cmux-browser Skill (cmux) for Claude Code: What It Does and How to Install It

Makes the agent drive the browser inside cmux, a macOS terminal for coding agents, without stealing focus.

Installs: 8.9K; on skills.sh
Stars: 28.6K; on GitHub
Files: 12; besides SKILL.md
Updated: Nov 2026; last repo commit

The agent uses the cmux browser command. It opens a browser surface, takes an interactive snapshot that gives each element a reference such as e1, then fills, clicks and waits for text, a selector, a URL part or the load state. Every command on an existing page carries an explicit surface handle, and SKILL.md says never to guess it from what has focus.

It needs the cmux app, which the README says is macOS only, and jq for the discovery snippets. cmux uses WKWebView, so trace and screencast recording, network interception and raw input injection return not_supported. Saved login state holds cookies and storage, so treat it as sensitive.

Good for

cmux users who run Claude Code or Codex and want the agent to use the built-in browser
Developers checking a page, filling a form or extracting data from a site
Anyone writing shell scripts around cmux browser commands

Install cmux-browser

Claude Code

Copy this prompt.

Install the cmux-browser skill in Claude Code. Run this command:

npx --yes skills@1.5.23 add manaflow-ai/cmux --global --yes --skill cmux-browser --agent claude-code codex --copy

Then check that it loaded and tell me how to use it.

2
Paste it into Claude Code. It shows the change and asks before running it.

Requires Node.js.

Install the cmux app first (the README documents a DMG, which auto-updates, and a Homebrew cask), because the skill only drives the browser inside cmux, which is macOS only. The skill install command is the one SKILL.md gives under Skill distribution and refresh. It needs jq for the surface-discovery snippets and runs local commands, so there is no Claude app upload route.

From the project README. Source.

What's in the skill

When it loads

End-user browser automation with cmux. Use when you need to open sites, inspect or interact with browser surfaces, wait for page state, and extract data without stealing focus.

Sections in SKILL.md

Read the CLI contract first
Find an existing browser surface without changing focus
Core workflow
Wait
Viewport sizing (WKWebView)
Limits (WKWebView)
Troubleshooting js_error
Skill distribution and refresh
Deep-dive references
Ready-to-use templates

Key facts

Author: Manaflow (vendor's own repo; the cmux LICENSE names Manaflow, Inc.)
License: GPL-3.0-or-later for the cmux project per LICENSE (README: "Free and GPL-licensed"); some server directories use BUSL-1.1
Installs on skills.sh: 8,885 (Nov 2, 2026)
GitHub stars: 28,635 for the whole cmux repo (Nov 2, 2026)
Last repo commit: Nov 1, 2026
Audits on skills.sh: Gen Agent Trust Hub: pass, Socket: pass, Snyk: warn
What it includes: 13 files including SKILL.md: AGENTS.md, an agent config, 7 reference guides and 3 shell templates
Needs: The cmux app (macOS only) and jq
Works in: Claude Code and Codex (the SKILL.md install command targets both)

What it includes

cmux browser --help and cmux --version before any exact command
Runs code · Read the CLI contract
cmux identify --json and cmux tree --all --json list browser surfaces
Both are read-only listings per SKILL.md, and neither changes focus. The output can hold page URLs and titles.
Runs code · Find a surface
cmux --json browser open <url> --focus false returns a surface ref
Runs code · Open a browser surface
snapshot --interactive, then fill e1 "text" and click e2 --snapshot-after
Refs go stale, so snapshot again after navigation or a modal.
Try: “Open example.com in cmux and fill in the contact form”
Runs code · Snapshot, fill and click
wait --selector, --text, --url-contains, --load-state and --function with a timeout
Runs code · Wait
viewport <w> <h>, viewport reset, and download list for saved files
Runs code · Viewport, zoom and downloads
form-automation.sh, authenticated-session.sh and capture-workflow.sh
capture-workflow.sh writes a snapshot text file and a screenshot file. authenticated-session.sh loads and saves a state file.
Runs code · Templates
Fall back to get text body or get html body when snapshot fails
Runs code · Troubleshoot js_error

From the official docs

Permissions

Medium risk

The agent can click and submit in a real browser, including logged-in pages, and can save session state to disk. Use throwaway accounts first and keep any state file private.

What it can do

Read your data
Create and change things
Delete things
Read-only mode

Watch out for

It can act in logged-in pages
The authentication guide covers login, OAuth and 2FA flows, cookie setting and state save and load. Only let it use accounts you are happy for an agent to act in. github.com
Saved state and cookies hold your session
references/authentication.md says saved state contains cookies, localStorage, sessionStorage and open tab metadata, and that state files hold auth tokens and must never be committed. Its examples keep them in a mode 700 directory with chmod 600 files, but the authenticated-session.sh template defaults to ./auth-state.json in the working directory. The guide also shows cookies set with a value from an environment variable, and cookies clear --all plus deleting the state file after sensitive tasks. AGENTS.md calls cookies and saved state from authenticated pages sensitive. github.com
Listings can expose page URLs and titles
SKILL.md says raw tree and list output can contain page URLs and titles, so filter or redact it before logging or pasting. github.com
The install command refreshes global skills
The pinned skills 1.5.23 command uses --global --yes --copy for claude-code and codex, which writes the full skill into both global skill folders. github.com

Security advisories

No published security advisories as of Nov 2, 2026.

Technical details

Turning on read-only	No read-only switch exists.
Sign-in and keys	Saved state files contain cookies, localStorage and sessionStorage; the guide says to keep them in a private directory.
Scopes	Controls the cmux browser: opens pages, clicks, fills forms and reads page content Can save and load browser state with cookies, localStorage and sessionStorage Runs shell templates that write snapshot, screenshot and state files

Questions

What does the cmux-browser skill do?

It teaches the agent to use the cmux browser command. The agent can open a site, read the page structure, click and fill elements, wait for page state and extract data, all inside cmux.

How do I install it?

Use the install box at the top of this page. Install the cmux app first, then add the skill with the command SKILL.md gives, which targets both Claude Code and Codex. The box lists the folder routes too.

Does it work without cmux?

No. The skill only drives the browser inside cmux, and the cmux README says the app is macOS only for now.

Is it official?

Yes. It lives in the manaflow-ai/cmux repository, the vendor's own repo.

Does it need an API key?

No key is named in SKILL.md. Login examples in the reference guides pass a password from an environment variable such as APP_PASSWORD.

Is it safe?

It can click, fill and submit in a real browser, and it can save and load browser state that holds cookies and storage. AGENTS.md says to treat URLs, titles, snapshots, cookies and saved state from authenticated pages as sensitive. Keep state files out of git.

What can it not do?

SKILL.md says trace and screencast recording, network route interception and mocking, and low-level raw input injection return not_supported in cmux's WKWebView browser.

Information

updated Nov 2, 2026

Repository: manaflow-ai/cmux
Folder: skills/cmux-browser

manaflow-ai/cmux skills.sh

Sources

Docs, issue trackers and discussions this page is built from.

Show all 6

cmux-browser SKILL.md · github.com
cmux README · github.com
cmux-browser AGENTS.md · github.com
cmux-browser references/authentication.md · github.com
capture-workflow.sh · github.com
authenticated-session.sh · github.com