LSP MCP Server for Rell

An MCP (Model Context Protocol) server for interacting with the Rell Language Server Protocol (LSP) interface. This server acts as a bridge that allows LLMs to query LSP Hover and Completion providers for Rell projects.

Note: This project is based on @Tritlo/lsp-mcp with slight modifications to remove all extensions and use it specifically with the default Rell language server. The Rell LSP JAR file is automatically downloaded and managed by this server.

Overview

The MCP Server works by:

1. Starting an LSP client that connects to a LSP server

2. Exposing MCP tools that send requests to the LSP server

3. Returning the results in a format that LLMs can understand and use

This enables LLMs to utilize the Rell LSP for more accurate code suggestions and analysis.

Related MCP server: VSCode LSP MCP Server

Configuration:

{
  "mcpServers": {
    "rell-lsp-mcp": {
      "type": "stdio",
      "command": "node",
      "args": [
        "/path/to/this/project/dist/index.js"
      ]
    }
  }
}

Features

MCP Tools

• get_info_on_location: Get hover information at a specific location in a file

• get_completions: Get completion suggestions at a specific location in a file

• get_code_actions: Get code actions for a specific range in a file

• open_document: Open a file in the LSP server for analysis

• close_document: Close a file in the LSP server

• get_diagnostics: Get diagnostic messages (errors, warnings) for open files

• start_lsp: Start the LSP server with a specified root directory

• restart_lsp_server: Restart the LSP server without restarting the MCP server

• set_log_level: Change the server's logging verbosity level at runtime

MCP Resources

• lsp-diagnostics:// resources for accessing diagnostic messages with real-time updates via subscriptions

• lsp-hover:// resources for retrieving hover information at specific file locations

• lsp-completions:// resources for getting code completion suggestions at specific positions

Additional Features

• Comprehensive logging system with multiple severity levels

• Colorized console output for better readability

• Runtime-configurable log level

• Detailed error handling and reporting

• Simple command-line interface

Prerequisites

• Node.js (v16 or later)

• npm

• Java JDK (for running the Rell LSP server)

Installation

Building the MCP Server

1. Clone this repository:

   git clone <-repository-url>
   cd lsp-mcp

2. Install dependencies:

   npm install

3. Build the MCP server:

   npm run build

Testing

The project includes integration tests for the Rell LSP support. These tests verify that the LSP-MCP server correctly handles LSP operations like hover information, completions, diagnostics, and code actions with the Rell language server.

Running Tests

To run the Rell LSP tests:

npm test

Test Coverage

The tests verify the following functionality:

• Automatic downloading and initialization of the Rell LSP server

• Opening Rell files for analysis

• Getting hover information for functions and types

• Getting code completion suggestions

• Getting diagnostic error messages

• Getting code actions for errors

Usage

Run the MCP server directly with Node.js:

node dist/index.js

The server automatically downloads and manages the Rell LSP server JAR file, so no additional configuration is needed. The Rell LSP server will be downloaded to ~/chromia/lsp-mcp/ on first use.

Important: Starting the LSP Server

You must explicitly start the LSP server by calling the start_lsp tool before using any LSP functionality. This ensures proper initialization with the correct root directory for your Rell project:

{
  "tool": "start_lsp",
  "arguments": {
    "root_dir": "/path/to/your/project"
  }
}

Logging

The server includes a comprehensive logging system with 8 severity levels:

• debug: Detailed information for debugging purposes

• info: General informational messages about system operation

• notice: Significant operational events

• warning: Potential issues that might need attention

• error: Error conditions that affect operation but don't halt the system

• critical: Critical conditions requiring immediate attention

• alert: System is in an unstable state

• emergency: System is unusable

By default, logs are sent to:

1. Console output with color-coding for better readability

2. MCP notifications to the client (via the notifications/message method)

Viewing Debug Logs

For detailed debugging, you can:

1. Use the claude --mcp-debug flag when running Claude to see all MCP traffic between Claude and the server:

   claude --mcp-debug

2. Change the log level at runtime using the set_log_level tool:

   {
     "tool": "set_log_level",
     "arguments": {
       "level": "debug"
     }
   }

The default log level is info, which shows moderate operational detail while filtering out verbose debug messages.

API

The server provides the following MCP tools:

get_info_on_location

Gets hover information at a specific location in a file.

Parameters:

• file_path: Path to the file

• line: Line number

• column: Column position

Example:

{
  "tool": "get_info_on_location",
  "arguments": {
    "file_path": "/path/to/your/file.rell",
    "line": 3,
    "column": 5
  }
}

get_completions

Gets completion suggestions at a specific location in a file.

Parameters:

• file_path: Path to the file

• line: Line number

• column: Column position

Example:

{
  "tool": "get_completions",
  "arguments": {
    "file_path": "/path/to/your/file.rell",
    "line": 3,
    "column": 10
  }
}

get_code_actions

Gets code actions for a specific range in a file.

