) or Fast (the guide's low-end set). "Custom shaders" runs your own .glsl files.

Pick one with the ✨ button in the player's control bar (also in fullscreen) or Ctrl+0..6; the change applies live and is saved, so the next session starts with it. The player's ⚙ and Settings → Player set it too, with HQ or Fast (used by Ctrl+1..6); Settings → Player holds the custom files. Shaders cost GPU time every frame and need hardware video rendering: on media_kit's software fallback they do nothing, and the app says so. A shader that is missing or fails to load shows a toast and switches shaders off.

Troubleshooting

• "host offline - check address and port": wrong address, or the port is not open.
• "wrong server password": password mismatch.
• "host runs a different version": update the app on both sides.
• Picture frozen but sound fine: Ctrl+R reloads the video; the app also does it automatically.
• Everyone waits at "waiting for X": X has not buffered 4 seconds yet or has not pressed Space.
• Windows: the build is new; if something is off there, say so.

For contributors: docs/DEVELOPMENT.md, docs/ARCHITECTURE.md, docs/PROTOCOL.md.