cmux_browser_open

THE RISK

High Risk

Why cmux_browser_open needs a policy

Opening a browser surface is an external operation that creates a new browser instance/pane within the workspace. It falls under Execute as it triggers an external operation (launching a browser surface) rather than simply reading data or writing to a data store. It is not Destructive or Financial, but it does initiate a potentially broad action (browser execution environment) that an agent could misuse.

From the tool's definition "Open a browser surface (split in the current workspace)" — triggers an external browser operation and modifies the workspace layout

Documented attack patterns abuse exactly the kind of access cmux_browser_open gives an agent:

POLICY

How to control cmux_browser_open

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_browser_open:

policy.json

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

cmux_browser_open 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.

Create a free account and register Cmux Agent — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Cmux Agent tools

Destructive cmux_clear_notifications Clear all notifications. Destructive cmux_close_all Close ALL workspaces (or all except specific ones). Use Destructive cmux_close_surface Close a surface (tab). Destructive cmux_close_window Close a window. Destructive cmux_close_workspace Close one or more workspaces and all their panes. Pass a single ref or an array of refs. Execute cmux_browser_console Get or clear browser console logs/errors. Execute cmux_batch Execute multiple CMUX operations in a single MCP call. Each step references an existing cm Execute cmux_browser_eval Execute JavaScript in the browser page.

All 63 Cmux Agent tools →

Execute tools on other servers

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Go deeper

High-risk tools in Cmux Agent → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about cmux_browser_open

What does the cmux_browser_open tool do? +

Open a browser surface (split in the current workspace). 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.

How do I enforce a policy on cmux_browser_open? +

Register the Cmux Agent MCP server in PolicyLayer and add a rule for cmux_browser_open: 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.

What risk level is cmux_browser_open? +

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

Can I rate-limit cmux_browser_open? +

Yes. Add a rate_limit block to the cmux_browser_open 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_browser_open completely? +

Set action: deny in the PolicyLayer policy for cmux_browser_open. 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_browser_open? +

cmux_browser_open 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.

Enforce policy on every Cmux Agent tool call.

Start from Cmux Agent, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →