BlockbenchMCP

Let an AI build Minecraft models, textures and animations directly inside Blockbench — through the Model Context Protocol.

BlockbenchMCP gives an AI assistant a live connection to a running Blockbench window. The model can start a project from the start screen, build geometry (bones + cubes), paint textures procedurally, author keyframe animations (including GeckoLib), install Blockbench plugins, move the camera, and take screenshots so it can look at its own work and refine it — all without you touching the editor.

It was used to model, texture and animate a full GeckoLib grizzly bear (walk / run / sleep / attack) end-to-end from a single prompt.

Table of contents

• How it works
• Features
• Requirements
• Installation
• Connecting your AI client
• Tool reference
• Example: an animated GeckoLib bear
• Example: procedural detail
• Troubleshooting
• Security
• Limitations
• Development
• License

How it works

There are two pieces:

┌────────────┐   stdio (MCP)   ┌──────────────────┐   HTTP 127.0.0.1:8787   ┌─────────────────────┐
│  AI client │ ──────────────▶ │  blockbench-mcp  │ ──────────────────────▶ │  Blockbench + plugin │
│ (Claude…)  │ ◀────────────── │   (Node server)  │ ◀────────────────────── │   (live editor)      │
└────────────┘                 └──────────────────┘                         └─────────────────────┘

Features

• 🧱 Modeling — create bones (groups) and cubes one at a time or in bulk (add_groups / add_cubes build a whole posed skeleton in one call), with full rotation and inflate; edit/move/reparent/delete, and read the outliner tree.
• 🏗️ Procedural detail — the fix for models that come out as 15 flat boxes, because an LLM cannot compute 200 sets of [from,to] in its head. voxelize_matrix extrudes a character matrix into cubes (draw a blade, an emblem or a horn profile as pixel art and get geometry); add_hollow_volume builds a shell with a real cavity instead of a solid box (hoods, helmets, pauldrons, cages); generate_array repeats an element along a line, ring or grid with jitter, taper, per-element rotation and an anti-z-fighting depth stagger (torn hems, scales, plates, teeth, rivets); extrude_chain builds a tapering, curving chain with one bone per segment (horns, tails, tentacles, braids). Each one enforces the model's own left/right and returns a compact report instead of 300 serialized cubes.
• 📏 Density gate — audit_complexity grades the model against a cube budget (prop 30-60, mob 100-180, hero 180-300+) and reports monolithic boxes (one cube holding >30% of the volume with nothing layered on it), how much geometry actually overlaps, micro-detail density, bare flat faces and bone-hierarchy depth. Verdict too_primitive / acceptable / high_detail, so a blockout never reaches the texturing pass.
• 🎨 Texturing — create textures, paint procedurally (pixels, rects, lines, circles, ellipses, polygons, dither, noise, gradients), auto-shade every cube face with detail_cubes (no flat or untextured gaps), and place features in face-relative coordinates with paint_faces.
• 🦴 Rigging — create_rig builds a properly segmented humanoid or quadruped skeleton (three bones per limb, so elbows and knees can actually bend), joints on the real pivots, correct *_left / *_right naming and an optional blocked-out body; check_rig refuses to call a two-bone-limb rig animation-ready.
• 🎬 Animation — generate_animation writes a complete, direction-correct base cycle (idle / walk / run / attack / cast / jump / hurt / death / fly) with proper gait phasing, bent elbows and knees, body counter-rotation, follow-through and seamless loops; add_keyframes refines it in bulk with interpolation control.
• 📐 Measured animation — analyze_animation evaluates the rig and reports how far each hand, foot and head really travels forward / back / left / right in the model's own axes, whether the loop closes and whether the lower limb segments move at all. It is what catches an attack that swings into the model's back instead of at its target.
• 🧭 Left/right that can't get flipped — the model faces -Z, so its own right is +X (verified against Blockbench's vanilla data). get_orientation, which_side and check_sides answer the question from coordinates; add_cube/add_group take side:"left"|"right" and refuse a call whose coordinate contradicts it; and every screenshot is stamped with which image edge is the model's right — because a front view is mirrored.
• 🙋 Human review gate — request_review renders labelled views (or animation poses), shows them in the MCP Copilot panel and blocks until the user presses a button, returning their verdict and comment inside the same tool call. ask_user does the same for a question. The AI stops grading its own homework without interrupting the conversation.
• 🎯 Reference matching — drag a reference image into the MCP Copilot panel; the AI reads it (get_reference) and scores its silhouette match with compare_reference (an IoU match_percent + a [reference | model | overlay] composite and concrete "add/trim mass here" advice) plus numeric proportions via measure_model. Turns "does it match?" from an opinion into a number, so models actually resemble the reference instead of "almost".
• 📸 Vision — screenshot, screenshot_views (several angles at once, named from the model's point of view and captioned) and get_texture return images inline so the model can see and iterate; check_model audits for problems.
• 🧠 Guidance — get_guide returns playbooks for modeling, detailing (the cube budget and the 4-layer doctrine), orientation, rigging, texturing, VFX, animation, review and reference-matching, so the AI builds detailed, rotated models instead of a few flat boxes.
• 🪟 In-app panel — the MCP Copilot side panel shows server status, the reference drop zone, a live activity log of the AI's commands, model stats, the last match score — and any review the AI is currently waiting on, with thumbnails, a comment box and approve / needs-changes buttons.
• 🧩 Plugins — search, install (by store id, URL, or file) and uninstall Blockbench plugins, so the AI can set up formats like GeckoLib itself.
• 📦 Export — export_project uses the format's own codec; export_model writes through any named codec (default glTF: a self-contained .gltf that imports straight into Godot / Unity / Blender).
• 🔧 Escape hatch — execute_script runs arbitrary Blockbench JS for anything not covered by a dedicated tool.
• 🟢 70 tools total, all over a single local connection.

Requirements

• Blockbench desktop app, version 4.8+ (the web app cannot host the bridge — see Limitations).
• Node.js 18+ (for global fetch).
• An MCP-capable client (Claude Code, Claude Desktop, Cursor, etc.).

Installation

1. Install the bridge plugin in Blockbench

1. Open the desktop Blockbench app.
2. File ▸ Plugins ▸ Load Plugin from File and select plugin/blockbench_mcp.js.
   (Alternatively copy it into your Blockbench plugins folder.)
3. On first start the plugin asks for network permission (it needs the net module to host the local server). Choose “Always allow for this plugin.”
4. A toast confirms “MCP server started on port 8787.”
   • Toggle the server any time: Tools ▸ Start / Stop MCP Server
   • Change the port: Settings ▸ General ▸ MCP Server Port (default 8787)

Verify it's up:

curl http://127.0.0.1:8787/ping
# {"ok":true,"protocol":1,"blockbench_version":"5.x.x","is_app":true,"has_project":false}

2. Build the MCP server

git clone https://github.com/sosadly/blockbench-mcp.git
cd blockbench-mcp
npm install
npm run build

Connecting your AI client

Point your client at dist/index.js over stdio. Use an absolute path.

Claude Code — .mcp.json in your project (or claude mcp add):

{
  "mcpServers": {
    "blockbench": {
      "command": "node",
      "args": ["/absolute/path/to/blockbench-mcp/dist/index.js"],
      "env": { "BLOCKBENCH_MCP_PORT": "8787" }
    }
  }
}

Claude Desktop — claude_desktop_config.json:

{
  "mcpServers": {
    "blockbench": {
      "command": "node",
      "args": ["C:\\path\\to\\blockbench-mcp\\dist\\index.js"]
    }
  }
}

Tool reference

Conventions: coordinates are Blockbench units; rotations are degrees; texture pixel ops use a top-left origin with y pointing down. add_cube returns each face's resolved UV rect; detail_cubes then base-coats every face and paint_faces lets you paint features (eyes, nose, claws) in coordinates relative to a face, so you don't compute absolute UVs by hand.

