Just an MCP Command Runner

A Model Context Protocol (MCP) server that exposes Just recipes as MCP tools, allowing AI assistants to discover and execute project commands defined in Justfiles.

Quick Start

Install and run the MCP server with npx:

# Install just command runner if not already installed
# See https://github.com/casey/just#installation

# Run the MCP server pointing to a directory with a Justfile
npx just-runner-mcp --justfile ./path/to/your/justfile

# Or run from current directory (will search for Justfile)
npx just-runner-mcp

# Test via MCP inspector web UI
npx -y @modelcontextprotocol/inspector -- npx -y just-runner-mcp --justfile tests/support/basic.justfile

MCP Client Configuration

Add to your MCP client configuration (e.g., Claude Desktop):

{
  "mcpServers": {
    "just-runner": {
      "command": "npx",
      "args": ["just-runner-mcp", "--justfile", "/path/to/your/project"]
    }
  }
}

Why Just?

Just is a command runner that saves and runs project-specific commands in a justfile. It's an effective choice for exposing commands/scripts to AI assistants because:

• Simple syntax: Justfiles use a clean, make-inspired syntax that's easy to read and write
• Cross-platform: Works on Linux, macOS, Windows, and other Unix-like systems
• Language agnostic: Recipes can be written in any language (bash, python, etc.)
• Self-documenting: Recipes can include documentation comments that describe what they do
• Parameterized: Recipes can accept arguments, making them flexible and reusable
• Dependency management: Recipes can depend on other recipes, ensuring proper execution order

Tool Generation

The MCP server automatically parses Justfiles and generates MCP tools for each recipe. Each tool is named after the recipe and includes:

• Name: The recipe name from the justfile
• Description: Extracted from documentation comments (lines starting with # above the recipe)
• Parameters: Automatically detected recipe parameters with their default values and types

For example, this justfile recipe:

# Compile Latex Document. FILEPATH must be an absolute path
compile-latex FILEPATH:
    tectonic {{FILEPATH}}

# Compile Typst Document. FILEPATH must be an absolute path
compile-typst FILEPATH:
    typst compile --format pdf {{FILEPATH}}
    typst compile --format png {{FILEPATH}} "page-{0p}-of-{t}.png"

Would expose the following tools to the MCP client:

{
  "tools": [
    {
      "name": "compile-latex",
      "description": "Run just recipe: compile-latex\nCompile Latex Document. FILEPATH must be an absolute path",
      "inputSchema": {
        "type": "object",
        "properties": {
          "FILEPATH": {
            "type": "string",
            "description": "Required"
          }
        },
        "required": ["FILEPATH"]
      }
    },
    {
      "name": "compile-typst",
      "description": "Run just recipe: compile-typst\nCompile Typst Document. FILEPATH must be an absolute path",
      "inputSchema": {
        "type": "object",
        "properties": {
          "FILEPATH": {
            "type": "string",
            "description": "Required"
          }
        },
        "required": ["FILEPATH"]
      }
    }
  ]
}

CLI Options

The MCP server supports several command-line options:

Usage: just-mcp [options]

Expose Justfile recipes as MCP tools

Options:
  --justfile <path>     Path to Justfile (default: "Justfile")
  --just-binary <path>  Path to just binary (default: "just")
  --timeout <ms>        Timeout for recipe execution in milliseconds (default: 30000)
  --list-tools          Print available tools and exit
  --help                Show help message
  -h                    display help for command

Examples

This section demonstrates a practical Justfile for a development project and the MCP tools it generates. Each recipe becomes a tool that AI assistants can discover and execute.

Simple Development Justfile

Here's a justfile for a typical development workflow:

# Install project dependencies
install:
    npm install

# Start the development server
dev port="3000":
    echo "Starting dev server on port {{port}}"
    npm run dev -- --port {{port}}

# Run all tests
test:
    npm test

# Deploy to staging environment
deploy-staging: build test
    echo "Deploying to staging..."
    ./scripts/deploy.sh staging

MCP Tools Output

When you run just-runner-mcp --list-tools, you get the following JSON response showing all available tools:

{
  "tools": [
    {
      "name": "install",
      "description": "Run just recipe: install\nInstall project dependencies",
      "inputSchema": {
        "type": "object",
        "properties": {},
        "required": []
      }
    },
    {
      "name": "dev",
      "description": "Run just recipe: dev\nStart the development server",
      "inputSchema": {
        "type": "object",
        "properties": {
          "port": {
            "type": "string",
            "description": "Parameter: port (default: 3000)"
          }
        },
        "required": []
      }
    },
    {
      "name": "test",
      "description": "Run just recipe: test\n",
      "inputSchema": {
        "type": "object",
        "properties": {},
        "required": []
      }
    },
    {
      "name": "deploy-staging",
      "description": "Run just recipe: deploy-staging\nDeploy to staging environment",
      "inputSchema": {
        "type": "object",
        "properties": {},
        "required": []
      }
    }
  ]
}

With these tools available, an AI assistant can help you with common development tasks like "install the dependencies", "start the dev server on port 8080", "run the tests", or "deploy to staging".

Alternatives

There are other approaches to integrating Just with MCP:

• just-mcp - A Rust-based MCP server that provides more comprehensive Justfile support and handling. This cargo package offers additional features beyond just exposing recipes as tools to the LLM.

This repository (just-runner-mcp) takes a minimal approach, focusing specifically on exposing Just recipes as MCP tools for AI assistants to discover and execute.

Development

# Run all tests
just test

# Run specific test file
just test tests/integration.test.ts

License

This project is licensed under the MIT License - see the LICENSE file for details.