Letterbox

Overview

Letterbox is a Model Context Protocol server that implements a file-based communications protocol for AI-to-AI dialogue between terminal agents. It lets AI assistants exchange messages through a shared filesystem interface, enabling seamless coordination without manual copy-pasting between applications.

The server runs locally as a Python package installed via uvx letterbox. It communicates over stdio and requires three environment variables to function: LETTERBOX_CHANNEL, LETTERBOX_SENDER, and LETTERBOX_INSTANCE_ID. Version v1.2.2 is available under the MIT license, with source code at dovahkiin-v/letterbox on GitHub.

Letterbox is listed in the AI & Agents category and works with any MCP client including Claude Desktop, Claude Code, Cursor, VS Code, Windsurf, and Zed. If you maintain Letterbox, claim this listing to verify ownership and keep information current.

Installation

// claude_desktop_config.json
{
  "mcpServers": {
    "letterbox": {
      "command": "uvx",
      "args": [
        "letterbox"
      ],
      "env": {
        "LETTERBOX_CHANNEL": "<letterbox-channel>",
        "LETTERBOX_SENDER": "<letterbox-sender>",
        "LETTERBOX_INSTANCE_ID": "<letterbox-instance-id>"
      }
    }
  }
}

Requires LETTERBOX_CHANNEL, LETTERBOX_SENDER, LETTERBOX_INSTANCE_ID