cmux

Product Introduction

1. Definition: cmux is a free, open-source, GPU-accelerated terminal emulator application for macOS. It is built natively with Swift and AppKit, utilizing libghostty as its core terminal rendering engine, analogous to how a browser uses WebKit.
2. Core Value Proposition: cmux exists to solve terminal clutter and inefficient context switching for developers and engineers, particularly those using AI coding agents. It provides a native, programmable workspace with vertical tabs, intelligent notifications, and an integrated browser to centralize multitasking workflows.

Main Features

1. Vertical Tabs with Context: The application features a persistent sidebar with vertical tabs. Each tab displays dynamic metadata including the current git branch, working directory, active network ports, and custom notification text, providing immediate project context without switching views.
2. Agent Notification System: cmux implements a multi-layer notification system for processes requiring user attention. It uses standard terminal escape sequences (OSC 9, 99, 777) to trigger visual indicators: colored rings around panes, unread badges on tabs, a popover, and macOS desktop notifications. This is critical for AI agents like Claude Code or OpenCode that pause for input.
3. Programmable In-App Browser: A key differentiator is the integrated, scriptable browser pane that can be split alongside any terminal. It exposes a full Unix socket API for automation: navigation, DOM snapshotting, clicking, typing, and JavaScript evaluation, allowing agents to self-verify web changes without leaving the terminal environment.
4. Native Multiplexing & Session Restore: cmux provides native horizontal and vertical split panes within tabs, eliminating the need for external multiplexers like tmux for basic splits. All workspace state—windows, panes, working directories, scrollback, and active agent sessions—is automatically saved and restored across app and system restarts.
5. Comprehensive Automation API: The entire application is controllable via a command-line interface (CLI) and a Unix domain socket. Users can programmatically create workspaces, manage splits, send input, capture screen output, and drive the browser, enabling deep integration with scripts and tools.

Problems Solved

1. Pain Point: Disorganized terminal sprawl and inefficient mental context switching between multiple terminal windows, SSH sessions, and browser tabs during complex development and agentic workflows.
2. Target Audience: Software engineers, DevOps professionals, and researchers who heavily utilize terminal-based AI coding agents (Claude Code, Cursor, Aider), manage multiple concurrent projects, and require automation. It's also ideal for users of tmux or iTerm2 seeking a more integrated, GUI-native experience.
3. Use Cases: Orchestrating a multi-agent coding team (e.g., Claude Code teams) where each subagent gets a dedicated, visible pane; monitoring long-running processes (servers, builds, scripts) with immediate visual notification upon completion or error; developing full-stack applications with a terminal and a programmable browser view side-by-side; managing workspaces for different git worktrees or microservices.

Unique Advantages

1. Strengths & Limitations (Pros & Cons):

   • Pros: Exceptional organization via vertical tabs and persistent state; unparalleled integration and visibility for AI agent workflows; powerful, unique browser automation API; true native macOS performance and efficiency (Swift/AppKit); extensive programmability; seamless session restore; active open-source development.
   • Cons: Currently macOS-only, limiting cross-platform teams; being a relatively new project, it may lack some niche configuration options of established terminals; the unique workflow requires adaptation from users of traditional terminals.

2. Key Alternatives & Differentiation:

   • iTerm2: The macOS terminal standard. iTerm2 is highly configurable with a vast feature set but lacks native vertical tabs, deep agent notification systems, and an integrated browser. cmux offers a more opinionated, workflow-centric organization layer.
   • Warp: A modern, GPU-accelerated terminal. Warp focuses on AI assistance, command palettes, and collaborative features within a single window. cmux differentiates with its multi-workspace/tab model, notification rings for external agents, and its programmable browser component.
   • Ghostty: The rendering engine upon which cmux is built. Ghostty is a minimalist, standalone terminal focused on speed and correctness. cmux is not a fork but an application that uses libghostty, adding a full GUI layer, workspace management, and the unique features described above. They are complementary projects.
   • tmux: A terminal multiplexer that runs inside any terminal. tmux is server-based, highly stable, and works over SSH. cmux is a native GUI application with built-in splits, a richer visual interface, and native macOS integration. They can be used together, with cmux attaching to remote tmux sessions.

Frequently Asked Questions (FAQ)

1. Is cmux just a fork of Ghostty? No, cmux is not a fork. It uses libghostty as a library for its rendering core, similar to how applications use WebKit. Ghostty is a standalone terminal application, while cmux is a separate macOS app built on top of that engine, adding a complete GUI and workflow features.
2. How does cmux handle notifications from AI agents? cmux automatically detects when an agent needs attention via standard terminal escape sequences (OSC 9, 99, 777). This triggers visual rings around the pane, tab badges, and system notifications. Most modern coding agents like Claude Code and OpenCode support these sequences or can be configured with hooks to trigger them.
3. Can I use cmux for remote development over SSH? Yes. cmux fully supports SSH connections. You can open workspaces directly over SSH and, in beta, natively attach to remote tmux sessions, allowing you to manage agent workflows running on remote servers from your local cmux interface.
4. Is the cmux browser a real browser, and what can it do? Yes, it is a real, embedded browser component. It is fully scriptable via a Unix socket API, allowing for automation tasks like navigation, taking screenshots, manipulating the DOM, and executing JavaScript. This enables AI agents to interact with and verify web applications directly within the terminal environment.
5. How customizable is cmux compared to other terminals? Terminal rendering (themes, fonts, colors) is controlled by your Ghostty configuration file (~/.config/ghostty/config). cmux's own UI and behavior (sidebar, splits, shortcuts) are configured via ~/.config/cmux/cmux.json. All keyboard shortcuts for cmux-specific actions are also customizable within the app's settings.