browser_eval · Byob MCP: Risk & Policy

WHAT IT DOES

What browser_eval does on Byob

AI agents invoke browser_eval to trigger actions in Byob. 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_eval needs a policy

This tool enables execution of arbitrary JavaScript code in the context of a user's authenticated browser session. The blast radius is critical because: (1) arbitrary code execution can access/exfiltrate sensitive data from any open webpage, (2) it can perform authenticated actions (e.g., financial transactions, data modification) on behalf of the user, (3) it can modify page content or inject malware-like payloads,…

From the tool's definition Tool allows 'Run arbitrary JavaScript in a browser tab via CDP Runtime.evaluate' - the word 'arbitrary' indicates unrestricted code execution with full access to the browser environment, DOM, and potentially sensitive data or actions available to the…

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

browser_eval Rate-limited

browser_eval 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_eval": {
      "limits": [
        {
          "counter": "browser_eval_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 Byob, so you can run agents against it safely.

FAQ

Questions about browser_eval

What does the browser_eval tool do? +

Run arbitrary JavaScript in a browser tab via CDP Runtime.evaluate. It is categorised as a Execute tool in the Byob 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_eval? +

Register the Byob MCP server in PolicyLayer and add a rule for browser_eval: 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 Byob. Nothing to install.

What risk level is browser_eval? +

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

Can I rate-limit browser_eval? +

Yes. Add a rate_limit block to the browser_eval 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_eval completely? +

Set action: deny in the PolicyLayer policy for browser_eval. 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_eval? +

browser_eval is provided by the Byob MCP server (wxtsky/byob). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.