browser_close_instance

THE RISK

Critical Risk

Why browser_close_instance needs a policy

Closing a browser instance is an irreversible action that terminates the instance and destroys its state (open tabs, session data, in-memory context). Unlike navigating back or modifying content, this cannot be undone — the instance and all its associated state are gone. Severity is medium because it affects only a single browser instance, not persistent user data or external systems.

From the tool's definition Close the specified browser instance

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

POLICY

How to control browser_close_instance

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_close_instance:

policy.json

{
  "version": "1",
  "default": "deny",
  "hide": [
    "browser_close_instance"
  ]
}

browser_close_instance disappears from the agent's tool list entirely, and any attempt to call it is denied. The rest of the server keeps working.

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.

RESTRICT 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 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 Execute browser_fill Fill a form field

All 20 Concurrent Browser MCP tools →

Destructive tools on other servers

Android Forensics ADB MCP Server create_full_backup 0xarchive web3_revoke_key Frida Game Hacking MCP clear_scan cPanel MCP Server clear_spam_box

Go deeper

Critical-risk tools in Concurrent Browser MCP → Every critical-risk tool this server exposes, in one view.
All critical-risk MCP tools → Every critical-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Blast radius → How much damage one compromised or confused agent can do — and how to bound it.
Tool call approval → Approval gates on individual tool calls: auto-allow, prompt, or deny.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about browser_close_instance

What does the browser_close_instance tool do? +

Close the specified browser instance. It is categorised as a Destructive tool in the Concurrent Browser MCP MCP Server, which means it can permanently delete or destroy data. Block by default and require explicit approval.

How do I enforce a policy on browser_close_instance? +

Register the Concurrent Browser MCP server in PolicyLayer and add a rule for browser_close_instance: 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_close_instance? +

browser_close_instance is a Destructive tool with critical risk. Critical-risk tools should be blocked by default and only enabled with explicit human approval.

Can I rate-limit browser_close_instance? +

Yes. Add a rate_limit block to the browser_close_instance 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_close_instance completely? +

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

browser_close_instance 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 →