carbonctl

carbonctl is an MCP server for my Elegoo Centauri Carbon 3D printer. An AI assistant can check on a print, look through the camera, read the print history, and pause, resume, or stop a print. It can’t do anything else to the printer.

Problem

I wanted to ask an assistant “is the print OK?” from anywhere, and let it stop a failing print. An assistant with direct access to real hardware needs limits that live in code, not in a prompt.

Work

flowchart LR
  A[MCP client<br/>Claude, agents] -->|loopback HTTP| C[carbonctl]
  C -->|WebSocket, SDCP| P[Printer]
  C -->|MJPEG| K[Camera]
  C -->|HTTP| H[Print history]

• Allowlist, not denylist. Only pause, stop, and resume can reach the printer. Every other command is rejected in one place.
• No one-shot stop. A stop needs a single-use token tied to the current print and valid for five minutes, followed by a human approval through MCP elicitation. A stale token, or one for a different print, fails.
• Loopback only. The server refuses to listen anywhere but localhost.
• Locked-down NixOS module. Runs as a sandboxed service that can reach only the printer and local clients. It runs on europa behind my sign-in gate.

Stack

Go, MCP (streamable HTTP), SDCP over WebSocket, NixOS module.

Status

Active, and in daily use from my agents.

Lessons

• Put the safety checks in the server, where the model can’t argue its way past them.
• Two-step confirmation with a human in the loop costs little and removes the scariest failure.