An Emacs plugin for sending file references to an AI CLI (Claude Code / Gemini CLI) running in another cmux surface.

cmux.el is an Emacs Lisp port of tanabee/cmux.vim, with stricter input validation and no shell invocation.

• Send the current buffer's path as an @mention reference (@src/main.ts ).
• Send the path with the current line (@src/main.ts:L42 ).
• Send the path with the selected line range (@src/main.ts:L42-55 ).
• Auto-detect the surface running Claude Code or Gemini CLI.
• Optional follow-up Enter key and target-pane focus after sending.
• Strict surface-ID validation; refuses TRAMP buffers.

Requires Emacs 27.1 or newer. No external Emacs Lisp dependencies.

(use-package cmux
  :ensure t
  :commands (cmux-send-file cmux-send-pos cmux-send-range
             cmux-dwim cmux-detect cmux-set-surface)
  :bind (:map prog-mode-map
         ("C-c m f" . cmux-send-file)
         ("C-c m p" . cmux-send-pos)
         ("C-c m r" . cmux-send-range)
         ("C-c m d" . cmux-detect)
         ("C-c m s" . cmux-set-surface)))

(straight-use-package
 '(cmux :type git :host github :repo "kexi/cmux.emacs"))

git clone https://github.com/kexi/cmux.emacs.git ~/.emacs.d/site-lisp/cmux.emacs

(add-to-list 'load-path "~/.emacs.d/site-lisp/cmux.emacs")
(require 'cmux)

Open two cmux tabs: one with Emacs, one with your AI CLI (Claude Code or Gemini CLI). On the first call, cmux.el will run cmux-detect automatically to discover the AI CLI's surface. You can also set it explicitly with M-x cmux-set-surface.

Note on cmux-detect false positives: the built-in patterns are simple word-level regexps (e.g. anthropic, Gemini), so any surface whose screen happens to mention those words — a shell scrollback, a man page, an open editor buffer — can be matched even when no AI CLI is actually running there. If detection picks the wrong surface, override it explicitly with M-x cmux-set-surface (e.g. surface:1).

cmux.el does not bind any keys by default. This is intentional: the original Vim plugin's C-\ clashes with Emacs's built-in toggle-input-method, and every convenient Emacs prefix is already taken by something else. Pick bindings that fit your own layout.

A suggested layout under C-c m:

(global-set-key (kbd "C-c m f") #'cmux-send-file)
(global-set-key (kbd "C-c m p") #'cmux-send-pos)
(global-set-key (kbd "C-c m r") #'cmux-send-range)
(global-set-key (kbd "C-c m d") #'cmux-detect)
(global-set-key (kbd "C-c m s") #'cmux-set-surface)

Or use the buffer-local minor mode via its keymap:

(define-key cmux-mode-map (kbd "C-c m f") #'cmux-send-file)
(define-key cmux-mode-map (kbd "C-c m r") #'cmux-send-range)
(global-cmux-mode 1)

(with-eval-after-load 'evil
  (evil-define-key 'normal 'global (kbd "SPC c f") #'cmux-send-file)
  (evil-define-key 'normal 'global (kbd "SPC c p") #'cmux-send-pos)
  (evil-define-key 'visual 'global (kbd "SPC c r") #'cmux-send-range)
  (evil-define-key 'normal 'global (kbd "SPC c d") #'cmux-detect)
  (evil-define-key 'normal 'global (kbd "SPC c s") #'cmux-set-surface))

All options live in the cmux customization group (M-x customize-group RET cmux RET).

• cmux must be installed and on PATH, and Emacs must be running inside a cmux surface.
• An AI CLI (Claude Code or Gemini CLI) must be running in another cmux surface.

• All cmux invocations use call-process with list-form arguments and an absolute executable path resolved via executable-find. No shell interpolation happens at any stage.
• TRAMP buffers are refused: any buffer whose file or default-directory is remote will raise a user-error.
• Surface references are validated against a strict regexp (surface:<digits>), trimmed, length-limited, and NUL-rejected before being passed to the CLI. This applies to user input, CMUX_SURFACE_ID, and the output of cmux identify alike.
• Each cmux invocation is bounded by cmux-call-timeout.
• File paths containing NUL or control characters are rejected.
• When cmux-path-style is set to 'absolute, the full absolute path of the current buffer — including your home directory and user name — is sent to the AI CLI. The default 'git-root (or 'relative-home) avoids exposing this information; switch to 'absolute only when you understand the implication.

• Original Vim implementation: tanabee/cmux.vim.
• cmux itself: manaflow-ai/cmux.

MIT. See LICENSE.