RunJS - A .NET MCP Server to Run JS Using Jint

This project contains an MCP server that can execute JavaScript in an isolated sandbox and return a result from the script.

This is extremely powerful as in many cases, you may want to run JavaScript, but it may be difficult to deploy it into live infrastructure (e.g. deploy a serverless function for each piece of code).

Jint is a C# library that embeds a JavaScript runtime into .NET and allows controlling the execution sandbox by specifying:

• Memory limits
• Number of statements
• Runtime
• Depth of calls (recursion)

This makes it easy to generate and run JavaScript dynamically within your prompt as a tool

Project Setup

The project is set up in the following structure:

📁 app
  📁 src                      # The client application using Vercel AI SDK
  .env                        # 👈 Make your own from the .env.sample
  .env.sample                 # Sample .envfile; make a copy as .env
📁 server
  Program.cs                  # A .NET MCP server exposing the Jint tool
builder-server.sh             # Simple script (command) to build the container
docker-compose.yaml           # Start the Aspire Dashboard container for OTEL
Dockerfile                    # Dockerfile for the .NET server app

Configuring Your Local Environment

You'll need to Install the .NET SDK if you don't have it. These are available for Windows, Linux, and macOS.

Once installed, you can run the following command to start the server:

dotnet run --project server

This starts the MCP server on port 5000. To use this with the LLM API call, you will need to expose it to OpenAI using a proxy like ngrok or the VS Code Ports tool.

👉 Be sure to set the port as public

Once you've done this, you will need to create a copy of the .env.sample file and as .env and set your OpenAI API key and the URL:

OPENAI_API_KEY=sk-proj-kSZWV-M7.......K_MMv8JZRmIA
MCP_ENDPOINT=https://mhjt5hqd-5000.use.devtunnels.ms/sse

From the /app directory, run the following:

cd app
npm i
npm run app -- "Use your echo tool wih my name: <YOUR_NAME_HERE>; write out your response"

This should invoke the .NET MCP endpoint and output your name!

Security

🚨 THERE IS CURRENTLY NO AUTH 🚨

See the workstream here

Running the Server

# To start the server
dotnet run --project server

# To start the server with hot reload
dotnet watch run --project server --non-interactive

Running the Client

cd app
npm run app -- "My prompt goes here"

To test this, you can run two types of prompts:

cd app

# Just test the echo
npm run app -- "Echo my name back to me: Charles"

# Generate and execute JavaScript
npm run app -- "Generate some JavaScript that will lowercase and return the string 'Hello, World' and execute it.  Give me the results; ONLY THE RESULTS"

If you run the following:

docker compose up

You will also get the Aspire Dashboard at http://localhost:18888 to trace the internal calls to the tools.