execute_js · Simple Console MCP: Risk & Policy

WHAT IT DOES

What execute_js does on Simple Console MCP

AI agents invoke execute_js to trigger actions in Simple Console MCP. 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 execute_js needs a policy

This tool permits execution of arbitrary JavaScript in a browser context, which can trigger any action available to JavaScript running on the page. While the examples (clicking buttons, filling forms) are benign, the capability extends to arbitrary code execution that could exfiltrate data, modify page state, trigger financial transactions, or interact with sensitive APIs—all depending on attacker-controlled input.

From the tool's definition Tool explicitly states it 'Execute[s] JavaScript code in the page context' with capabilities to perform 'clicking buttons, filling forms, or calling page functions.' These are arbitrary code execution actions whose effects depend entirely on the JavaScript…

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

execute_js Rate-limited

execute_js 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": {
    "execute_js": {
      "limits": [
        {
          "counter": "execute_js_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 Simple Console MCP, so you can run agents against it safely.

FAQ

Questions about execute_js

What does the execute_js tool do? +

Execute JavaScript code in the page context. Returns the result of the expression. Useful for clicking buttons, filling forms, or calling page functions. It is categorised as a Execute tool in the Simple Console MCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on execute_js? +

Register the Simple Console MCP server in PolicyLayer and add a rule for execute_js: 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 Simple Console MCP. Nothing to install.

What risk level is execute_js? +

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

Can I rate-limit execute_js? +

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

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

execute_js is provided by the Simple Console MCP server (tznthou/simple-console-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.