evaluate_script

THE RISK

High Risk

Why evaluate_script needs a policy

This tool executes arbitrary JavaScript in the context of a browser page. An AI agent could use it to run any code — exfiltrate data, manipulate the DOM, steal credentials, perform actions on behalf of the user, or interact with any APIs accessible from the page context. The blast radius is critical because arbitrary code execution in a browser page has no inherent scope limitations.

From the tool's definition Evaluate JavaScript code inside the currently selected page, similar to DevTools Console

Documented attack patterns abuse exactly the kind of access evaluate_script gives an agent:

POLICY

How to control evaluate_script

PolicyLayer is an MCP gateway — it sits between your AI agents and ReverseCraft DevTools MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for evaluate_script:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "evaluate_script": {
      "limits": [
        {
          "counter": "evaluate_script_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

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.

Create a free account and register ReverseCraft DevTools MCP — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More ReverseCraft DevTools MCP tools

Destructive clear_cookies Clear browser cookies for all sites or a specific domain. Destructive clear_persistent_scripts Remove all registered persistent scripts for the current page at once. This is useful for Destructive clear_script_replacements Remove all script replacement rules for the current page. Destructive remove_persistent_script Remove a previously registered persistent script by its identifier. After removal, the sc Destructive remove_script_replacement Remove a script replacement rule by its ID. Destructive remove_xhr_breakpoint Remove a previously set XHR/Fetch breakpoint by its URL pattern, or remove all XHR breakpo Destructive close_page Closes the page by its index. The last open page cannot be closed. Execute navigate_page Navigates the currently selected page to a URL.

All 46 ReverseCraft DevTools MCP tools →

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Go deeper

High-risk tools in ReverseCraft DevTools MCP → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about evaluate_script

What does the evaluate_script tool do? +

Evaluate JavaScript code inside the currently selected page, similar to DevTools Console. It is categorised as a Execute tool in the ReverseCraft DevTools 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 ReverseCraft 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 ReverseCraft DevTools 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 ReverseCraft DevTools MCP server (reverse-craft/rc-devtools-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every ReverseCraft DevTools MCP tool call.

Start from ReverseCraft DevTools MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →