arc_execute_javascript · Arc Browser MCP: Risk & Policy

WHAT IT DOES

What arc_execute_javascript does on Arc Browser MCP

AI agents invoke arc_execute_javascript to trigger actions in Arc Browser MCP. 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 arc_execute_javascript needs a policy

Executing arbitrary JavaScript in a browser tab can trigger side effects including reading sensitive data from web pages, modifying DOM content, exfiltrating credentials, performing unauthorized actions on behalf of the user on websites, and interacting with third-party services.

From the tool's definition Tool name 'arc_execute_javascript' and description 'Execute JavaScript in an Arc tab' explicitly indicate arbitrary code execution capabilities within a browser tab context.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

arc_execute_javascript Rate-limited

arc_execute_javascript 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": {
    "arc_execute_javascript": {
      "limits": [
        {
          "counter": "arc_execute_javascript_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 Arc Browser MCP, so you can run agents against it safely.

FAQ

Questions about arc_execute_javascript

What does the arc_execute_javascript tool do? +

Execute JavaScript in an Arc tab. Disabled unless ARC_MCP_ENABLE_JAVASCRIPT=1. It is categorised as a Execute tool in the Arc Browser 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 arc_execute_javascript? +

Register the Arc Browser MCP server in PolicyLayer and add a rule for arc_execute_javascript: 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 Arc Browser MCP. Nothing to install.

What risk level is arc_execute_javascript? +

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

Can I rate-limit arc_execute_javascript? +

Yes. Add a rate_limit block to the arc_execute_javascript 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 arc_execute_javascript completely? +

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

arc_execute_javascript is provided by the Arc Browser MCP server (jasoncronje/arc-browser-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.