BlockHand

BlockHand — MCP server for Minecraft Education

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BlockHand gives AI agents hands and eyes inside Minecraft Education Edition. Claude Code, Codex, Gemini CLI, Grok or any other Model Context Protocol client can drive the Agent robot, build structures, read blocks, compare regions and react to game events.

It talks to the game only through Minecraft Education's built-in /connect (/wsserver) WebSocket command. There is no add-on, behavior pack, mod or Script API to install in the game, nothing is injected into the game process, and no screen reading is involved. On a school computer, the only thing to install is the AI client.

• 44 tools and 2 resources: Agent robot control, world edits, 24 build shapes, blueprints, block reading, region comparison, symmetry checks and event subscription.
• Fast building: shapes are merged into the fewest possible /fill commands (a solid sphere of radius 8 goes from 2,000+ blocks to under 200 commands).
• Local by default: the game WebSocket listens on 127.0.0.1 unless MINECRAFT_EDU_WS_HOST changes it, and no account or online service is involved.
• One fixed address: every AI session on the machine shares one background hub, so the game always connects with /connect 127.0.0.1:19131.
• Installers for Claude Code, Codex, Gemini CLI and Grok CLI when you run from source.

The /connect command is an official interface. The WebSocket message protocol behind it has no published stability guarantee, so behavior should be re-checked after game updates.

Contents

• Quick start
• Install from source
• Tools
• Connecting and staying connected
• How building stays fast
• Reading blocks
• Undo (off by default)
• Configuration
• Safety
• Troubleshooting
• Known limitations
• Development
• License and trademarks

Quick start

Requirements:

• Minecraft Education Edition, in a world with Cheats on. The player who connects needs operator rights. Verified on 1.26.32 (Windows desktop and macOS 14+).
• Node.js 22 or newer on the machine that runs the AI client.

1. Register the server with your AI client

Claude Code:

claude mcp add blockhand --scope user -- npx -y @gjlmotea/blockhand-mcp

Codex:

codex mcp add blockhand -- npx -y @gjlmotea/blockhand-mcp

Clients that read an mcpServers JSON config (Claude Desktop, Cursor, Gemini CLI):

{
  "mcpServers": {
    "blockhand": {
      "command": "npx",
      "args": ["-y", "@gjlmotea/blockhand-mcp"]
    }
  }
}

VS Code uses a top-level servers key instead, and each server has a type:

{
  "servers": {
    "blockhand": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@gjlmotea/blockhand-mcp"]
    }
  }
}

Where each client keeps the file:

Client	Config file
Claude Desktop	macOS: ~/Library/Application Support/Claude/claude_desktop_config.json; Windows: %APPDATA%\Claude\claude_desktop_config.json. Settings → Developer → Edit Config opens it
Cursor	~/.cursor/mcp.json for every project, .cursor/mcp.json inside one project
Gemini CLI	~/.gemini/settings.json (user), .gemini/settings.json (project)
VS Code	.vscode/mcp.json in the workspace, or the user mcp.json that the MCP: Open User Configuration command opens

Sources: Claude Desktop, Cursor, Gemini CLI, VS Code.

Desktop apps started from Finder or the Start menu often do not see your shell's PATH. If the server fails to start with a vague "server not responding" error, replace npx with its absolute path (which npx on macOS, where npx on Windows).

Restart the AI client after registering.

2. Connect the game

The game connects to the server, not the other way round.

1. Open a world in Minecraft Education. The main menu is not enough.

2. Make sure Cheats are on.

3. Type in the chat:

   /connect 127.0.0.1:19131
   

When the connection is made, a green "✔ BlockHand connected" line appears in the chat, in the game's own language: English, Traditional Chinese or Simplified Chinese, and English for any other game language. You can also ask the AI to call mc_status, which returns the exact connectCommand.

3. Ask for something

Build a hollow glass sphere with radius 6 in front of me.

Summon the Agent, walk it five blocks forward and plant a row of wheat.

Check whether the house at 0 64 0 to 20 80 20 is mirror-symmetric and show me which blocks break the symmetry.

