LSP (Language Server Protocol)

The built-in language server was introduced in RuboCop 1.53. This experimental feature has been under consideration for a while.

The Language Server Protocol is the modern standard for providing cross-editor support for various programming languages.

This feature enables extremely fast interactions through the LSP.

Offense detection and autocorrection are performed in real-time by editors and IDEs using the language server. The Server Mode is primarily used to speed up RuboCop runs in the terminal. Therefore, if you want real-time feedback from RuboCop in your editor or IDE, opting to use this language server instead of the server mode will not only provide a fast and efficient solution, but also offer a straightforward setup for integration.

Run as a Language Server

Run rubocop --lsp command from LSP client.

When the language server is started, the command displays the language server’s PID:

$ ps aux | grep rubocop
user             17414   0.0  0.2  5557716 144376   ??  Ss    4:48PM   0:02.13 /Users/user/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/bin/rubocop --lsp


The language server supports textDocument/formatting method and is autocorrectable. The autocorrection is safe.

Examples of LSP Client

Here are examples of LSP client configurations.

VS Code

vscode-rubocop integrates RuboCop into VS Code.

You can install this VS Code extension from the Visual Studio Marketplace.

Emacs (Eglot)

Eglot is a client for Language Server Protocol servers on Emacs.

Add the following to your Emacs configuration file (e.g. ~/.emacs.d/init.el):

(require 'eglot)

(add-to-list 'eglot-server-programs '(ruby-mode . ("bundle" "exec" "rubocop" "--lsp")))
(add-hook 'ruby-mode-hook 'eglot-ensure)

Below is an example of additional setting for autocorrecting on save:

(add-hook 'ruby-mode-hook (lambda () (add-hook 'before-save-hook 'eglot-format-buffer nil 'local)))

If you run into problems, first use "M-x eglot-reconnect" to reconnect to the language server.

See Eglot’s official documentation for more information.