cast_url · Kef MCP: Risk & Policy

WHAT IT DOES

What cast_url does on Kef

AI agents invoke cast_url to trigger actions in Kef. 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 cast_url needs a policy

This tool initiates an external operation (casting audio to a physical speaker device) whose effects depend on the URL argument provided. It's not a simple read or write to a data store, but an execution of a media streaming action on external hardware. Misuse could involve playing arbitrary or malicious audio streams on the device.

From the tool's definition "Play a direct audio stream URL on the KEF via Google Cast" — triggers external playback operation on a physical device via Google Cast protocol

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

cast_url Rate-limited

cast_url 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": {
    "cast_url": {
      "limits": [
        {
          "counter": "cast_url_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 Kef, so you can run agents against it safely.

FAQ

Questions about cast_url

What does the cast_url tool do? +

Play a direct audio stream URL on the KEF via Google Cast. It is categorised as a Execute tool in the Kef MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on cast_url? +

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

What risk level is cast_url? +

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

Can I rate-limit cast_url? +

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

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

cast_url is provided by the Kef MCP server (nqrwhal/kef-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.