Blender MCP

# Blender MCP

Control a running Blender instance from Hermes via socket on TCP port 9876.

## Setup (one-time)

### 1. Install the Blender addon

    curl -sL https://raw.githubusercontent.com/ahujasid/blender-mcp/main/addon.py -o ~/Desktop/blender_mcp_addon.py

In Blender:
    Edit > Preferences > Add-ons > Install > select blender_mcp_addon.py
    Enable "Interface: Blender MCP"

### 2. Start the socket server in Blender

Press N in Blender viewport to open sidebar.
Find "BlenderMCP" tab and click "Start Server".

### 3. Verify connection

    nc -z -w2 localhost 9876 && echo "OPEN" || echo "CLOSED"

## Protocol

Plain UTF-8 JSON over TCP -- no length prefix.

Send:     {"type": "<command>", "params": {<kwargs>}}
Receive:  {"status": "success", "result": <value>}
          {"status": "error",   "message": "<reason>"}

## Available Commands

| type                    | params            | description                     |
|-------------------------|-------------------|---------------------------------|
| execute_code            | code (str)        | Run arbitrary bpy Python code   |
| get_scene_info          | (none)            | List all objects in scene       |
| get_object_info         | object_name (str) | Details on a specific object    |
| get_viewport_screenshot | (none)            | Screenshot of current viewport  |

## Python Helper

Use this inside execute_code tool calls:

    import socket, json

    def blender_exec(code: str, host="localhost", port=9876, timeout=15):
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.connect((host, port))
        s.settimeout(timeout)
        payload = json.dumps({"type": "execute_code", "params": {"code": code}})
        s.sendall(payload.encode("utf-8"))
        buf = b""
        while True:
            try:
                chunk = s.recv(4096)
                if not chunk:
                    break
                buf += chunk
                try:
                    json.loads(buf.decode("utf-8"))
                    break
                except json.JSONDecodeError:
                    continue
            except socket.timeout:
                break
        s.close()
        return json.loads(buf.decode("utf-8"))

## Common bpy Patterns

### Clear scene
    bpy.ops.object.select_all(action='SELECT')
    bpy.ops.object.delete()

### Add mesh objects
    bpy.ops.mesh.primitive_uv_sphere_add(radius=1, location=(0, 0, 0))
    bpy.ops.mesh.primitive_cube_add(size=2, location=(3, 0, 0))
    bpy.ops.mesh.primitive_cylinder_add(radius=0.5, depth=2, location=(-3, 0, 0))

### Create and assign material
    mat = bpy.data.materials.new(name="MyMat")
    mat.use_nodes = True
    bsdf = mat.node_tree.nodes.get("Principled BSDF")
    bsdf.inputs["Base Color"].default_value = (R, G, B, 1.0)
    bsdf.inputs["Roughness"].default_value = 0.3
    bsdf.inputs["Metallic"].default_value = 0.0
    obj.data.materials.append(mat)

### Keyframe animation
    obj.location = (0, 0, 0)
    obj.keyframe_insert(data_path="location", frame=1)
    obj.location = (0, 0, 3)
    obj.keyframe_insert(data_path="location", frame=60)

### Render to file
    bpy.context.scene.render.filepath = "/tmp/render.png"
    bpy.context.scene.render.engine = 'CYCLES'
    bpy.ops.render.render(write_still=True)

## Pitfalls

- Must check socket is open before running (nc -z localhost 9876)
- Addon server must be started inside Blender each session (N-panel > BlenderMCP > Connect)
- Break complex scenes into multiple smaller execute_code calls to avoid timeouts
- Render output path must be absolute (/tmp/...) not relative
- shade_smooth() requires object to be selected and in object mode