browser_evaluate

Evaluate JavaScript expression on page or element

SERVERCowork Bench SOURCE0717376/cowork_bench

High RISK CLASS

Category Execute

Parameters 00 required

Recommended Rate-limitedsee the rule below

Registry record Grade F, identity unverified Pull the record →

This record as markdown: /tools/cowork-bench/browser-evaluate.md

WHAT IT DOES

What browser_evaluate does on Cowork Bench

AI agents invoke browser_evaluate to trigger actions in Cowork Bench. 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.

RISK

Why browser_evaluate is rated High

This tool executes arbitrary JavaScript in a browser context, which can perform virtually any action in the page including reading sensitive data, modifying DOM, making network requests, or triggering other side effects. Arbitrary code execution in a browser context is a high-severity Execute risk.

From the tool's definition "Evaluate JavaScript expression on page or element"

Attacks that exploit this kind of access

RECOMMENDED RULE

The rule that runs browser_evaluate safely

PolicyLayer is an MCP gateway: it sits between your AI agents and Cowork Bench, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For browser_evaluate, this is the rule to start with:

browser_evaluate Rate-limited

browser_evaluate 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_evaluate": {
      "limits": [
        {
          "counter": "browser_evaluate_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL → Instant setup, no code required.

The button opens the PolicyLayer dashboard: create your workspace, connect Cowork Bench, apply this rule, and every browser_evaluate call is checked against it from then on.

FAQ

Questions about browser_evaluate

What does the browser_evaluate tool do? +

Evaluate JavaScript expression on page or element. It is categorised as a Execute tool in the Cowork Bench 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_evaluate? +

Register the Cowork Bench MCP server in PolicyLayer and add a rule for browser_evaluate: 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 Cowork Bench. Nothing to install.

What risk level is browser_evaluate? +

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

Can I rate-limit browser_evaluate? +

Yes. Add a rate_limit block to the browser_evaluate 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_evaluate completely? +

Set action: deny in the PolicyLayer policy for browser_evaluate. 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_evaluate? +

browser_evaluate is provided by the Cowork Bench MCP server (0717376/cowork_bench). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

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