Just MCP - MCP Server

just-mcp

👋 A way to let LLMs speak Just

A production-ready MCP server that provides seamless integration with Just command runner, enabling AI assistants to discover, execute, and introspect Justfile recipes through the standardized MCP protocol.

🎯 Why Just + MCP = Better Agent Execution

Context-Saving Abstraction

If it isn't immediately obvious, the benefit of having LLMs use Just vs. bash is that running Just commands (via MCP) provides a context-saving abstraction where they don't need to waste context opening/reading bash files, Python scripts, or other build artifacts. The LLM via MCP simply gets the command, parameters, and hints - it's in their memory as "these are commands available to you."

Eliminates the Justfile Learning Curve

No more watching LLMs execute just -l to get command lists, inevitably start reading the justfile, then try to write justfile syntax (like it's a Makefile), corrupt the justfile, and create a bad experience. Just's evolving syntax simply doesn't have a large enough corpus in frontier models today - we need more popular repos with justfiles in the training dataset.

Safer Than Raw Bash Access

Just-mcp is fundamentally safer than bash. If you read HackerNews, there's a story at least once daily about operators whose LLMs start forgetting, hallucinating, and eventually breaking down - deleting files and doing nasty unwanted things. Giving LLMs unsupervised, unrestricted bash access without carefully monitoring context consumption is a recipe for disaster.

Using Justfile fixes that. Even if the LLM modifies its own justfile, the next context is memoized by the justfile (hopefully in an idempotent git repo). This abstraction shields the llm from the command line complexity where hallucinations or attention tracking the current working directory cause it to go over the rails and off the cliff.

Powerful Agent Execution Tool

Just-mcp is perfect for anybody doing agent execution:

Ultra-low overhead - probably better than every other tool
Human-friendly - justfiles are easy for humans and low overhead for LLMs
Quick and dirty - while some prefer full Python FastAPI servers, just-mcp is just easy-as
sm0l model friendly - works great with self-hostable GPU/CPU open source models with 8k-32k context limits

Built-in Safety Patterns

Just has useful patterns for introducing:

Transparent logging without distracting the agent
Secondary model inspection - use sm0l models to scan commands asking "is this harmful?" before execution
Python decorator-like patterns for command validation
Idempotent execution backed by git repos

b00t

b00t mcp create just-mcp -- bash just-mcp --stdio "${REPO_ROOT}"
b00t mcp export just-mcp

🚀 Current Status: 67% Complete (8/12 core tasks)

✅ Implemented Features

🏗️ Complete MCP Server - Full rmcp 0.3.0 integration with MCP 2024-11-05 protocol
📋 Recipe Discovery - Parse and list all available Justfile recipes
⚡ Recipe Execution - Execute recipes with parameters and capture structured output
🔍 Recipe Introspection - Get detailed recipe information, parameters, and documentation
✅ Justfile Validation - Syntax and semantic validation with error reporting
🌍 Environment Management - Comprehensive .env file support and variable expansion
🧪 Full Test Coverage - 33 passing tests across integration and unit test suites

🎯 MCP Tools Available

list_recipes - List all available recipes in the justfile
run_recipe - Execute a specific recipe with optional arguments
get_recipe_info - Get detailed information about a specific recipe
validate_justfile - Validate the justfile for syntax and semantic errors

🏃 Quick Start

Installation & Setup

# Clone and build
git clone <repository-url>
cd just-mcp
cargo build --release

# Test the server
cargo run -- --stdio

Claude Desktop Integration

Add to your Claude Desktop MCP configuration:

{
  "mcpServers": {
    "just-mcp": {
      "command": "/path/to/just-mcp",
      "args": ["--stdio"]
    }
  }
}

Usage Examples

# Run as MCP server
just-mcp --stdio

# Run in specific directory  
just-mcp --directory /path/to/project --stdio

🧪 Testing

Comprehensive Test Suite

# Run all tests (33 tests)
cargo test

# Run specific test suites
cargo test --test basic_mcp_test      # Protocol compliance testing
cargo test --test mcp_integration_working  # SDK integration testing

Test Architecture

basic_mcp_test.rs - Direct protocol compliance testing using raw JSON-RPC
mcp_integration_working.rs - Type-safe SDK integration testing with rmcp client
Unit tests - 25+ tests covering parser, executor, validator, and environment modules