Install from source

Use this route to work on BlockHand itself, or to use the installers and the doctor check.

git clone https://github.com/gjlmotea/blockhand-mcp.git
cd blockhand-mcp
corepack pnpm install --frozen-lockfile
corepack pnpm run build

Register with one of the installers:

corepack pnpm run setup:claude    # or setup:codex / setup:gemini / setup:grok
corepack pnpm run doctor          # add --client=claude etc. to check another client

The installers do more than write a config entry:

• They fill in the absolute path of this machine's Node, so desktop apps that cannot read nvm, Homebrew or your shell PATH still start the server.
• Before changing any setting, they run a real MCP initialize with the exact command, arguments and environment they are about to save, and check that all 44 tools are there.
• They do nothing when the server is already registered correctly, so re-running is safe.
• When an entry with the same name exists but differs, they stop and show the difference instead of overwriting someone else's settings.
• They write only through each client's official mcp add / mcp remove command, never by editing config files directly.

Remove with corepack pnpm run uninstall:claude (or uninstall:codex, and so on). It refuses to remove entries it cannot identify as belonging to this checkout.

Client	Writes to	Then
Codex	~/.codex/config.toml	Quit and restart; desktop app, CLI and IDE share it
Claude Code	~/.claude.json (user scope)	Restart the session
Gemini CLI	~/.gemini/settings.json (user scope)	Restart the CLI
Grok CLI	~/.grok/config.toml	Restart the CLI

To register by hand, point the client at <NODE> <REPO>/dist/index.js, where <NODE> is the absolute path from node -p "process.execPath" and <REPO> is the absolute path of the checkout. Claude Code and Gemini CLI place the command differently:

claude mcp add blockhand --scope user -- <NODE> <REPO>/dist/index.js
gemini mcp add --scope user blockhand <NODE> <REPO>/dist/index.js

Claude Code wants -- before the command. Gemini CLI takes the command right after the name (gemini mcp add [options] <name> <commandOrUrl> [args...]); a -- there would leave it without a command. Gemini only needs -- after the command, in front of server arguments that collide with its own options, and BlockHand takes no arguments. Gemini's default scope is project, so use --scope user to make it global. Claude Code's default scope is local; --scope project writes a shareable .mcp.json, which is handy for a whole class. Sources: Claude Code, Gemini CLI.

Tools

Connection and fallback (6)

Tool	Purpose
mc_status	Bridge state, the /connect command, subscribed events, command counters. Check this first when anything fails
mc_await_connection	Wait for the game to connect (up to 120 seconds per call)
mc_run_command	One raw slash command, for anything without a dedicated tool
mc_run_commands	Several raw commands in order
mc_undo_control	Inspect, clear or configure single-step undo (off by default)
mc_undo	Undo the last protected build, after checking for conflicting changes

Agent robot (10)

Tool	Purpose
mc_agent_create	Summon the Agent
mc_agent_move	Walk N blocks in a direction
mc_agent_turn	Turn left or right by 90 degrees
mc_agent_teleport	Bring a lost Agent back to the player
mc_agent_act	Attack, destroy or till, optionally repeated
mc_agent_place	Place a block from an inventory slot
mc_agent_collect	Pick up dropped items
mc_agent_inventory	Count, free space, item detail, drop, drop all, transfer
mc_agent_sense	Inspect, inspect data, detect, detect redstone: the Agent's eyes
mc_agent_program	Send a whole sequence of actions in one call and get a result per step

Agent directions are relative to the way the Agent is facing, not compass directions.

