MCPlexer

MCPlexer is a tiny MCP proxy for running multiple labeled instances of the same MCP server without tool-name collisions.

If an upstream server exposes get-calendar-events, MCPlexer can expose it as:

• personal:get-calendar-events

• work:get-calendar-events

It also appends a label note to every tool description, for example:

Note: this is one of multiple instances of this MCP, labeled "Personal".

Install from npm

Install MCPlexer globally when you want to use it from .mcp.json files:

npm install --global @oztamir/mcplexer

The installed command is mcplexer:

mcplexer --help

If you prefer pnpm for global packages:

pnpm add --global @oztamir/mcplexer

Related MCP server: Multi MCP

Configure your MCP client

Use mcplexer as the MCP server command. Put the upstream local MCP command after --.

{
  "mcpServers": {
    "google-personal": {
      "command": "mcplexer",
      "args": [
        "--prefix",
        "personal",
        "--label",
        "Personal",
        "--",
        "npx",
        "-y",
        "@example/google-workspace-mcp"
      ],
      "env": {
        "GOOGLE_ACCOUNT": "personal"
      }
    },
    "google-work": {
      "command": "mcplexer",
      "args": [
        "--prefix",
        "work",
        "--label",
        "Work",
        "--",
        "npx",
        "-y",
        "@example/google-workspace-mcp"
      ],
      "env": {
        "GOOGLE_ACCOUNT": "work"
      }
    }
  }
}

The wrapper passes its environment through to the upstream command, so per-account .mcp.json env values still work.

For remote MCP endpoints, pass --url. By default MCPlexer tries Streamable HTTP first and falls back to legacy SSE.

For hosted MCPs that use a normal user OAuth flow, such as Notion MCP, use browser OAuth:

{
  "mcpServers": {
    "notion-work": {
      "command": "mcplexer",
      "args": [
        "--prefix",
        "work",
        "--label",
        "Work Notion",
        "--url",
        "https://mcp.notion.com/mcp",
        "--oauth-flow",
        "browser"
      ]
    }
  }
}

On first use, MCPlexer prints the authorization URL to stderr and opens it in your browser. After you approve access, it receives the local callback on 127.0.0.1, stores the OAuth tokens under ~/.config/mcplexer/oauth/, and reconnects automatically. The downstream agent only sees the prefixed MCP tools.

The callback port is not tied to Notion. By default MCPlexer picks a deterministic high loopback port from the upstream URL and prefix, so multiple labeled instances do not all fight for one fixed port. If your OAuth provider or pre-registered client requires an exact redirect URI, set it explicitly with --oauth-callback-port. If the provider requires a known public client ID, pass it with --oauth-client-id. If you cannot open a browser automatically, add --oauth-no-open and open the printed URL manually. Use --oauth-store when you need a custom token cache path.

If you already have an OAuth access token, let MCPlexer pass it through the MCP SDK auth provider:

{
  "mcpServers": {
    "calendar-work-remote": {
      "command": "mcplexer",
      "args": [
        "--prefix",
        "work",
        "--label",
        "Work",
        "--url",
        "https://mcp.example.com/mcp",
        "--oauth-bearer-env",
        "WORK_MCP_TOKEN"
      ],
      "env": {
        "WORK_MCP_TOKEN": "replace-me"
      }
    }
  }
}

For machine-to-machine OAuth, use client credentials:

{
  "mcpServers": {
    "calendar-work-remote": {
      "command": "mcplexer",
      "args": [
        "--prefix",
        "work",
        "--label",
        "Work",
        "--url",
        "https://mcp.example.com/mcp",
        "--oauth-client-id",
        "your-client-id",
        "--oauth-client-secret-env",
        "WORK_MCP_CLIENT_SECRET",
        "--oauth-scope",
        "calendar.read calendar.write"
      ],
      "env": {
        "WORK_MCP_CLIENT_SECRET": "replace-me"
      }
    }
  }
}

For non-OAuth remote authentication, you can still pass static headers with --header or --header-env. Do not combine OAuth options with an Authorization header.

You can force a remote transport when needed:

mcplexer --prefix work --url https://mcp.example.com/mcp --transport http
mcplexer --prefix legacy --url https://mcp.example.com/sse --transport sse

Other ways to run MCPlexer

Run with npx

You can also run MCPlexer through npx without a global install:

npx -y @oztamir/mcplexer --help

For .mcp.json, use npx as the command and put MCPlexer arguments after the package name:

{
  "mcpServers": {
    "google-personal": {
      "command": "npx",
      "args": [
        "-y",
        "@oztamir/mcplexer",
        "--prefix",
        "personal",
        "--label",
        "Personal",
        "--",
        "npx",
        "-y",
        "@example/google-workspace-mcp"
      ]
    }
  }
}

Link a local checkout

From a local checkout of this repo:

pnpm install
pnpm build
pnpm link --global

Agent skill

This repo exposes an agent skill that teaches AI coding agents when and how to use MCPlexer — adding a second labeled instance of an MCP server, and migrating an existing MCP entry to a prefixed setup.

Install it with npx skills:

npx skills add OzTamir/mcplexer

Or install it as a Claude Code plugin:

/plugin marketplace add OzTamir/mcplexer
/plugin install mcplexer@mcplexer

CLI reference

mcplexer --prefix <name> [--label <label>] -- <command> [args...]
mcplexer --prefix <name> [--label <label>] --url <mcp-url> [--transport auto|http|sse]

Options:

• --prefix <name>: tool-name prefix. Must contain only letters, numbers, _, or -.

• --label <label>: human label used in the description note. Defaults to the prefix.

• --separator <text>: separator between prefix and tool name. Defaults to :.

• --note <text>: custom note appended to every tool description.

• --url <url>: remote upstream MCP endpoint.

• --transport <mode>: stdio, auto, http, or sse.

• --header <name: value>: literal remote header. Repeatable.

• --header-env <name=ENV>: remote header whose value is read from an environment variable. Repeatable.

• --oauth-bearer-env <ENV>: remote OAuth bearer token read from an environment variable.

• --oauth-flow browser: remote OAuth authorization-code flow with a local browser callback.

• --oauth-callback-port <port>: local browser OAuth callback port. Defaults to a deterministic high port based on upstream URL and prefix.

• --oauth-store <path>: OAuth token/client cache path for browser flow.

• --oauth-no-open: print the OAuth URL without trying to open a browser.

• --oauth-client-id <id>: OAuth client ID. In browser flow, use this for a pre-registered public client. In client_credentials, pair it with --oauth-client-secret-env.

• --oauth-client-secret-env <ENV>: OAuth client_credentials client secret environment variable.

• --oauth-scope <scope>: optional OAuth scope for client_credentials.

• --oauth-client-name <name>: optional OAuth client display name for metadata.

How it works

MCPlexer is itself a stdio MCP server. It connects to one upstream MCP server, lists upstream tools, rewrites each tool name and description, then forwards tools/call requests by stripping the configured prefix before calling upstream.