evaluate_js · InSite MCP: Risk & Policy

WHAT IT DOES

What evaluate_js does on InSite

AI agents invoke evaluate_js to trigger actions in InSite. 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 evaluate_js needs a policy

JavaScript execution in a browser context is a classic Execute primitive that can interact with the DOM, fetch data, exfiltrate credentials, modify page state, trigger requests, or perform any action the page can perform.

From the tool's definition Tool description explicitly states 'Execute JavaScript code in the page context' — this permits arbitrary code execution within the browser environment.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

evaluate_js Rate-limited

evaluate_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": {
    "evaluate_js": {
      "limits": [
        {
          "counter": "evaluate_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 InSite, so you can run agents against it safely.

FAQ

Questions about evaluate_js

What does the evaluate_js tool do? +

Execute JavaScript code in the page context. It is categorised as a Execute tool in the InSite MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on evaluate_js? +

Register the InSite MCP server in PolicyLayer and add a rule for evaluate_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 InSite. Nothing to install.

What risk level is evaluate_js? +

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

Can I rate-limit evaluate_js? +

Yes. Add a rate_limit block to the evaluate_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 evaluate_js completely? +

Set action: deny in the PolicyLayer policy for evaluate_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 evaluate_js? +

evaluate_js is provided by the InSite MCP server (jowharshamshiri/insite). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.