Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/jtsternberg/claude-plugins/using-cmux-clinpx skills add jtsternberg/claude-plugins --skill using-cmux-cligit clone --depth 1 https://github.com/jtsternberg/claude-pluginsWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00049 | $0.15282 |
| Opus 5 | $0.00024 | $0.07641 |
| Sonnet 5 | $0.00010 | $0.03056 |
| Haiku 4.5 | $0.00005 | $0.01528 |
Grade A, and why
using-cmux-cli scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 676 lines — stays where its author put it; the contents beside it link to each section on GitHub.
cmux CLI
Drive cmux (cmux) from the command line: windows, workspaces, panes, surfaces, tabs, terminal I/O, notifications, and the embedded browser. The CLI talks to the running cmux.app over a Unix socket.
This skill only applies when the user is working with cmux specifically — not generic terminal multiplexers like tmux or screen.
The golden rule: let --help be the source of truth
cmux ships frequently and its flags evolve. Rather than mirroring option lists into this skill (which bitrots), run cmux <cmd> --help before constructing any real invocation. The top-level overview below is inlined live; for everything else, call cmux <cmd> --help on demand. That single rule replaces a dozen "remember to pass --foo" footnotes.
Current environment (resolved at skill load)
cmux identify --json 2>/dev/null || echo '{"error":"cmux identify failed — see Troubleshooting"}'
identify returns three things:
caller.*— your own surface/workspace/window/pane refs (where the agent is running). Use these as defaults when the user says "here" / "this pane".focused.*— where the user is currently looking. Often different fromcaller. When the user says "do that in the other tab I'm looking at", targetfocusedexplicitly.socket_path— the actual socket the CLI is talking to.
If identify fails, you're either outside cmux or the socket is unreachable — see Troubleshooting. When outside cmux, every targeted command needs explicit handles; discover them with cmux tree --all --json.
Is cmux actually running?
cmux ping
Silent success means yes. An error means the app isn't running, the socket path is wrong, or auth is misconfigured. See Troubleshooting.
Overview: every subcommand at a glance
cmux --help
This is the master index. Every subcommand appears here with its full signature — enough to construct most invocations on sight. For anything non-obvious, drill in with cmux <cmd> --help.
Capabilities of the running build (what flags the current app actually supports):
What ships with it
7 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago First seen · 676 lines · 49 tokens per session scan A bebfac23d0d9
using-cmux-cli is a skill published in the GitHub repository jtsternberg/claude-plugins (5 stars, last pushed 5d ago), licensed MIT. It adds 49 tokens to every session and 15,282 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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