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Overview

Minecraft Web Client includes a fully integrated JavaScript server (based on Flying Squid) that runs entirely in your browser. This enables true singleplayer gameplay with world generation, saving, and all the features you’d expect from a local Minecraft server.
Your singleplayer worlds run completely in your browser - no external server required!

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Creating a New World

There are multiple ways to start a singleplayer world:
Use the ?singleplayer=1 or ?sp=1 query parameter to create an empty world instantly:
https://mcraft.fun/?singleplayer=1
Worlds created this way are not saved automatically. Use /export to download your progress.

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World Generation

The integrated local server provides basic world generation capabilities:
  • Flat terrain generation - Simple world generation for quick building
  • Custom world types - Support for different generator options
  • Chunk loading - Generates new chunks as you explore
World generation is simpler than vanilla Minecraft. Complex terrain features may generate differently.

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Generation Limitations

// From src/mcTypes.ts - World generation configuration
interface LevelDat {
  generatorName?: string;
  generatorVersion?: number;
  generatorOptions?: string;
  // ...
}
New chunks may be generated with basic terrain. For complex worlds, it’s recommended to:
  1. Import existing world files
  2. Stay within pre-generated areas
  3. Use /export frequently to backup progress

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The Local Server

The integrated server is a full implementation that:
  • Runs entirely in JavaScript (via Flying Squid)
  • Handles all game logic server-side
  • Manages world data and persistence
  • Supports multiplayer via P2P (see P2P Multiplayer)
// From src/createLocalServer.ts
export const startLocalServer = (serverOptions) => {
  const passOptions = { ...serverOptions, Server: LocalServer }
  const server = createMCServer(passOptions)
  server.formatMessage = (message) => `[server] ${message}`
  return server
}

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Unsupported Features

Some features are not available in the local server:
The following features are not supported:
  • Transaction packet handling
  • Some advanced teleport mechanics

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Saving Your World

Worlds can be saved in multiple ways:

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Automatic Saving

By default, worlds are saved:
  • On a regular interval during gameplay
  • When you disconnect from the world
  • When you close the browser (if supported)
// Worlds are saved to browser memory
// From src/builtinCommands.ts
command: ['/save'],
async invoke () {
  await saveServer(false)
  writeText('Saved to browser memory')
}

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Manual Saving

Use the /save command in chat to manually save your world: 
/save
You’ll see a confirmation message: “Saved to browser memory”

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Disable Auto-Save

To prevent automatic saving (useful for testing), use the ?noSave=true parameter: 
https://mcraft.fun/?singleplayer=1&noSave=true
With noSave=true, only manual /save commands will persist your world!

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Exporting Worlds

Export your world to download it as a ZIP file:
1

Save Your World

First, ensure your world is saved:
/save
2

Export Command

Use either command to export:
/export
or
/download
3

Download

Your browser will download a ZIP file named world-prismarine-exported.zip containing:
  • level.dat - World metadata
  • region/ - All region files with terrain data
  • playerdata/ - Player data files
  • All other world files
// Export implementation from src/builtinCommands.ts
export const exportWorld = async (path: string, type: 'zip' | 'folder', zipName = 'world-prismarine-exported') => {
  if (type === 'zip') {
    setLoadingScreenStatus('Generating zip, this may take a few minutes')
    const zip = new JSZip()
    await addFolderToZip(path, zip, '')
    const zipContent = await zip.generateAsync({ type: 'blob' })
    
    const downloadLink = document.createElement('a')
    downloadLink.href = URL.createObjectURL(zipContent)
    downloadLink.download = `${zipName}.zip`
    downloadLink.click()
  }
}
Large worlds may take several minutes to export. Don’t close the browser while exporting!

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Server Configuration

Advanced users can configure the local server with query parameters:

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Server Settings

Use ?serverSetting=<key>:<value> to configure server options: 
?serverSetting=noInitialChunksSend:true
This disables initial chunk loading on the loading screen.
Server settings are defined in the Flying Squid module options. See the Space Squid repository for available options.

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Gameplay Modes

Switch between game modes using F3+F4 or the /gamemode command:

Creative

Full flying, instant breaking, unlimited resources

Survival

Standard gameplay with health and hunger

Adventure

Limited block breaking, good for custom maps

Spectator

Fly through blocks, observe without interaction
// Cycle through game modes with F3+F4
// From src/controls.ts:702
nextGameMode = 'creative' | 'survival' | 'adventure' | 'spectator'
bot.chat(`/gamemode ${nextGameMode}`)

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World Data Access

For debugging and advanced usage, access world data via the browser console: 
// Access the local server instance
localServer

// View loaded region files
localServer.overworld.storageProvider.regions

// Change world name
localServer.levelData.LevelName = 'My World'
localServer.writeLevelDat()
See Global Variables for more console commands.

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Best Practices

Export your world frequently using /export. Browser data can be cleared accidentally, so keep offline backups!
Stick to first-class versions (1.19.4, 1.21.4) for the most stable experience.
Large worlds consume browser memory. If you experience lag:
  • Limit exploration to reduce loaded chunks
  • Export and reload to clear memory
  • Use smaller render distances
Worlds created in the web client can be exported and used in:
  • Vanilla Minecraft Java Edition
  • Other Minecraft servers (with compatible versions)
  • Imported back into the web client later

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Troubleshooting

level.dat not found: Ensure you’re loading a valid world folder containing level.dat
Version mismatch: If your world is from an unsupported version, you’ll be prompted to select a compatible version. Choose the closest supported version.
World generation issues: If new chunks appear broken, reload chunks with F3+A or export and reimport the world.

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