World (13)

mc_set_block, mc_fill, mc_clone, mc_test_block, mc_read_block, mc_verify_reading, mc_compare_regions, mc_analyze_symmetry, mc_query_target, mc_summon, mc_world_settings (time, weather, game rules, difficulty), mc_structure (save and load structures), mc_ticking_area.

mc_query_target parses the JSON that querytarget returns. It is the way to find where the player or the Agent is before building.

mc_compare_regions compares two regions with a single testforblocks. Its masked mode ignores air in the source, which is how you check that a student's build contains everything it should, whatever else is around it.

mc_analyze_symmetry checks whether a region is mirror-symmetric and, when it is not, points to the cells that differ. It works by saving the region, loading a mirrored copy into a scratch area, comparing, and restoring the scratch area afterwards. The scratch area may not overlap the region being analyzed; this is checked before any command is sent. If backing up the scratch area fails, nothing is placed.

mc_structure saves to memory by default (gone when the game closes). Use saveMode: "disk" only when the user asks to keep a structure; it writes into the world folder. The game has no command that lists saved structures, so mc_status reports the names saved during the current session.

Player and feedback (7)

mc_teleport, mc_give, mc_gamemode, mc_effect, mc_player_action (kill, clear, xp, ability), mc_message (say, tell, title, subtitle, action bar), mc_feedback (sounds and particles).

Building (4)

Tool	Purpose
mc_build_preview	Plan only: block count, bounding box, number of /fill batches
mc_build_shape	24 geometric shapes; most support hollow
mc_blueprint_preview	Preview a block-by-block blueprint
mc_build_blueprint	Any shape from a list of "coordinate → block"; identical blocks are merged

Group	Shapes
Solids	box, sphere, ellipsoid, cylinder, cone, pyramid, prism
Lines and surfaces	line, curve (smooth curve), disk, tube (cylinder along any direction)
Building parts	wedge (single slope), roof (gable or hip), arch, stairs, spiralStairs, polywall (polyline wall)
Terrain and paths	heightfield (rolling ground), ribbon (road or bridge deck)
Decorative	torus, helix, cross, star, revolution (solid of revolution)

With 24 shapes, the real risk is the AI picking the wrong one: a wrong pick does not fail, it builds something plausible but wrong. The tool description is therefore organized by what you want to build, with the most commonly confused pairs spelled out:

You want	Use	Not
Slanted pillars, beams, pipes	tube	cylinder (axis-aligned only)
Walls with sharp corners	polywall	curve (rounds the corners)
A roof with a ridge	roof	wedge (one slope only)
Stairs winding up a tower	spiralStairs	helix (no treads) or stairs (straight)
Towers or chimneys that taper	cone / pyramid with topRadius	stacked boxes
Domes, vases, cooling towers	revolution with a profile	a new shape per object
Uneven ground	heightfield	many boxes

A few parameters are easy to misread: the radius of prism and pyramid is the distance from the center to an edge, not to a corner; the ridgeAxis of roof is the direction the ridge runs, not the way the slopes face; heightfield corners are thicknesses in blocks; and arch draws the arch itself, with the opening (width 2 × radius − 1) in the middle.

Events (4)

mc_events_catalog, mc_events_subscribe, mc_events_unsubscribe, mc_events_poll.

Events go into a ring buffer that you read with a cursor. dropped > 0 means polling was too slow and some events were lost. Subscriptions are restored automatically after a reconnect. Known event names include PlayerMessage, BlockPlaced, BlockBroken, ItemAcquired, ItemCrafted, MobKilled, PlayerTravelled, AgentCommand and more (see mc_events_catalog). Events only fire when the thing actually happens: BlockPlaced is sent when a player places a block by hand, not for /setblock or /fill.

Resources (2)

minecraft-edu://connection (live connection state) and minecraft-edu://capabilities (what the bridge can and cannot do).

Connecting and staying connected

All AI sessions on one machine (desktop apps, CLIs, IDEs, parallel tasks) share a background hub that owns port 19131 and the single game connection. Each session's MCP server is a thin proxy to it. The first session that needs the hub starts it; every open session checks in once a minute, so port 19131 stays served while any session is open, and a hub that dies is restarted right away. The hub exits about 10 minutes after the last session and the game are both gone.

