surface_new · Wmux MCP: Risk & Policy

WHAT IT DOES

What surface_new does on Wmux

AI agents invoke surface_new to trigger actions in Wmux. 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 surface_new needs a policy

This tool creates a new execution environment (terminal/shell process) whose effects depend on what commands are subsequently executed within it. While the tool itself doesn't directly run commands, it enables command execution.

From the tool's definition Tool opens a new surface (terminal) in a workspace and returns surface/pty IDs. The verb 'open' combined with 'terminal' in a Windows tmux environment indicates spawning a new shell/process whose behavior depends on subsequent commands run within it.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

surface_new Rate-limited

surface_new 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": {
    "surface_new": {
      "limits": [
        {
          "counter": "surface_new_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 Wmux, so you can run agents against it safely.

FAQ

Questions about surface_new

What does the surface_new tool do? +

Open a new surface (terminal) in the active pane of a workspace. Returns the new surfaceId + ptyId. Omit workspaceId to open in your own (the caller\. It is categorised as a Execute tool in the Wmux MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on surface_new? +

Register the Wmux MCP server in PolicyLayer and add a rule for surface_new: 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 Wmux. Nothing to install.

What risk level is surface_new? +

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

Can I rate-limit surface_new? +

Yes. Add a rate_limit block to the surface_new 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 surface_new completely? +

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

surface_new is provided by the Wmux MCP server (openwong2kim/wmux). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

// THE FULL RECORD

surface_new is one line of Wmux's registry record.

The record carries the whole server: verified identity, auth posture, risk grade, every tool classified, recommended policy — re-checked continuously.

Teams ship this data inside their own products. See what a licence covers →