cmux_send_key · Cmux MCP: Risk & Policy

WHAT IT DOES

What cmux_send_key does on Cmux

AI agents invoke cmux_send_key to trigger actions in Cmux. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why cmux_send_key needs a policy

This tool sends keystrokes to terminal panes, including control sequences like C-c (interrupt) and C-d (EOF/logout). These can trigger arbitrary program behavior, terminate processes, or interact with running shells/applications in ways that depend on context. The blast radius is high because sending keys like C-d to a shell could close sessions, or Enter could confirm destructive prompts.

From the tool's definition Send a named key such as Enter, Tab, Escape, C-c, C-d. Ctrl chords accept either form

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

cmux_send_key Rate-limited

cmux_send_key stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "cmux_send_key": {
      "limits": [
        {
          "counter": "cmux_send_key_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Cmux, so you can run agents against it safely.

FAQ

Questions about cmux_send_key

What does the cmux_send_key tool do? +

Send a named key such as Enter, Tab, Escape, C-c, C-d. Ctrl chords accept either form:. It is categorised as a Execute tool in the Cmux MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on cmux_send_key? +

Register the Cmux MCP server in PolicyLayer and add a rule for cmux_send_key: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Cmux. Nothing to install.

What risk level is cmux_send_key? +

cmux_send_key is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit cmux_send_key? +

Yes. Add a rate_limit block to the cmux_send_key rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block cmux_send_key completely? +

Set action: deny in the PolicyLayer policy for cmux_send_key. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides cmux_send_key? +

cmux_send_key is provided by the Cmux MCP server (puchkoff/cmux-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.