• Switching sessions, models or AI tools does not drop the game connection.
• /reload all and moving to another world do not drop it either during a normal load. At the main menu the connection stays open, but every command answers "Unknown command".
• You only need to type /connect again after restarting Minecraft, or after the hub side closes the connection (the chat shows "Connection closed").
• You do not need to type /connect while connected. Doing so shows nothing at all, which looks like a hang; typing and commands keep working as usual.
• mc_run_command can send reload all. For development packs, scripts and behavior pack JSON reload this way; resource packs only reload when a player types /reload all in the chat. Wait until the world has finished loading before sending more commands. While a load hangs, the bridge's 30-second keepalive probe keeps running, and after three unanswered probes in a row the bridge closes the connection.
• If an old BlockHand without hub support already holds 19131, new sessions fall back to standalone mode on a random port; use the address mc_status reports. Standalone mode never takes 19131 itself.

To reconnect quickly, press T in the game, press ↑ to recall the last command, then Enter.

How building stays fast

Sending one setblock per block would mean tens of thousands of WebSocket round trips for a medium-sized sphere. BlockHand's pipeline is:

shape parameters → inside() test over the bounding box → set of block coordinates
  → merge runs along X → merge rectangles along Z → merge boxes along Y (3-stage greedy)
  → split by the 32,768-block /fill limit
  → send

The merge is deterministic, and tests pin that the merged batches cover exactly the original set of blocks, no more and no less. Hollow shapes all come from the same "inside test plus shell-neighbor test", so a new shape only needs an inside() function. tests/domain/fill-efficiency.test.ts sets an upper bound on fill commands per block for every shape.

For very large terrain work, compute the coordinates yourself and send many /fill commands with mc_run_commands (200 per call), or write a .mcfunction file and run it with /function. See agents/docs/large-builds.md for the limits involved.

Reading blocks

Minecraft Education has no command that simply returns the block at a position, so reading has built-in limits:

• mc_test_block is a yes/no question: you have to guess a block ID.
• mc_read_block does not need a guess. It asks whether the block is air, and when the answer is no, the game's error message names the actual block. The result is the localized display name ("Dirt"), not the block ID (dirt).
• When the message cannot be parsed, mc_read_block returns an error, never a successful response with block: null. The consumer is an AI, and an AI that sees a successful null will confidently treat the spot as empty and may build over a student's work.
• mc_verify_reading checks that the parsing still works, with at most two read-only commands. Run it after a game update, after changing the game language (supported: Traditional Chinese, Simplified Chinese, English), or whenever mc_read_block starts failing on blocks you can see. Mutation tests make sure the probe and the parser turn red when the parsing rules are broken.

Reading a whole region block by block would need a behavior pack and the Script API, which BlockHand deliberately avoids so that school computers need no extra installs.

Undo (off by default)

Automatic undo is off by default. On Minecraft Education 1.26.32 (macOS), the native region check that undo sends before the first small build (body.version=25) never got a reply; after that, every command on the connection timed out, including what the player typed in chat, and only restarting Minecraft recovered. This was reproduced three times. Turn it on with MINECRAFT_EDU_UNDO_ENABLED=1 or mc_undo_control with configure enabled=true only if you accept that risk. Design notes: agents/docs/undo/region-undo.md.

When enabled, undo keeps one operation of up to 4,096 blocks, snapshotted in game memory with structure save ... memory (nothing is written to disk). There is no redo. It covers the overworld, absolute integer coordinates and basic static building blocks; unsupported builds are rejected before anything is changed.

Configuration

Everything has a default; no .env file is needed.

