@qwen-code/node-repl-mcp

A standalone Model Context Protocol server that exposes a session-persistent Node.js REPL as three MCP tools. It runs a real Node.js kernel in a dedicated child process; top-level bindings, closures, and module state persist across calls within a session.

This package is fully independent of @qwen-code/qwen-code-core — any MCP client (Qwen Code via mcpServers, Claude, Codex, etc.) can run it.

Tools

Tool	Description
`node_repl`	Execute JavaScript. `{ code, timeout_ms?, title? }`. Bindings persist across calls; top-level `await` supported.
`node_repl_reset`	Terminate the kernel process and discard all bindings/module state.
`node_repl_add_node_module_dir`	Register an extra `node_modules` directory for bare-package resolution.

Cell semantics

Explicit output only: nodeRepl.write(value) for text, nodeRepl.emitImage(png|jpeg|webp) for images; console.* is captured. Plain expression results are not returned.
nodeRepl.cwd / homeDir / tmpDir and nodeRepl.getHeapStatus() are available.
Top-level static import is not allowed — use dynamic await import().
Bare packages resolve from the session cwd node_modules plus any directory registered via node_repl_add_node_module_dir; package entrypoints use Node singleton caching. Local .js/.mjs reload on each execution.
Node builtins are importable except process/node:process. Use (await import('node:module')).createRequire(import.meta.url) for CommonJS or native (N-API) addons.
Timeout, cancellation, node_repl_reset, or a crash replaces the kernel process and discards all bindings.

Isolation note: the VM context provides lifecycle/namespace isolation, not an OS security sandbox. Imported packages and builtins run with ordinary Node.js authority and inherit the parent environment. Grant this server only in trusted contexts.

Usage

Once published, the packaged bin is the simplest entry point:

jsonc

// qwen-code settings.json (or any MCP client)
{
  "mcpServers": {
    "node-repl": {
      "command": "npx",
      "args": ["-y", "@qwen-code/node-repl-mcp"],
      "cwd": "/your/workspace",
      "env": { "QWEN_NODE_REPL_ROOTS": "/extra/node_modules/parent" },
    },
  },
}

For local development against a build in this repo, point at dist/index.js:

jsonc

{
  "mcpServers": {
    "node-repl": {
      "command": "node",
      "args": ["/path/to/packages/node-repl/dist/index.js"],
      "cwd": "/your/workspace",
    },
  },
}

The cwd you set is the kernel's working directory and the base for bare-package resolution (<cwd>/node_modules).

Environment:

QWEN_NODE_REPL_ROOTS — extra readable roots (path-list, OS delimiter).
QWEN_NODE_REPL_DEBUG — set truthy for stderr debug logging.

Build & test

bash

npm run build         # tsc (tsconfig.build.json) + copy runtime assets into dist/runtime
npm run typecheck     # includes the test files
npm test              # vitest: kernel integration, N-API, 100-cell + 10-kernel scale,
                      # line fidelity, hoisting semantics, MCP surface
npm run smoke           # kernel manager + output adapter, in-process
npm run smoke:mcp       # real MCP client <-> built stdio server
npm run smoke:lifecycle # proves the kernel child is reaped on stdin EOF / signals

Run vitest from inside this package. A bare npx vitest run from the repo root executes every workspace project (tens of thousands of tests) and can exhaust the default heap.

Provenance

Ported from the Qwen Code PR #9499 Node REPL core (kernel, module loader, cell transform, protocol, security policy, kernel manager). The qwen-coupled result converter was replaced by output-adapter.ts, which emits MCP content blocks. The empty-in-production trusted-package / sha256-pinning layer was removed entirely: this runtime has no trusted-package or capability mechanism.