add_mcp_server · MCP Router MCP: Risk & Policy

WHAT IT DOES

What add_mcp_server does on MCP Router

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

WHY IT NEEDS A POLICY

Why add_mcp_server needs a policy

This is a Write operation—it creates and registers new MCP server entries in the discovery service, which is reversible (servers can be deregistered). The severity is medium because unauthorized registration could allow an attacker to inject malicious MCP servers into the routing fabric, potentially redirecting subsequent tool requests to attacker-controlled services.

From the tool's definition Tool description states 'Register a new MCP server' which is a create/add operation that modifies the service discovery registry by adding new server entries.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

add_mcp_server Rate-limited

add_mcp_server 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": {
    "add_mcp_server": {
      "limits": [
        {
          "counter": "add_mcp_server_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 MCP Router, so you can run agents against it safely.

FAQ

Questions about add_mcp_server

What does the add_mcp_server tool do? +

Register a new MCP server with the discovery service. It is categorised as a Write tool in the MCP Router MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on add_mcp_server? +

Register the MCP Router MCP server in PolicyLayer and add a rule for add_mcp_server: 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 MCP Router. Nothing to install.

What risk level is add_mcp_server? +

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

Can I rate-limit add_mcp_server? +

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

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

add_mcp_server is provided by the MCP Router MCP server (maverick-ljxuan/mcp-router). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.