browser_execute_js · Browser MCP: Risk & Policy

WHAT IT DOES

What browser_execute_js does on BrowserMCP

AI agents invoke browser_execute_js to trigger actions in BrowserMCP. 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 browser_execute_js needs a policy

browser_execute_js triggers real processes with real consequences. An agent gone sideways doesn't fire it once — it starts dozens of builds, sends mass notifications, or burns through compute before anyone looks up.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

browser_execute_js Rate-limited

browser_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": {
    "browser_execute_js": {
      "limits": [
        {
          "counter": "browser_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 BrowserMCP, so you can run agents against it safely.

FAQ

Questions about browser_execute_js

What does the browser_execute_js tool do? +

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

How do I enforce a policy on browser_execute_js? +

Register the Browser MCP server in PolicyLayer and add a rule for browser_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 BrowserMCP. Nothing to install.

What risk level is browser_execute_js? +

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

Can I rate-limit browser_execute_js? +

Yes. Add a rate_limit block to the browser_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 browser_execute_js completely? +

Set action: deny in the PolicyLayer policy for browser_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 browser_execute_js? +

browser_execute_js is provided by the Browser MCP server (smotree/browsermcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.