MCP Language Server (TypeScript)

A Model Context Protocol (MCP) server that wraps Language Server Protocol (LSP) servers, making language server capabilities available to LLMs through MCP.

Overview

This project bridges two important protocols:

• LSP (Language Server Protocol): Provides semantic understanding of code (definitions, references, diagnostics, etc.)
• MCP (Model Context Protocol): Makes these capabilities accessible to LLMs

By wrapping LSP servers with MCP, LLMs can gain deep semantic understanding of codebases, enabling them to navigate, analyze, and refactor code more effectively.

Architecture

High-Level Structure

src/
├── index.ts              # Main entry point and MCP server setup
├── logging/              # Logging infrastructure
│   └── logger.ts         # Component-based logging system
├── protocol/             # LSP protocol types
│   ├── types.ts          # Type definitions and wrappers
│   └── uri.ts            # URI utilities
├── lsp/                  # LSP client implementation
│   ├── client.ts         # LSP client and process management
│   ├── transport.ts      # JSON-RPC message transport
│   └── methods.ts        # LSP method wrappers
├── watcher/              # File system watching
│   ├── watcher.ts        # Workspace file watcher
│   └── gitignore.ts      # Gitignore pattern matching
└── tools/                # MCP tool implementations
    ├── utilities.ts      # Shared utility functions
    ├── definition.ts     # Get symbol definitions
    ├── references.ts     # Find symbol references
    ├── hover.ts          # Get hover information
    ├── diagnostics.ts    # Get diagnostics (errors/warnings)
    ├── edit.ts           # Apply text edits
    └── rename.ts         # Rename symbols

Component Details

1. Logging System (logging/)

Purpose: Provides structured, component-based logging with configurable levels.

Key Features:

• Component-based filtering (Core, LSP, Wire, LSP Process, Watcher, Tools)
• Configurable log levels (DEBUG, INFO, WARN, ERROR, FATAL)
• Environment variable configuration (LOG_LEVEL, LOG_COMPONENT_LEVELS, LOG_FILE)

Example:

const logger = createLogger(Component.LSP);
logger.debug('Processing request: %s', requestId);
logger.error('Failed to initialize: %s', err);

2. Protocol Layer (protocol/)

Purpose: Type definitions and utilities for LSP protocol.

Components:

• types.ts: Re-exports VSCode LSP types and provides wrapper interfaces
• uri.ts: URI conversion utilities (pathToUri, uriToPath)

Key Abstractions:

• ISymbol: Unified interface for SymbolInformation and WorkspaceSymbol
• SymbolKindNames: Human-readable names for symbol kinds
• FileChangeType, WatchKind: File watching enums

3. LSP Client (lsp/)

Purpose: Manages communication with LSP server processes.

Components:

transport.ts - JSON-RPC Message Transport

• Implements LSP message framing (Content-Length headers)
• Handles message serialization/deserialization
• Supports both requests and notifications

Message Flow:

Client Request → JSON-RPC → LSP Server
              ← Response ←

client.ts - LSP Client Implementation

• Spawns and manages LSP server process
• Handles three types of communication:
  1. Client → Server requests: Tool calls (definition, hover, etc.)
  2. Server → Client requests: Capability registration, workspace edits
  3. Server → Client notifications: Diagnostics, messages

Key Methods:

• initialize(): Initialize LSP server with workspace configuration
• openFile(): Open files for analysis
• call(): Make LSP requests (with response)
• notify(): Send LSP notifications (no response)
• registerServerRequestHandler(): Handle server-initiated requests
• registerNotificationHandler(): Handle server notifications

Process Management:

• Automatic process cleanup on exit
• Graceful shutdown with timeout and fallback kill
• stderr capture for logging

methods.ts - LSP Method Wrappers

Provides typed wrappers for common LSP methods:

• symbol(): Workspace symbol search
• references(): Find all references
• hover(): Get hover information
• rename(): Rename symbol
• definition(): Go to definition

4. File Watcher (watcher/)

Purpose: Monitor workspace files and sync changes with LSP server.

Components:

gitignore.ts - Gitignore Matching

• Loads and parses .gitignore files
• Determines if files/directories should be ignored
• Uses ignore library for pattern matching

watcher.ts - Workspace Watcher

Responsibilities:

1. File System Monitoring: Uses chokidar to watch for file changes
2. Pattern Matching: Supports LSP glob patterns (**/*.ts, *.{js,ts})
3. Smart Filtering: Excludes node_modules, .git, build artifacts
4. Debouncing: Reduces notification spam
5. File Opening: Automatically opens files matching registered patterns

Event Flow:

File Change → Filter → Debounce → Notify LSP Server
                                → Open/Close Files

Configuration:

• excludedDirs: Directories to skip (e.g., node_modules)
• excludedFileExtensions: File types to ignore (e.g., .pyc)
• maxFileSize: Skip large binary files
• debounceTime: Delay before sending notifications

5. Tools (tools/)

Purpose: Implement MCP tools that expose LSP capabilities.

utilities.ts - Shared Utilities

• addLineNumbers(): Format code with line numbers
• getFullDefinition(): Expand definition to include comments/body
• getLineRangesToDisplay(): Calculate context ranges
• formatLinesWithRanges(): Format output with line ranges

Tool Implementations

Each tool follows a similar pattern:

1. Parse and validate inputs
2. Open necessary files in LSP
3. Call appropriate LSP methods
4. Format results for display
5. Return formatted text

definition.ts - Symbol Definition Lookup

readDefinition(client, 'MyClass.myMethod')
→ Searches workspace symbols
→ Opens files containing matches
→ Expands definition range (includes comments)
→ Returns formatted code with location info

references.ts - Find All References

findReferences(client, 'myFunction')
→ Finds symbol via workspace/symbol
→ Calls textDocument/references
→ Groups by file
→ Shows context around each reference

hover.ts - Get Hover Information

getHoverInfo(client, 'file.ts', 10, 5)
→ Calls textDocument/hover
→ Formats hover content (markdown/plaintext)
→ Returns type info and documentation

diagnostics.ts - Get Diagnostics

getDiagnosticsForFile(client, 'file.ts')
→ Opens file
→ Retrieves cached diagnostics
→ Groups by severity
→ Shows context around each diagnostic

edit.ts - Apply Text Edits

applyTextEdits(client, 'file.ts', [
  { startLine: 10, endLine: 12, newText: 'new code' }
])
→ Opens file
→ Converts to LSP TextEdit format
→ Applies edits from bottom to top
→ Writes back to filesystem

rename.ts - Rename Symbol

renameSymbol(client, 'file.ts', 10, 5, 'newName')
→ Calls textDocument/rename
→ Receives WorkspaceEdit
→ Applies changes across all files
→ Returns summary of changes

6. Main Server (index.ts)

Purpose: Orchestrates all components and exposes MCP tools.

Lifecycle:

1. Parse Configuration: Command-line arguments (workspace, LSP command)
2. Initialize LSP: Start LSP server process, send initialize request
3. Start Watcher: Monitor workspace files
4. Register Tools: Define available MCP tools
5. Start MCP Server: Listen on stdio for MCP requests
6. Handle Requests: Route tool calls to appropriate handlers
7. Graceful Shutdown: Close files, shutdown LSP, cleanup

Configuration Options:

mcp-language-server \
  --workspace /path/to/project \
  --lsp typescript-language-server \
  -- --stdio

• --workspace: Project directory
• --lsp: LSP server command
• --: Arguments after this are passed to LSP server

Data Flow

Tool Call Flow

MCP Client (LLM)
    ↓ (stdio)
MCP Server (index.ts)
    ↓
Tool Handler (tools/definition.ts)
    ↓
LSP Client (lsp/client.ts)
    ↓ (JSON-RPC over stdio)
LSP Server Process (e.g., typescript-language-server)
    ↓
File System / Code Analysis
    ↑
Results flow back up the chain

File Change Flow

File System Change
    ↓
chokidar (watcher.ts)
    ↓
Filter & Debounce
    ↓
LSP Client
    ↓ workspace/didChangeWatchedFiles
LSP Server
    ↓
Diagnostics Published
    ↓
Cached in LSP Client
    ↓
Available to Tools

Key Design Patterns

1. Layered Architecture

• Clear separation between transport, protocol, and tool layers
• Each layer has well-defined responsibilities
• Abstractions allow for testing and modification

2. Event-Driven Communication

• LSP uses bidirectional JSON-RPC
• Server can send requests/notifications to client
• Handlers registered for different message types

3. Resource Management

• Files opened/closed explicitly
• Process cleanup on shutdown
• Timeout-based fallbacks for cleanup

4. Debouncing & Caching

• File change events debounced to reduce noise
• Diagnostics cached in client
• Open file state tracked

5. Error Handling

• Errors propagated with context
• Logging at appropriate levels
• Graceful degradation where possible

Configuration

Environment Variables

• LOG_LEVEL: Set global log level (DEBUG, INFO, WARN, ERROR, FATAL)
• LOG_COMPONENT_LEVELS: Set per-component levels (e.g., lsp:DEBUG,tools:INFO)
• LOG_FILE: Write logs to file in addition to stderr
• LSP_CONTEXT_LINES: Lines of context for references (default: 5)

Example: Debug Mode

export LOG_LEVEL=DEBUG
mcp-language-server --workspace . --lsp gopls

Testing

Unit Tests

Focus on individual components:

• Logging configuration
• URI conversion
• Message framing/parsing
• Pattern matching
• Text edit application

Integration Tests

Test end-to-end flows:

• LSP initialization
• File opening/closing
• Tool execution
• Workspace watching

Extension Points

The codebase is designed for extensibility:

1. New Tools: Add files to tools/ and register in index.ts
2. Custom Filters: Modify watcher configuration
3. LSP Methods: Add wrappers in lsp/methods.ts
4. Custom Logging: Add new components or log sinks

Comparison with Go Version

This TypeScript implementation maintains architectural parity with the Go version while leveraging TypeScript/Node.js ecosystems:

Aspect	Go Version	TypeScript Version
LSP Types	Generated from gopls	VSCode LSP types
File Watching	fsnotify	chokidar
Gitignore	go-gitignore	ignore
Concurrency	Goroutines & channels	Async/await & Promises
Process Management	os/exec	child_process
MCP SDK	mcp-go	@modelcontextprotocol/sdk

Dependencies

• @modelcontextprotocol/sdk: MCP protocol implementation
• vscode-languageserver-protocol: LSP type definitions
• chokidar: File system watcher
• ignore: Gitignore pattern matching

Building and Running

# Install dependencies
npm install

# Build
npm run build

# Run
node dist/index.js --workspace /path/to/project --lsp typescript-language-server -- --stdio

License

BSD-3-Clause (same as original Go implementation)