AI agents invoke cmux_new_window to trigger actions in Cmux Agent. 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.
This tool executes an action that triggers external effects—window creation within the CMUX environment. Although window creation is not inherently destructive or financial, it is an Execute-category action because it runs a system operation whose outcome depends on context (which session, what configuration).
From the tool's definition Tool creates a new window in a programmable terminal control plane that orchestrates multiple parallel AI CLI sessions and browser instances. The description states it 'Create[s] a new window' which is an operational effect on the system state.
Documented attack patterns abuse exactly the kind of access cmux_new_window gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Cmux Agent, and nothing reaches the server without passing your rules. This is the rule we recommend for cmux_new_window:
{
"version": "1",
"default": "deny",
"tools": {
"cmux_new_window": {
"limits": [
{
"counter": "cmux_new_window_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}cmux_new_window 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.
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Create a new window. Returns the window ref. It is categorised as a Execute tool in the Cmux Agent MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Cmux Agent MCP server in PolicyLayer and add a rule for cmux_new_window: 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 Agent. Nothing to install.
cmux_new_window is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the cmux_new_window 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.
Set action: deny in the PolicyLayer policy for cmux_new_window. 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.
cmux_new_window is provided by the Cmux Agent MCP server (multiagentcognition/cmux-agent-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Cmux Agent, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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63 Cmux Agent tools catalogued and risk-classified — across an index of 43,000+ MCP servers.