subscribe_to_channel · Spline MCP: Risk & Policy

WHAT IT DOES

What subscribe_to_channel does on Spline

AI agents invoke subscribe_to_channel to trigger actions in Spline. 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 subscribe_to_channel needs a policy

Subscribing to a WebSocket channel initiates an ongoing external connection/operation that triggers real-time data streaming. This goes beyond a simple read since it establishes a persistent connection and registers a listener for continuous updates, qualifying as an external operation execution. The blast radius is medium as misuse could result in unintended persistent connections or data exposure.

From the tool's definition Subscribe to a WebSocket channel for real-time updates

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

subscribe_to_channel Rate-limited

subscribe_to_channel 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": {
    "subscribe_to_channel": {
      "limits": [
        {
          "counter": "subscribe_to_channel_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

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

FAQ

Questions about subscribe_to_channel

What does the subscribe_to_channel tool do? +

Subscribe to a WebSocket channel for real-time updates. It is categorised as a Execute tool in the Spline MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on subscribe_to_channel? +

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

What risk level is subscribe_to_channel? +

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

Can I rate-limit subscribe_to_channel? +

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

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

subscribe_to_channel is provided by the Spline MCP server (lesleslie/spline-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.