Orientation. A model faces -Z, so its own right is +X and its left is -X. This is verified against Blockbench's bundled vanilla data: Bedrock's rightArm, stored at x = -5, is loaded by Blockbench at x = +5 (the Bedrock/Java codecs mirror X on import and export). Camera views are named from the model's point of view — front shows its face — and every capture is captioned with which image edge is the model's right, because a front view is mirrored just like facing a person.

Rotation signs. +X rotation lifts a bone's front, so a down-pointing bone (arm, leg) swings its tip forward while an up-pointing bone (torso, neck) tips backward. Elbows bend +X, knees bend -X, and +Y turns the model toward its own left. generate_animation applies these for you and analyze_animation verifies the result by measurement.

Example: an animated GeckoLib bear

The sequence an AI follows for “make a textured GeckoLib bear that can walk, run, sleep and attack”:

// 0. Read the playbook so the model comes out detailed and rotated, not boxy
get_guide {}

// 1. Install GeckoLib (adds the `geckolib_model` format)
install_plugin { "id": "geckolib" }

// 2. New project, straight from the start screen
new_project { "format": "geckolib_model", "name": "bear",
              "texture_width": 64, "texture_height": 64 }

// 3. Lay out the whole posed skeleton in one call — note the rotated bones
add_groups { "groups": [
  { "name": "body",   "origin": [0, 12, 0] },
  { "name": "head",   "origin": [0, 14, -7], "parent": "body", "rotation": [10, 0, 0] }, // nose down
  { "name": "leg_fl", "origin": [3, 8, -5],  "parent": "body", "rotation": [-6, 0, 0] },
  { "name": "leg_fr", "origin": [-3, 8, -5], "parent": "body", "rotation": [-6, 0, 0] }
  // ...hind legs, ears, muzzle, tail...
] }

// 4. Build many cubes at once (segment limbs, taper with inflate, mirror left/right)
add_cubes { "cubes": [
  { "name": "torso",   "from": [-5, 8, -7], "to": [5, 16, 7], "parent": "body" },
  { "name": "head",    "from": [-3.5, 10, -13], "to": [3.5, 16, -7], "parent": "head" },
  { "name": "muzzle",  "from": [-2, 10, -16], "to": [2, 13, -13], "parent": "head", "inflate": -0.5 }
  // ...
] }

// 5. Texture: shaded base coat on EVERY face (no gaps), then features face-relative
create_texture { "name": "bear", "width": 64, "height": 64, "fill": "#6e4a2b" }
detail_cubes   { "base": "#6e4a2b", "noise": 0.12, "bottom_dark": 0.3 }
paint_faces    { "faces": [
  { "cube": "head",   "face": "north", "ops": [
    { "type": "rect",    "x": 2, "y": 2, "width": 1, "height": 1, "color": "#0f0a05" }, // eye
    { "type": "ellipse", "x": 3, "y": 4, "width": 2, "height": 2, "color": "#140e08" }  // nose
  ] }
] }

// 6. Animate (bulk keyframes, smooth interpolation)
create_animation { "name": "animation.bear.walk", "loop": "loop", "length": 1.2 }
add_keyframes {
  "animation": "animation.bear.walk",
  "keyframes": [
    { "bone": "leg_fl", "channel": "rotation", "time": 0.0, "value": [28, 0, 0], "interpolation": "catmullrom" },
    { "bone": "leg_fl", "channel": "rotation", "time": 0.6, "value": [-28, 0, 0], "interpolation": "catmullrom" },
    { "bone": "leg_fl", "channel": "rotation", "time": 1.2, "value": [28, 0, 0], "interpolation": "catmullrom" }
  ]
}

// 7. Review from every side + audit, fix what you see, then repeat 4–7
screenshot_views { "views": ["isometric_right_front", "front", "left", "back"] }
check_model {}

// 8. Save / export
save_project   { "path": "D:/models/bear.bbmodel" }
export_project { "path": "D:/models/bear.geo.json" }

Example: procedural detail

The same four calls turn a 20-cube blockout into a 150-cube model. None of the tools knows what
a hood or a scale is — they are extrusion, shells, arrays and chains.

