MCP Rubber Duck — Setup Knowledge

What is mcp-rubber-duck?

An MCP server that bridges to multiple OpenAI-compatible LLMs. It provides "rubber duck debugging" with AI — query multiple LLM providers simultaneously and get different perspectives. Supports OpenAI, Gemini, Groq, Ollama, Together AI, and CLI coding agents (Claude, Codex, Gemini CLI, Aider).

Supported tools and config paths

Tool	Config File (macOS)	Key	Notes
Claude Desktop	`~/Library/Application Support/Claude/claude_desktop_config.json`	`mcpServers`	Windows: `%APPDATA%\Claude\claude_desktop_config.json`
Claude Code (user)	`~/.claude.json`	`mcpServers`	Prefer `claude mcp add` CLI
Claude Code (project)	`.mcp.json`	`mcpServers`	Shared via VCS
Cursor (project)	`.cursor/mcp.json`	`mcpServers`
Cursor (global)	`~/.cursor/mcp.json`	`mcpServers`
Windsurf	`~/.codeium/windsurf/mcp_config.json`	`mcpServers`	Supports `${env:VAR}`
VS Code	`.vscode/mcp.json`	`servers`	Different key name! Uses `${env:VAR}`
Continue	`.continue/config.yaml`	`mcpServers` (array)	YAML format

ChatGPT supports MCP via UI only (Developer Mode > Add MCP Server), not a config file.

Critical requirement

MCP_SERVER=true must be set as an environment variable. Without it, the server won't start in MCP mode. This is the most common setup mistake.

Install methods

•npm global: npm install -g mcp-rubber-duck → command is mcp-rubber-duck
•npx: npx -y mcp-rubber-duck → no install needed
•From source: node /path/to/mcp-rubber-duck/dist/index.js

When users ask about setup

If a user asks how to add rubber duck to any tool, you have two options:

•Suggest the /setup command: Tell the user to run /setup <tool-name> for an interactive guided setup
•
Answer directly: Use the reference files for detailed config templates:
- •.claude/skills/setup-mcp-server/references/tool-configs.md — per-tool JSON/YAML templates
- •.claude/skills/setup-mcp-server/references/provider-env-vars.md — env var reference
- •.claude/skills/setup-mcp-server/references/troubleshooting.md — common issues

Quick example (Claude Desktop)

json

{
  "mcpServers": {
    "rubber-duck": {
      "command": "npx",
      "args": ["-y", "mcp-rubber-duck"],
      "env": {
        "MCP_SERVER": "true",
        "OPENAI_API_KEY": "sk-...",
        "DEFAULT_PROVIDER": "openai"
      }
    }
  }
}