Variable	Default	Meaning
MINECRAFT_EDU_WS_HOST	127.0.0.1	Listen address of the game WebSocket; loopback only by default (see Safety). The hub's control port always listens on 127.0.0.1
MINECRAFT_EDU_WS_PORT	19131	Port the game connects to. The default uses the shared hub; any other value (such as 0) runs an isolated standalone bridge
MINECRAFT_EDU_HUB	auto	0 disables the hub (one bridge per session); 1 forces the hub on a custom port
MINECRAFT_EDU_HUB_PORT	19132	Local control port the sessions use to reach the hub; also the lock that decides which process is the hub
MINECRAFT_EDU_HUB_DIR	~/.blockhand	Folder for the hub's state file hub.json (holds the hub's access token) and its log hub.log; see Safety
MINECRAFT_EDU_HUB_IDLE_MS	600000	The hub exits after this long with no session check-ins and no game connection
MINECRAFT_EDU_HUB_ATTACH_MS	8000	How long a session waits for a hub it has just started to answer (500–60,000 ms); if the hub does not answer in time, the session runs a standalone bridge on a random port
MINECRAFT_EDU_CONNECT_NOTICE	on	Show the green "✔ BlockHand connected" line in chat, in the game language (detected with one read-only time query daytime first); 0 turns it off
MINECRAFT_EDU_WS_PORT_FALLBACK	1	Pick a free port when the preferred one is taken; 0 fails instead
MINECRAFT_EDU_ENCRYPTION	on	When the game connects, try the WebSocket encryption handshake; if it is not negotiated, the connection continues in plaintext. 0 skips the handshake
MINECRAFT_EDU_COMMAND_TIMEOUT_MS	10000	How long to wait for the game to answer one command
MINECRAFT_EDU_KEEPALIVE_INTERVAL_MS	30000	Interval of the idle keepalive probe (time query daytime); three unanswered probes in a row close the connection
MINECRAFT_EDU_EVENT_BUFFER	500	Event ring buffer size
MINECRAFT_EDU_MAX_BUILD_BLOCKS	40000000	Maximum blocks per build; non-solid shapes are also capped at an 8,000,000-block scan volume
MINECRAFT_EDU_UNDO_ENABLED	0	1 turns on automatic single-step undo (see the warning above)
MINECRAFT_EDU_UNDO_MAX_BLOCKS	4096	Undo budget, 1–32,768 blocks, in at most 64 regions
MINECRAFT_EDU_STEP_DELAY_MS	100	Default delay between Agent program steps
MINECRAFT_EDU_DEBUG_FRAMES	unset	1 prints every raw frame from the game to stderr

Safety

• By default the game WebSocket listens on 127.0.0.1 only. MINECRAFT_EDU_WS_HOST changes that address. The game port does not authenticate connections, and a new connection replaces the current one, so any other address lets every machine that can reach it connect in place of the game. The hub's control port (MINECRAFT_EDU_HUB_PORT, 19132 by default) always listens on 127.0.0.1. BlockHand itself makes no connections to the internet.
• The shared hub keeps two files in ~/.blockhand (or MINECRAFT_EDU_HUB_DIR). When the hub is off (MINECRAFT_EDU_HUB=0, or a custom MINECRAFT_EDU_WS_PORT without MINECRAFT_EDU_HUB=1), neither file is written.
  • hub.json holds the hub's process ID, ports, build ID, start time and a random access token. The hub creates a new token each time it starts and removes the file when it shuts down. Every request to the control port must present the token as a bearer token; sessions read it from this file to reach the hub. The folder is created with mode 0700 and the file with mode 0600. Node.js cannot set owner/group/other permissions on Windows, so these modes only take effect on macOS and Linux.
  • hub.log is the hub's timestamped log. When a new hub starts and the log is larger than 1 MiB, it is renamed to hub.log.1. With MINECRAFT_EDU_DEBUG_FRAMES=1, every raw frame from the game is logged there too.
• Apart from those files, the server writes nothing on the host. mc_structure with saveMode: "disk" makes the game write a structure into its world folder; the tool reply says where.
• No accounts or credentials for any online service are involved; the hub token only guards the local control port.
• mc_run_command accepts slash commands, which only act inside the local game world. Its gate is structural rather than a keyword blocklist:
  1. One line only. Newlines and NUL are rejected, so one request cannot become two commands.
  2. Commands that would cut or redirect the bridge are refused: wsserver, connect and closewebsocket, including when wrapped in execute.
  3. Everything else is tagged read-only, world-write or wide-effect, so the MCP host can ask for confirmation based on the tool annotations.
• Block IDs, selectors and state strings that go into a command line are checked against allow-list patterns first, so spaces cannot smuggle in extra arguments.
• On a shared hub, only one AI session can change the world at a time. A second session's write tool is refused immediately with a clear message; read-only tools are not affected.