📚 Architecture

Project Structure

just-mcp/
├── src/main.rs              # CLI binary
├── just-mcp-lib/           # Core library
│   ├── parser.rs           # Justfile parsing
│   ├── executor.rs         # Recipe execution  
│   ├── validator.rs        # Validation logic
│   ├── environment.rs      # Environment management
│   └── mcp_server.rs       # MCP protocol implementation
├── tests/                  # Integration tests
└── justfile               # Demo recipes

Tech Stack

Rust 1.82+ with async/await support
rmcp 0.3.0 - Official MCP SDK for Rust
serde/serde_json - JSON serialization
snafu - Structured error handling
tokio - Async runtime

🔄 Development Roadmap

🎯 Next Priority Tasks (Remaining 33%)

LSP-Style Completion System - Intelligent autocompletion for recipes and parameters
Enhanced Diagnostics - Advanced syntax error reporting and suggestions
Virtual File System - Support for stdin, remote sources, and in-memory buffers
Release Preparation - Documentation, CI/CD, and crate publication

🚀 Future Enhancements

Plugin system for custom recipe types
Integration with other build tools
Performance optimizations for large justfiles
Advanced dependency visualization

📖 Usage Patterns

Recipe Execution

// List available recipes
await client.callTool("list_recipes", {});

// Execute recipe with parameters  
await client.callTool("run_recipe", {
  "recipe_name": "build",
  "args": "[\"--release\"]"
});

// Get recipe information
await client.callTool("get_recipe_info", {
  "recipe_name": "test"
});

Validation

// Validate justfile
await client.callTool("validate_justfile", {
  "justfile_path": "./custom.justfile"  
});

🤝 Contributing

This project follows the b00t development methodology:

TDD Approach - Tests first, implementation second
Feature Branches - Never work directly on main branch
Structured Errors - Use snafu for error management
Git Workflow - Clean commits with descriptive messages

Development Commands

just build    # Build the project
just test     # Run tests  
just server   # Start MCP server
just clean    # Clean build artifacts

📄 License

This project is licensed under LICENSE.

🚀 Release Setup & CI/CD

✅ Completed Setup

Cocogitto & Conventional Commits

Installed cocogitto for conventional commit enforcement
Configured cog.toml with proper commit types and changelog settings
Set up git hooks for commit message linting (commit-msg) and pre-push testing

GitHub Actions CI/CD

CI Pipeline (ci.yml): Multi-platform testing (Ubuntu, Windows, macOS), formatting, clippy, commit linting
Release Pipeline (release.yml): Automated versioning, changelog generation, GitHub releases, and crates.io publishing

Crates.io Preparation

Updated both Cargo.toml files with complete metadata (description, keywords, categories, license, etc.)
Added proper exclusions for development-only files
Verified MIT license is in place

Documentation & Structure

README.md is production-ready with installation and usage instructions
Created initial CHANGELOG.md for release tracking
Updated .gitignore with Rust-specific entries

🚀 Production Deployment

Development Workflow:

All commits must follow conventional commit format (enforced by git hooks)
Use feat:, fix:, docs:, etc. prefixes for automatic versioning
Push to main branch triggers automated releases and crates.io publishing
Library tests pass ✅ (25/25) with comprehensive test coverage

Release Process:

Automated Versioning: Cocogitto analyzes commit messages for semantic versioning
GitHub Releases: Automatic changelog generation and GitHub release creation
Crates.io Publishing: Library crate (just-mcp-lib) publishes first, then binary crate (just-mcp)
CI/CD Pipeline: Multi-platform testing (Ubuntu, Windows, macOS) with formatting and clippy checks

Installation:

# Install from crates.io
cargo install just-mcp

# Or download from GitHub releases
wget https://github.com/promptexecution/just-mcp/releases/latest/download/just-mcp

🔗 Related Projects

Just - The command runner this integrates with
Model Context Protocol - The protocol specification
rmcp - Official Rust MCP SDK

Friends of just-mcp

just-vscode - VSCode extension with LSP integration for enhanced Just authoring
just-awesome-agents - Collection of patterns and tools for agent execution with Just# Test change to trigger pre-push hook