set_user_agent · JS Reverse MCP: Risk & Policy

WHAT IT DOES

What set_user_agent does on JS Reverse MCP

AI agents use set_user_agent to create or update resources in JS Reverse MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your JS Reverse MCP environment.

WHY IT NEEDS A POLICY

Why set_user_agent needs a policy

This tool modifies HTTP headers sent by the browser (user-agent string), which affects how the page is processed and tracked. While reversible (can be changed back), it alters the browser's presentation and may affect behavior of web applications, logging, analytics, and request handling.

From the tool's definition Tool name 'set_user_agent' and description 'Set custom user-agent for active page' indicate modification of browser state/headers for the active page.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_user_agent Rate-limited

set_user_agent stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "set_user_agent": {
      "limits": [
        {
          "counter": "set_user_agent_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about set_user_agent

What does the set_user_agent tool do? +

Set custom user-agent for active page. It is categorised as a Write tool in the JS Reverse MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_user_agent? +

Register the JS Reverse MCP server in PolicyLayer and add a rule for set_user_agent: 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 JS Reverse MCP. Nothing to install.

What risk level is set_user_agent? +

set_user_agent is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit set_user_agent? +

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

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

set_user_agent is provided by the JS Reverse MCP server (noone-hub/jsreverser-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.