Parameters:

• file_path: Path to the file

• start_line: Start line number

• start_column: Start column position

• end_line: End line number

• end_column: End column position

Example:

{
  "tool": "get_code_actions",
  "arguments": {
    "file_path": "/path/to/your/file.rell",
    "start_line": 3,
    "start_column": 5,
    "end_line": 3,
    "end_column": 10
  }
}

start_lsp

Starts the LSP server with a specified root directory. This must be called before using any other LSP-related tools.

Parameters:

• root_dir: The root directory for the LSP server (absolute path recommended)

Example:

{
  "tool": "start_lsp",
  "arguments": {
    "root_dir": "/path/to/your/project"
  }
}

restart_lsp_server

Restarts the LSP server process without restarting the MCP server. This is useful for recovering from LSP server issues or for applying changes to the LSP server configuration.

Parameters:

• root_dir: (Optional) The root directory for the LSP server. If provided, the server will be initialized with this directory after restart.

Example without root_dir (uses previously set root directory):

{
  "tool": "restart_lsp_server",
  "arguments": {}
}

Example with root_dir:

{
  "tool": "restart_lsp_server",
  "arguments": {
    "root_dir": "/path/to/your/project"
  }
}

open_document

Opens a file in the LSP server for analysis. This must be called before accessing diagnostics or performing other operations on the file.

Parameters:

• file_path: Path to the file to open

Example:

{
  "tool": "open_document",
  "arguments": {
    "file_path": "/path/to/your/file.rell",
  }
}

close_document

Closes a file in the LSP server when you're done working with it. This helps manage resources and cleanup.

Parameters:

• file_path: Path to the file to close

Example:

{
  "tool": "close_document",
  "arguments": {
    "file_path": "/path/to/your/file"
  }
}

get_diagnostics

Gets diagnostic messages (errors, warnings) for one or all open files.

Parameters:

• file_path: (Optional) Path to the file to get diagnostics for. If not provided, returns diagnostics for all open files.

Example for a specific file:

{
  "tool": "get_diagnostics",
  "arguments": {
    "file_path": "/path/to/your/file"
  }
}

Example for all open files:

{
  "tool": "get_diagnostics",
  "arguments": {}
}

set_log_level

Sets the server's logging level to control verbosity of log messages.

Parameters:

• level: The logging level to set. One of: debug, info, notice, warning, error, critical, alert, emergency.

Example:

{
  "tool": "set_log_level",
  "arguments": {
    "level": "debug"
  }
}

MCP Resources

In addition to tools, the server provides resources for accessing LSP features including diagnostics, hover information, and code completions:

Diagnostic Resources

The server exposes diagnostic information via the lsp-diagnostics:// resource scheme. These resources can be subscribed to for real-time updates when diagnostics change.

Resource URIs:

• lsp-diagnostics:// - Diagnostics for all open files

• lsp-diagnostics:///path/to/file - Diagnostics for a specific file

Important: Files must be opened using the open_document tool before diagnostics can be accessed.

Hover Information Resources

The server exposes hover information via the lsp-hover:// resource scheme. This allows you to get information about code elements at specific positions in files.

Resource URI format:

lsp-hover:///path/to/file?line={line}&column={column}&language_id={language_id}

Parameters:

• line: Line number (1-based)

• column: Column position (1-based)

Example:

lsp-hover:///home/user/project/src/main.rell?line=42&column=10&language_id=rell

Code Completion Resources

The server exposes code completion suggestions via the lsp-completions:// resource scheme. This allows you to get completion candidates at specific positions in files.

Resource URI format:

lsp-completions:///path/to/file?line={line}&column={column}&language_id={language_id}

Parameters:

• line: Line number (1-based)

• column: Column position (1-based)

Example:

lsp-completions:///home/user/project/src/main.rell?line=42&column=10

Listing Available Resources

To discover available resources, use the MCP resources/list endpoint. The response will include all available resources for currently open files, including:

• Diagnostics resources for all open files

• Hover information templates for all open files

• Code completion templates for all open files

Subscribing to Resource Updates

Diagnostic resources support subscriptions to receive real-time updates when diagnostics change (e.g., when files are modified and new errors or warnings appear). Subscribe to diagnostic resources using the MCP resources/subscribe endpoint.

Note: Hover and completion resources don't support subscriptions as they represent point-in-time queries.

Working with Resources vs. Tools

You can choose between two approaches for accessing LSP features:

1. Tool-based approach: Use the get_diagnostics, get_info_on_location, and get_completions tools for a simple, direct way to fetch information.

2. Resource-based approach: Use the lsp-diagnostics://, lsp-hover://, and lsp-completions:// resources for a more RESTful approach.

Both approaches provide the same data in the same format and enforce the same requirement that files must be opened first.

Troubleshooting

• If the server fails to start, make sure the path to the LSP executable is correct

• Check the log file (if configured) for detailed error messages

License

MIT License

Acknowledgments

• @Tritlo/lsp-mcp for the original implementation

• Chromaway for the Rell language and LSP server

• Anthropic for the Model Context Protocol specification