qnexus-mcp

A community Model Context Protocol (MCP) server for Quantinuum Nexus, wrapping the official qnexus Python SDK so any MCP-speaking agent (Claude Code, Cursor, VS Code, Codex, …) can inspect Nexus and, opt-in, run circuits on the free emulator.

Not affiliated with, endorsed by, or an official product of Quantinuum. "Quantinuum" and "Nexus" are trademarks of their respective owners, used here nominatively to describe compatibility.

Status

Early development. Read-only by default. Design and rationale live in docs/DESIGN.md; the research behind it is in docs/research/.

Requirements

• Python 3.10+ and uv (the uvx command). On Windows, note the full path to uvx.exe (where uvx in a terminal) — GUI clients don't inherit your shell PATH.
• A Quantinuum Nexus account.

1. Authenticate (once)

qnexus-mcp never handles your Nexus token. Authenticate out-of-band with the qnexus CLI (it opens your browser):

uvx --from qnexus qnx login

Inside Nexus JupyterHub, authentication is automatic; do not run qnx login there.

2. Add the server to your MCP client

The launch command is the same everywhere — only the config file differs per client:

uvx qnexus-mcp==0.2.0                              # read-only (default)
uvx qnexus-mcp==0.2.0 --toolsets read,execute      # + run circuits on the free H2-1LE emulator

Pin a version (==0.2.0): uvx otherwise resolves the latest PyPI release on every launch — unpinned installs are neither reproducible nor auditable. Avoid 0.1.0 on Windows: its first tool call hangs (fixed in 0.2.0).

What to expect on startup: the first ever launch downloads the quantum SDK stack (1–3 min); every launch takes ~30 s before the server responds — the SDK is imported up front, before the MCP handshake. "Waiting for server to respond to initialize" during that window is normal.

Claude Desktop

Settings → Developer → Local MCP servers → Edit Config (always use this button — the Microsoft Store build keeps the file under %LOCALAPPDATA%\Packages\Claude_*\..., not %APPDATA%\Claude), then add:

{
  "mcpServers": {
    "nexus": {
      "command": "C:\\Users\\<you>\\.local\\bin\\uvx.exe", // or plain "uvx" on macOS/Linux
      "args": ["qnexus-mcp==0.2.0", "--toolsets", "read,execute"]
    }
  }
}

Then quit Claude Desktop fully and reopen — the config is only read at cold start, and closing the window leaves the old process running (tray icon → Quit, or taskkill /F /IM claude.exe).

VS Code

Command Palette → MCP: Open User Configuration (or a workspace .vscode/mcp.json):

{
  "servers": {
    "nexus": {
      "type": "stdio",
      "command": "C:\\Users\\<you>\\.local\\bin\\uvx.exe", // or plain "uvx" on macOS/Linux
      "args": ["qnexus-mcp==0.2.0", "--toolsets", "read,execute"]
    }
  }
}

Claude Code

claude mcp add nexus -- uvx qnexus-mcp==0.2.0 --toolsets read,execute

Other MCP clients (Cursor, Codex, …)

Same command; the config shape is one of the two JSON forms above (mcpServers vs servers + "type": "stdio") — check your client's docs for which file to put it in.

Something not working? See docs/TROUBLESHOOTING.md — it covers every failure mode observed in real client setups (wrong config path, slow first start, auth, rate limits, known Nexus-side errors).

Configuration

... View full README on GitHub