The authoritative game server, without the netcode.
Define how your world behaves. Crux runs the authoritative loop, keeps every connected player in sync, and scales instances on demand.
The part of multiplayer that takes months - the server that holds the world and moves every player - is now a feature you switch on.
Free developer instance. No credit card required.
import { defineRoom } from "@supercraft/crux-realtime";
// Crux runs this as an authoritative server and
// streams state changes to every connected player.
export default defineRoom({
state: { players: {} }, // the authoritative world
onJoin(player) {
this.state.players[player.id] = { x: 0, y: 0, hp: 100 };
},
onAction(player, action) { // validated server-side
if (action.type === "move") this.move(player, action.dir);
},
onTick(dt) { // advance the world, 20x/sec
this.world.step(dt);
},
});$ crux realtime describe
"4-player co-op survival. Players share one world, gather
wood and stone, build structures, and survive night waves.
The server keeps inventory and base state. Reconnecting
players rejoin the same world."
✓ Authoritative world generated
✓ State model: players, inventory, structures, wave
✓ Live instance ready
▸ playground: open in browser to play it nowPlayers and data were the easy part. The world is the hard part.
The authoritative game server receives player actions, holds the true state of the world, advances it every tick, resolves conflicts, and streams the result back to everyone - without drift and without trusting the client. Studios spend months on it, then rebuild it for every title. Realtime Servers removes that work.
The world is the hard part
Auth, storage, and leaderboards are solved. The authoritative server that holds the world and moves every player is not - and it is where months go.
You rebuild it every title
State sync, instancing, reconnection, and anti-cheat get reinvented for each game. Crux runs that layer once, for all of them.
Netcode is its own discipline
Most teams do not have a networking engineer. Realtime Servers means you do not need one to ship a synced, server-authoritative world.
Nothing to operate
No servers to provision, no sockets to manage, no sync layer to maintain. You ship the rules of your world, Crux runs the world.
A hosted authoritative server, ready to run your world.
Everything the runtime side of a multiplayer game needs, without a networking library to wrangle or a fleet to babysit.
Authoritative world loop
Your server holds the single source of truth. Player actions are validated server-side and never trusted from the client, so the world stays consistent and cheat-resistant.
Automatic state sync
Connected players receive only what changed, in real time. No manual replication, no hand-written diffing, no socket plumbing to maintain.
Rooms and instances
Spin up isolated worlds per match, party, or shard. Instances are created, filled, and torn down automatically as players come and go.
Matchmaking handoff
Plugs straight into Crux matchmaking. A matched group lands in a fresh authoritative instance, ready to play, with no glue code in between.
Reconnection and resilience
Players drop and rejoin without losing the world. State survives brief disconnects and instance handovers so a flaky network does not end the session.
Built on the Crux backend
Auth, player data, leaderboards, economy, and live config are already there. Your world reads and writes them through the same platform, with no extra integration.
Write it, or describe it.
Bring your own game logic, or let Crux scaffold a working world from a plain-language description and refine from there.
Build mode
Write your world against a small, focused server model: handle a player joining, handle an incoming action, advance the world each tick. Drop in an engine SDK and ship. You own the rules, Crux owns the runtime.
- Server-authoritative state and validation by default
- Automatic state sync to every connected client
- Instances, matchmaking, and reconnection handled for you
Describe mode Preview
Tell Crux what your game does in plain language. Crux turns that into a running authoritative server you can play against in minutes, then refine. The fastest path from idea to a live multiplayer world, with no server code at all.
- Describe players, rules, and shared state in words
- Get a live, playable instance to iterate on
- Drop into Build mode any time to take full control
Same projects, environments, and credentials.
API Key
Admin scripts, CI, deploys, and control-plane access.
Server Token
Your world logic and trusted runtime services.
Player JWT
Connected players, issued by the auth system.
Realtime Servers is part of the Crux backend, so production, staging, and test stay isolated by environment - exactly like the rest of the platform.
A hosted server, not a library.
- Nothing to operate. No servers, sockets, or sync layer to maintain.
- Authoritative by default. The world is decided server-side, so clients cannot cheat the state.
- One backend, end to end. The platform that authenticates players also runs the world they play in.
- Scales with sessions. Instances follow demand, not your on-call rotation.
Built for games with a shared, live world.
Best suited for
- Realtime co-op, survival, and sandbox games
- Session-based competitive matches
- Persistent shared worlds and social spaces
- Unity, Godot, and web/JavaScript clients
- Indie and mid-size studios without a netcode team
- Prototypes and game jams that need a real server fast
Less suited for
- Purely single-player games
- Twitch-critical esports titles that need fully custom rollback netcode from day one
- Games with no shared state between players
Why say this clearly? The offer is stronger when it reads like a tool built for a specific class of multiplayer game, not a vague "server for everything."
Common questions
Do I have to write netcode?
No. In Build mode you write game rules - who joins, what an action does, how the world advances each tick. In Describe mode you write nothing, you describe the game. Networking, sync, and instancing are handled for you.
Which engines work?
Unity, Godot, and web/JavaScript clients today, with more SDKs on the way. Any client that can hold a realtime connection can connect.
Is the server really authoritative?
Yes. State lives on the server and every player action is validated there, so clients cannot fabricate the world. Server-authoritative is the default, not something you wire up.
Can players reconnect mid-session?
Yes. The world persists across brief disconnects and instance moves, so a dropped connection does not lose progress.
How does it relate to the rest of Crux?
Realtime Servers is part of the same platform. Auth, player data, leaderboards, economy, matchmaking, and config are already integrated - your world uses them directly with the same credentials and environments.
The realtime game servers series.
Five guides on authoritative multiplayer, from first principles to scaling.
Ship a synced, authoritative world this week.
Start free on a developer instance. Write your world, or describe it. No netcode, no sockets, no ops.