Borrowing it
Nothing to install: this file belongs to tmux-python/libtmux-mcp. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/tmux-python/libtmux-mcp/main/.agents/skills/testing-mcp-with-cli-agents/SKILL.mdgit clone --depth 1 https://github.com/tmux-python/libtmux-mcpWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/tmux-python/libtmux-mcp/testing-mcp-with-cli-agents)<a href="https://agentmods.dev/skills/tmux-python/libtmux-mcp/testing-mcp-with-cli-agents"><img src="https://agentmods.dev/badge/skills/tmux-python/libtmux-mcp/testing-mcp-with-cli-agents/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/tmux-python/libtmux-mcp/testing-mcp-with-cli-agents"><img src="https://agentmods.dev/badge/skills/tmux-python/libtmux-mcp/testing-mcp-with-cli-agents.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector warn
SkillSpector: 5 findings, up to high
These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →
- high Memory Poisoning · line 230 Skill manipulates agent memory, state, or stored context. Memory corruption can alter personality, override safety rules, or cause unpredictable behavior.Fix: Protect agent memory and state from modification by untrusted content. Use read-only memory for critical instructions and validate all state changes.
- high Memory Poisoning · line 240 Skill manipulates agent memory, state, or stored context. Memory corruption can alter personality, override safety rules, or cause unpredictable behavior.Fix: Protect agent memory and state from modification by untrusted content. Use read-only memory for critical instructions and validate all state changes.
- high Memory Poisoning · line 247 Skill manipulates agent memory, state, or stored context. Memory corruption can alter personality, override safety rules, or cause unpredictable behavior.Fix: Protect agent memory and state from modification by untrusted content. Use read-only memory for critical instructions and validate all state changes.
- high Memory Poisoning · line 266 Skill manipulates agent memory, state, or stored context. Memory corruption can alter personality, override safety rules, or cause unpredictable behavior.Fix: Protect agent memory and state from modification by untrusted content. Use read-only memory for critical instructions and validate all state changes.
- medium Rogue Agent · line 233 Skill establishes unauthorized persistence across sessions via cron jobs, startup scripts, or state files. Session persistence allows an attacker to maintain access beyond the current interaction.Fix: Remove any persistence mechanisms (cron jobs, startup scripts, state files). Skills should not maintain state across sessions without explicit user consent.
What 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 5.1 | $0.00181 | $0.03999 |
| Opus 5.5 | $0.00072 | $0.01600 |
| Sonnet 5.5 | $0.00036 | $0.00800 |
| Haiku 4.5 | $0.00018 | $0.00400 |
Grade A, and why
testing-mcp-with-cli-agents 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 1mo 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 — 287 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Testing an MCP server through real CLI agents
Unit tests prove the server's internals; they don't prove a real agent can discover a tool, get past its approval gate, call it, and survive cancelling it mid-flight. This skill exercises that whole path by pointing installed CLI agents at a checkout and driving them — with the tmux tool surface (libtmux-mcp) as the running example, though the shape generalizes to any MCP server.
The core idea: three tmux servers, never one
The single biggest mistake is running everything on one tmux socket. Keep three distinct servers, each on its own socket, and the whole thing becomes safe and observable:
| Role | Socket | Who touches it | Why separate |
|---|---|---|---|
| Your real session | default | you, interactively | must never be mutated by a test |
| Harness | tmux -L cli-harness |
the driver: send-keys prompts in, capture-pane render out |
isolates the CLI TUI you're driving |
| MCP-target | tmux -L mcp-target, exported as LIBTMUX_SOCKET=mcp-target to the server |
the MCP server, when the agent calls tmux tools | independent ground-truth; a destructive tool here can't kill the agent you're driving |
The server resolves its socket from LIBTMUX_SOCKET and runs tmux -L <name>
(src/libtmux_mcp/_utils.py builds argv with -L server.socket_name, defaulted
from that env var). Exporting LIBTMUX_SOCKET=mcp-target into the server's
config env fully sandboxes every tmux tool call onto a scratch server.
Pre-create it so the agent has something to see:
$ tmux -L mcp-target new-session -d -s scratch
Keeping harness separate from MCP-target is the load-bearing part: if the
agent's own TUI pane lived on the socket its MCP server mutates, one
kill-server / kill-session tool call would tear down the agent mid-test, and
your captures would be polluted by the agent's own UI redraws.
Climb only as high as the question needs — three fidelity layers
Layer 0 — Direct MCP smoke, no CLI at all
What ships with it
1 file 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.
- 1mo ago First seen · 287 lines · 181 tokens per session scan A faa9a702a3c2
testing-mcp-with-cli-agents is a skill published in the GitHub repository tmux-python/libtmux-mcp (14 stars, last pushed 3d ago), licensed MIT. It adds 181 tokens to every session and 3,999 once invoked, about $0.0007 per session on Opus 5.5. 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 2026-08-30.
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