MCP Language Server

This is an MCP server that runs and exposes a language server to LLMs. Not a language server for MCP, whatever that would be.

Demo

mcp-language-server helps MCP enabled clients navigate codebases more easily by giving them access semantic tools like get definition, references, rename, and diagnostics.

Setup

Install Go: Follow instructions at https://golang.org/doc/install
Install or update this server: go install github.com/isaacphi/mcp-language-server@latest
Install a language server: follow one of the guides below
Configure your MCP client: follow one of the guides below

Tools

definition: Retrieves the complete source code definition of any symbol (function, type, constant, etc.) from your codebase.
references: Locates all usages and references of a symbol throughout the codebase.
diagnostics: Provides diagnostic information for a specific file, including warnings and errors.
hover: Display documentation, type hints, or other hover information for a given location.
rename_symbol: Rename a symbol across a project.
edit_file: Allows making multiple text edits to a file based on line numbers. Provides a more reliable and context-economical way to edit files compared to search and replace based edit tools.

About

This codebase makes use of edited code from gopls to handle LSP communication. See ATTRIBUTION for details. Everything here is covered by a permissive BSD style license.

mcp-go is used for MCP communication. Thank you for your service.

This is beta software. Please let me know by creating an issue if you run into any problems or have suggestions of any kind.

Contributing

Please keep PRs small and open Issues first for anything substantial. AI slop OK as long as it is tested, passes checks, and doesn't smell too bad.

Setup

Clone the repo:

git clone https://github.com/isaacphi/mcp-language-server.git
cd mcp-language-server

A justfile is included for convenience:

just -l
Available recipes:
    build    # Build
    check    # Run code audit checks
    fmt      # Format code
    generate # Generate LSP types and methods
    help     # Help
    install  # Install locally
    snapshot # Update snapshot tests
    test     # Run tests

Configure your Claude Desktop (or similar) to use the local binary:

{
  "mcpServers": {
    "language-server": {
      "command": "/full/path/to/your/clone/mcp-language-server/mcp-language-server",
      "args": [
        "--workspace",
        "/path/to/workspace",
        "--lsp",
        "language-server-executable"
      ],
      "env": {
        "LOG_LEVEL": "DEBUG"
      }
    }
  }
}

Rebuild after making changes.

Logging

Setting the LOG_LEVEL environment variable to DEBUG enables verbose logging to stderr for all components including messages to and from the language server and the language server's logs.

LSP interaction

internal/lsp/methods.go contains generated code to make calls to the connected language server.
internal/protocol/tsprotocol.go contains generated code for LSP types. I borrowed this from gopls's source code. Thank you for your service.
LSP allows language servers to return different types for the same methods. Go doesn't like this so there are some ugly workarounds in internal/protocol/interfaces.go.

Local Development and Snapshot Tests

There is a snapshot test suite that makes it a lot easier to try out changes to tools. These run actual language servers on mock workspaces and capture output and logs.

You will need the language servers installed locally to run them. There are tests for go, rust, python, and typescript.

integrationtests/
├── tests/        # Tests are in this folder
├── snapshots/    # Snapshots of tool outputs
├── test-output/  # Gitignored folder showing the final state of each workspace and logs after each test run
└── workspaces/   # Mock workspaces that the tools run on

To update snapshots, run UPDATE_SNAPSHOTS=true go test ./integrationtests/...

MCP Language Server

Integrations