MCP Server: Expose eg library through Model Context Protocol

[nikomatsakis]

Overview

Create an MCP (Model Context Protocol) server that exposes the eg library functionality to LLMs and AI assistants.

MCP Tools to Implement

1. search_crate_examples

Search for patterns in Rust crate examples and source code.

2. get_crate_source

Get the full path to an extracted crate for detailed exploration.

3. explore_crate_usage

Find how specific APIs are used throughout a crate's codebase.

Benefits for LLMs

• On-demand example discovery: 'Show me how to use tokio::spawn'
• Real-world usage patterns: See how APIs are actually used in practice
• Full source context: Access to complete crate source, not just examples
• Version-specific help: Get examples for specific crate versions
• Pattern exploration: 'Find all uses of async/await in this crate'

Implementation Plan

Phase 1: Basic MCP Server

• Set up MCP server framework
• Implement search_crate_examples tool
• Basic error handling and response formatting
• Integration testing with MCP client

Phase 2: Enhanced Tools

• Implement get_crate_source tool
• Add explore_crate_usage tool
• Rich response formatting with code snippets
• Performance optimization

Phase 3: Advanced Features

• Streaming responses for large results
• Caching and session management
• Configuration options
• Comprehensive documentation

This will make the eg library accessible to the entire MCP ecosystem!

[nikomatsakis]

Phase 1 Complete ✅

Successfully implemented the basic MCP server with the following achievements:

✅ Set up MCP server framework

• Added binary target to Cargo.toml
• Integrated official Rust MCP SDK (rmcp)
• Configured feature flags for optional MCP dependencies
• Set up proper async/stdio transport

✅ Implement search_crate_examples tool

• Tool: search_crate_examples
• Parameters: crate_name (required), pattern (optional regex)
• Functionality: Searches for patterns in Rust crate examples and source code
• Output: Formatted markdown with code snippets, file paths, and context

✅ Basic error handling and response formatting

• Proper JSON-RPC error responses with structured error data
• Input validation for regex patterns
• Graceful handling of crate resolution failures
• Rich markdown formatting for search results

✅ Integration testing with MCP client

• Created integration test that verifies server initialization
• Tested with MCP Inspector (npx @modelcontextprotocol/inspector)
• Verified proper MCP protocol compliance
• Confirmed tool registration and capabilities

Additional Achievements

• Bonus tool: get_crate_source - Returns full path to extracted crate
• Documentation: Created README-MCP.md with usage examples
• Testing: Integration test suite with proper feature flags
• Architecture: Clean separation using feature flags for optional MCP functionality

Usage Example

# Start the server
cargo run --bin eg-mcp --features mcp

# Test with MCP Inspector
npx @modelcontextprotocol/inspector cargo run --bin eg-mcp --features mcp

The server successfully exposes eg library functionality through the Model Context Protocol, making Rust crate examples searchable by any MCP-compatible LLM or AI assistant.

Ready for Phase 2: Enhanced tools and rich response formatting.