Troubleshooting

The player says chat stopped responding. Do not restart the game first. Check, in order:

1. What the game screen shows (in a world, main menu, or a loading screen).
2. mc_status: commandState: "timed-out" means the bridge's own commands are not being answered either.
3. Subscribe to PlayerMessage and ask the player to type a line. Player chat produces a type: "chat" event. Not receiving one is not proof by itself, because a subscription is not confirmed by the game; compare with the screen.
4. The last lines of ~/.blockhand/hub.log (every line is timestamped, rejected handshakes are logged too).

When a built-in command such as time answers "Unknown command", the tools add a hint: the game is most likely not in a world.

mc_status shows a port other than 19131. Another process holds 19131. Find it with lsof -nP -iTCP:19131 -sTCP:LISTEN (macOS) and check whether the hub is running with ls ~/.blockhand.

Windows, Microsoft Store (UWP) version. Windows app isolation blocks loopback connections. Grant an exemption with CheckNetIsolation LoopbackExempt.

A large build gets cut off. MCP clients usually time out requests after about 60 seconds, while the tool keeps sending fills. Check fillBatches with mc_build_preview first and split large builds.

Known limitations

• Text from the game stays in the game's language. Tool descriptions, server instructions and error messages are in English, but what the game itself returns (statusMessage, and the block names from mc_read_block) comes back in the game language.
• The world must have Cheats on; otherwise the game rejects every command.
• The Agent exists only in Education Edition.
• Event names and the agent subcommands are not formally documented by Mojang; they come from public observation and may change with game updates. mc_events_subscribe accepts names outside the known list but marks them unverified.
• @s does not always resolve for commands sent over WebSocket, so mc_query_target defaults to @p.
• Some responses arrive with a request ID that matches no request. When exactly one request is pending and no timeout has happened on the connection, the bridge may attribute the response by inference and logs it; after any timeout it only accepts exact matches. Modifying commands are never resent automatically.
• Commands are sent one at a time with a queue of at most 64; anything beyond that is refused with command-queue-full, and queued commands that outlive their own timeout are dropped and reported as not sent.
• After rebuilding, the running hub keeps the old version until it restarts; a hub restart drops the game connection.

Development

corepack pnpm run verify   # typecheck, tests, build, stdio smoke, shared-hub smoke
corepack pnpm run live     # end-to-end check against a running game (prints the /connect to type)

live walks through connect, read player position, chat, set time, summon the Agent, sense, walk an L-shaped path, preview and build a hollow glass sphere, verify the blocks, merge a blueprint, receive events, test the command gate and clean up. Add --keep to node scripts/live-check.mjs to keep the demo build.

src/
  domain/          pure logic, no MCP, ws or Node dependencies
    commands.ts            slash command builders and injection allow-lists
    command-policy.ts      structural gate for raw commands
    build/shapes.ts        the 24 shapes: inside() plus shell-neighbor test
    build/fill-planner.ts  3-stage greedy merge and batch splitting
    build/undo-plan.ts     undo regions, budgets and native region checks
  adapters/ws-minecraft-connection.ts   WebSocket server, request matching, events, reconnects
  application/     services: Agent programs, querytarget parsing, building, undo
  hub/             shared hub, session proxy, cross-session write lock
  server/          MCP server, schemas and the tools
  composition.ts   wiring; accepts a fake connection for tests
  index.ts         stdio entry point

The domain layer knows nothing about WebSockets, so the whole MCP tool pipeline is tested with in-memory fakes, without a running game.

License and trademarks

MIT.

NOT AN OFFICIAL MINECRAFT PRODUCT. NOT APPROVED BY OR ASSOCIATED WITH MOJANG OR MICROSOFT. Minecraft and Minecraft Education are trademarks of Mojang Studios and Microsoft. BlockHand is an independent open-source project.