evaluate_script · Chrome Devtools MCP: Risk & Policy

WHAT IT DOES

What evaluate_script does on Chrome Devtools

AI agents invoke evaluate_script to trigger actions in Chrome Devtools. 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 executes arbitrary JavaScript in a live browser page. An AI agent could use it to exfiltrate data, manipulate the DOM, make network requests, access cookies/localStorage, or perform any browser-level action. The blast radius is critical because the script runs with full page privileges and can do virtually anything the browser allows.

From the tool's definition "Evaluate a JavaScript function inside the currently selected page"

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 Chrome Devtools, 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 be JSON-serializable. It is categorised as a Execute tool in the Chrome Devtools 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 Chrome Devtools 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 Chrome Devtools. 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 Chrome Devtools MCP server (nimbus21/chrome-devtools-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.