// A hood: a shell with a real cavity, open at the face (north = -Z) and the neck (down)
add_hollow_volume {
  "bounds": { "from": [-5.5, 23, -5.5], "to": [5.5, 34, 5.5] },
  "wall_thickness": 1.5, "open_faces": ["north", "down"],
  "name": "hood", "parent": "head"
}

// A torn hem: 11 panels hanging off the cloak, tapering, jittered, staggered in depth so
// neighbours cannot z-fight
generate_array {
  "mode": "linear", "count": 11, "element_size": [2.2, 5, 1.2],
  "start": [-6, 7, 3.2], "end": [6, 7, 3.2],
  "anchor": "top", "jitter": [0.15, 0.4, 0], "size_decay": [0, -0.15, 0],
  "depth_stagger": 0.12, "rotation_range": { "min": [-4, 0, -9], "max": [4, 0, 9] },
  "seed": 12, "name_prefix": "hem", "parent": "body"
}

// A horn: 6 tapering segments, each bone bending 14° more than the last, animatable
extrude_chain {
  "segments": 6, "base_origin": [3.5, 32, -1], "segment_length": 2.4,
  "initial_size": [2.4, 2.4], "taper": 0.8, "curvature": [14, 0, -6],
  "direction": "up", "name": "horn", "side": "right", "parent": "head"
}

// A blade: drawn as pixel art in the side plane, extruded into cubes
voxelize_matrix {
  "matrix": ["......####", ".....#####", "...#######", "..######..",
             ".#####....", "######....", "#####.....", "####......"],
  "palette": { "#": { "name": "blade", "depth": 1 } },
  "plane": "yz", "origin": [0, 18, -4], "merge_adjacent": true, "parent": "hand_right"
}

// The gate: is this actually a model yet?
audit_complexity { "target": "hero" }
// -> { "verdict": "acceptable", "ready_for_texturing": true, "cubes": 152,
//      "metrics": { "micro_pct": 34, "overlapping_pct": 61, "bone_depth": 6 }, "issues": [] }

Troubleshooting

Blockbench shows “server stopped” and Start does nothing.
The plugin needs the net module. When you click Start MCP Server, Blockbench shows a permission dialog — choose “Always allow for this plugin.” If you previously denied it, revoke and retry from the plugin's context menu, or restart Blockbench.

MCP tools fail with “Cannot reach Blockbench on 127.0.0.1:8787”.
Make sure Blockbench is open, the plugin is loaded, and the server is running (green toast / curl .../ping works). Confirm the port in the plugin settings matches BLOCKBENCH_MCP_PORT.

“Unknown format … the matching plugin must be installed.”
Plugin formats like GeckoLib's geckolib_model require install_plugin { "id": "geckolib" } first. Run list_formats to confirm the id appeared.

Console 404s about about.md / the plugin store.
Harmless — Blockbench tries to fetch store metadata for the side-loaded plugin and gets a 404. It does not affect the bridge.

Security

• The bridge binds to 127.0.0.1 only — it is not reachable from your network.
• execute_script runs unsandboxed JavaScript inside Blockbench. Only connect MCP clients you trust, and prefer the dedicated tools over execute_script where possible.

Limitations

• Desktop only. The bridge relies on Node modules (net), which the Blockbench web app does not expose to plugins.
• One Blockbench window. The bridge talks to whichever project is currently active.
• Box-UV packing is up to the caller; add_cube returns resolved face UVs to make this manageable.

Development

npm run dev     # tsc --watch
npm run build   # one-off compile to dist/
npm test        # build, then run the tool-catalogue tests against dist/

• MCP tool definitions live in src/tools.ts; the HTTP client in src/client.ts; the server entry in src/index.ts; tests in test/tools.test.js (Node's built-in runner, no extra dependencies).
• Bridge command handlers live in plugin/blockbench_mcp.js under the commands object — add a handler there and a matching tool in tools.ts to extend the surface.
• The Blockbench API used by the bridge is documented in the official type definitions.

License

MIT © sosadly

Built with the Model Context Protocol. Not affiliated with Blockbench or GeckoLib.