code-runner

name	code-runner
description	Use this skill to execute JavaScript/TypeScript or Python code with any npm, JSR, or PyPI package — no local installation needed. Triggers on: "run code", "execute", "try package X", "test this snippet", "verify this logic".

code-runner-mcp Skill

This skill helps you use the code-runner-mcp tools and CLI to execute JavaScript/TypeScript (via Deno) or Python (via Pyodide/WASM) code in a secure, isolated sandbox with on-demand package installation.

Available Tools

Two MCP tools are registered when the server is running:

Tool	Language	Runtime
javascript-code-runner	JS / TS	Deno (subprocess)
python-code-runner	Python	Pyodide (WASM)

Both tools return combined stdout + stderr as text.

CLI Usage

No Deno?

• Install: npm i -g deno
• Or skip install entirely: replace deno run --allow-all with npx -y deno run --allow-all

Silent mode — suppress Deno's download/progress output:

deno run --allow-all --quiet jsr:@mcpc/code-runner-mcp/cli js "console.log('hello')"

# Inline code
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli js "console.log('hello')"
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py "print(1 + 2)"

# From a file
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli js --file ./script.ts
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py --file ./analysis.py

# Piped from stdin
echo "print('piped')" | deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py
cat app.ts | deno run --allow-all jsr:@mcpc/code-runner-mcp/cli js

# Python with package mapping
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py --file analysis.py \
  --packages '{"sklearn":"scikit-learn","PIL":"Pillow"}'

# Python with host filesystem mount
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py --file script.py \
  --nodefs-root /tmp --nodefs-mount /tmp

CLI Flags

Flag	Alias	Description
--file <path>	-f	Read code from a file
--packages <json>	-p	Python: map import→PyPI names (JSON)
--nodefs-root <dir>		Python: host dir to mount
--nodefs-mount <dir>		Python: mount point in Pyodide FS
--help	-h	Show help

JavaScript / TypeScript

Key rules

• Use console.log() to produce output — no output = (no output) returned
• Import npm packages: import { x } from "npm:package-name"
• Import JSR packages: import { y } from "jsr:@scope/pkg"
• Node built-ins: import fs from "node:fs"
• Code runs server-side in Deno, not in a browser
• File system access is limited to the temp cwd directory unless DENO_PERMISSION_ARGS=--allow-all is set

Example

import { chunk } from "npm:lodash-es";
import { z } from "npm:zod";

const schema = z.object({ name: z.string(), age: z.number() });
const result = schema.safeParse({ name: "Alice", age: 30 });
console.log(result.success); // true
console.log(chunk([1, 2, 3, 4, 5], 2));

Python

Key rules

• Use print() to produce output
• Packages are auto-installed via micropip — just import them
• When the import name differs from the PyPI name, pass the packages map: {"sklearn": "scikit-learn", "PIL": "Pillow", "cv2": "opencv-python"}
• Runs in WASM (Pyodide) — not a real CPython; some C-extension packages won't be available
• Timeout: 3 minutes (heavy imports like pandas can take a moment on first run)

Example

import pandas as pd
import numpy as np

data = {"x": np.arange(5), "y": np.random.rand(5)}
df = pd.DataFrame(data)
print(df.describe())

Packages map examples

Import	PyPI name
sklearn	scikit-learn
PIL	Pillow
cv2	opencv-python
bs4	beautifulsoup4
yaml	PyYAML

Environment Variables

Set these when starting the MCP server or before running the CLI:

Variable	Effect
ALLOWED_TOOLS	all / python / javascript / js
DENO_PERMISSION_ARGS	Extra Deno permissions, e.g. --allow-net or --allow-all
NODEFS_ROOT	Host directory to expose to Python
NODEFS_MOUNT_POINT	Where to mount it in Pyodide FS
PYODIDE_PACKAGE_CACHE_DIR	Custom cache dir for Pyodide packages
PYODIDE_PACKAGE_BASE_URL	Custom CDN mirror for Pyodide packages
DEBUG	Set to any value to enable verbose runner logs

Gotchas

JavaScript

• No output? Always add console.log(). Bare expressions don't print.
• Permission denied? JS code can only write to the runner's temp cwd unless DENO_PERMISSION_ARGS=--allow-all is set.
• --allow-all conflicts with other --allow-* flags — set only one mode.
• Imports must be ESM — require() is not supported; use import.
• TypeScript types are erased at runtime; type errors won't block execution.

Python

• No output? Always use print(). Expression results are not echoed.
• Package not found? Use the packages map when import name ≠ PyPI name.
• Syntax errors? Avoid triple-quoted strings when sending code as JSON. Use single quotes or string concatenation instead.
• Slow first run? Pyodide and packages are downloaded/cached on first use. Subsequent runs in the same session are fast.
• C extensions unavailable — packages like torch or lxml that require compiled C code may not work in Pyodide.
• File access — only accessible if NODEFS_ROOT is configured.
• await at top-level is supported (runPythonAsync is used internally).

CLI specific

• Inline code with special shell characters should be single-quoted: deno task cli py 'print(f"value: {1+1}")'
• The --packages flag expects valid JSON — double-quote all keys/values.
• Piped stdin is only read when stdin is not a TTY (i.e. when actually piped), so interactive use won't hang waiting for input.

Quick Reference

# JS: test an npm package
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli js 'import { format } from "npm:date-fns"; console.log(format(new Date(), "yyyy-MM-dd"))'

# Python: data analysis
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py 'import pandas as pd; df = pd.DataFrame({"a":[1,2,3]}); print(df.sum())'

# Python: package with name mismatch
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli py --packages '{"sklearn":"scikit-learn"}' --file model.py

# JS: read/write a file in sandbox
deno run --allow-all jsr:@mcpc/code-runner-mcp/cli js 'import fs from "node:fs"; import os from "node:os"; const p = os.tmpdir()+"/.deno_runner_tmp/out.txt"; fs.writeFileSync(p,"hi"); console.log(fs.readFileSync(p,"utf8"))'