browser_evaluate · Yaver MCP: Risk & Policy

WHAT IT DOES

What browser_evaluate does on Yaver

AI agents invoke browser_evaluate to trigger actions in Yaver. 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.

PARAMETERS

Parameter	Type	Required	Description
`javascript`	string	Yes	JavaScript code to execute. Return value is sent back as JSON.
`session_id`	string	Yes	Session ID

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why browser_evaluate needs a policy

browser_evaluate 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.

Risk signalsAccepts freeform code/query input (javascript)

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

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.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Yaver, so you can run agents against it safely.

FAQ

Questions about browser_evaluate

What does the browser_evaluate tool do? +

Execute JavaScript in the browser and return the result. Use for complex interactions, reading localStorage/cookies, or extracting data that CSS selectors can't reach. It is categorised as a Execute tool in the Yaver MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

What parameters does browser_evaluate accept? +

browser_evaluate accepts 2 parameters: javascript, session_id. Required: javascript, session_id. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on browser_evaluate? +

Register the Yaver 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 Yaver. 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 Yaver MCP server (yaver-cli). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.