browser_list_instances

THE RISK

Low Risk

Why browser_list_instances needs a policy

This tool retrieves information about existing browser instances without creating, modifying, deleting, or executing any actions. It is a read-only operation that returns state information. The blast radius is minimal since listing instances cannot harm systems or data.

From the tool's definition Tool name is 'browser_list_instances' and description states 'List all browser instances' — a pure query operation with no side effects.

Documented attack patterns abuse exactly the kind of access browser_list_instances gives an agent:

POLICY

How to control browser_list_instances

PolicyLayer is an MCP gateway — it sits between your AI agents and Concurrent Browser MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for browser_list_instances:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "browser_list_instances": {}
  }
}

browser_list_instances is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.

Create a free account and register Concurrent Browser MCP — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

CAP THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Concurrent Browser MCP tools

Destructive browser_close_all_instances Close all browser instances Destructive browser_close_instance Close the specified browser instance Execute browser_evaluate Execute JavaScript code in the page context Execute browser_navigate Navigate to a specified URL Execute browser_wait_for_element Wait for an element to appear Execute browser_wait_for_navigation Wait for page navigation to complete Execute browser_click Click on a page element Execute browser_create_instance Create a new browser instance

All 20 Concurrent Browser MCP tools →

Read tools on other servers

M-Team MCP Server get_torrent_detail YouTube MCP Server search_channel Android Forensics ADB MCP Server parse_browser_history 0xarchive web3_challenge

Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Data exfiltration → How read access becomes an exfiltration channel when chained with untrusted content.
MCP token cost → What connecting this server costs in context-window tokens on every request.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about browser_list_instances

What does the browser_list_instances tool do? +

List all browser instances. It is categorised as a Read tool in the Concurrent Browser MCP MCP Server, which means it retrieves data without modifying state.

How do I enforce a policy on browser_list_instances? +

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

What risk level is browser_list_instances? +

browser_list_instances is a Read tool with low risk. Read-only tools are generally safe to allow by default.

Can I rate-limit browser_list_instances? +

Yes. Add a rate_limit block to the browser_list_instances 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_list_instances completely? +

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

browser_list_instances is provided by the Concurrent Browser MCP server (sailaoda/concurrent-browser-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Concurrent Browser MCP tool call.

Start from Concurrent Browser MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →