pubsub.subscribe · MCP: Risk & Policy

WHAT IT DOES

What pubsub.subscribe does on Mcp

AI agents invoke pubsub.subscribe to trigger actions in Mcp. 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 pubsub.subscribe needs a policy

Subscribing registers an endpoint to receive future push notifications/callbacks, which constitutes triggering an external operation with side effects beyond simple data retrieval or creation. The blast radius is medium: a misused subscription could cause repeated callbacks to arbitrary URLs or rack up pay-per-call charges on the x402/USDC billing system.

From the tool's definition 'subscribe a callbackUrl to a topic by its topicId' — triggers an external operation (subscription registration) that causes ongoing callbacks to be sent to an external URL

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

pubsub.subscribe Rate-limited

pubsub.subscribe 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": {
    "pubsub.subscribe": {
      "limits": [
        {
          "counter": "pubsub.subscribe_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 Mcp, so you can run agents against it safely.

FAQ

Questions about pubsub.subscribe

What does the pubsub.subscribe tool do? +

PUBSUB: subscribe a callbackUrl to a topic by its topicId. It is categorised as a Execute tool in the Mcp MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on pubsub.subscribe? +

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

What risk level is pubsub.subscribe? +

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

Can I rate-limit pubsub.subscribe? +

Yes. Add a rate_limit block to the pubsub.subscribe 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 pubsub.subscribe completely? +

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

pubsub.subscribe is provided by the MCP server (@2sio/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.