Multiplayer for AI-generated games

the prompt

The one prompt that makes a vibe-coded game multiplayer

Paste this into Cursor, Claude Code, GitHub Copilot, Windsurf, Replit Agent, Lovable, Bolt or any assistant that can fetch a URL. It works on a game the agent just generated as well as on an existing codebase.

The agent needs nothing else. llms-full.txt is a single self-contained reference — every route, rule, recipe and failure mode — written so a model does not have to guess. openapi.json gives it the machine schema for the same routes.

the problem

Why AI-built games stay single-player

Ask an agent to “make it multiplayer” with no backend and you usually get one of three results, none of which ships:

• A Socket.IO or WebSocket server that works on localhost and then needs a host, a process manager, TLS, scaling and someone to restart it at 3 a.m.
• A hand-rolled WebRTC setup that forgets signaling, NAT traversal and the relay for players behind strict networks — so it works between two tabs and fails between two homes.
• A database used as a game server, polling or writing positions sixty times a second, with a bill that grows with every player and no protection against a modified client.

The Multiplayer API removes the server from the problem. It is a hosted HTTP + WebRTC service with a 7 KB JavaScript SDK, so the only code the agent writes is game code: log the player in, join a room, send positions, write health.

workflow

What the agent does, step by step

1. Reads the reference

   One fetch of llms-full.txt. The mental model fits in a paragraph: three channels, declarative rules, a single sync loop.

2. Creates the game — no account needed

   POST /v1/games with a name and a preset (realtime_arena, turn_based, coop, strict) returns a public key for the client and a secret key for your server. The sandbox runs 2 live rooms for 24 hours; claim it to your console when you subscribe.

3. Wires the SDK

   One import from the CDN, mp.login() for an instant guest account, mp.quickmatch() for a room, then the three channels. No build step, no dependency, no server file.

4. You open two tabs

   Load the game twice and watch the second player appear in the first. Send the link to a friend: WebRTC connects you directly, with an automatic relay when it cannot.

code

The code an agent should write

This is the complete realtime loop — nothing omitted. If your agent writes something much longer, point it back at the reference.

import { Multiplayer } from 'https://multiplayerapi.com/sdk/mp.esm.min.js';

const mp = new Multiplayer({ key: 'mp_pk_...', displayName: 'Player' });
await mp.login();                                   // instant guest account
const room = await mp.quickmatch({ mode: 'ffa', max_players: 8 });

room.on('members', (members) => syncAvatars(members));
room.on('peer', (p, from) => others[from]?.moveTo(p.x, p.y));   // direct channel
room.state.watch('players.*.hp', (hp, path) => drawHealth(path, hp));

setInterval(() => room.send({ x: me.x, y: me.y }), 50);         // positions, P2P
room.mine('hp', 100);                                            // health, validated

Three rules that keep AI-generated netcode correct

The most common mistake in generated code is routing positions through shared state. The reference says so explicitly, and the server's rules reject the worst abuses — a client cannot award itself points it is not allowed to write.

compatibility

Works with the tools people actually use

Not affiliated. Tool names are trademarks of their owners and are listed for compatibility only: the integration relies on nothing more than fetching a URL and editing JavaScript.

context

Why this matters now

AI game generation turned “make a game” from a months-long project into a prompt. The result is a wave of small, fun, single-player browser games — because multiplayer has always needed infrastructure that a prompt cannot provision. The games people remember are the ones they play with friends. An API an agent can set up on its own closes that gap: the same afternoon that produced the game can produce the online version.

Pricing is built for that pattern too: one flat subscription at $19.99/month covers every game in your console, so shipping ten experiments costs the same as shipping one.

Next: pick your engine — Phaser, Three.js, vanilla JavaScript, Godot or Unity — or read how to make a multiplayer browser game.