evaluate_script · JS Reverse MCP: Risk & Policy

WHAT IT DOES

What evaluate_script does on JS Reverse MCP

AI agents invoke evaluate_script to trigger actions in JS Reverse 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 evaluate_script needs a policy

This tool runs arbitrary JavaScript inside a browser page, which can access DOM, cookies, localStorage, make network requests, exfiltrate data, or perform any browser-accessible action. The blast radius is critical: a misused or malicious script could compromise session tokens, perform CSRF attacks, steal credentials, or pivot to further exploitation.

From the tool's definition "Evaluate a JavaScript function inside the currently selected page" — directly executes arbitrary JavaScript in a live browser page context

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

evaluate_script Rate-limited

evaluate_script 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_script": {
      "limits": [
        {
          "counter": "evaluate_script_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 JS Reverse MCP, so you can run agents against it safely.

FAQ

Questions about evaluate_script

What does the evaluate_script tool do? +

Evaluate a JavaScript function inside the currently selected page. Returns the response as JSON so returned values have to JSON-serializable. It is categorised as a Execute tool in the JS Reverse 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 evaluate_script? +

Register the JS Reverse MCP server in PolicyLayer and add a rule for evaluate_script: 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 JS Reverse MCP. Nothing to install.

What risk level is evaluate_script? +

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

Can I rate-limit evaluate_script? +

Yes. Add a rate_limit block to the evaluate_script 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_script completely? +

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

evaluate_script is provided by the JS Reverse MCP server (noone-hub/jsreverser-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.