lemmy_follow_community

THE RISK

Medium Risk

Why lemmy_follow_community needs a policy

Following/subscribing to a community is a reversible write action that modifies the user's subscription state. It creates a relationship between the user and a community but does not delete data or execute code. Can be undone by unfollowing.

From the tool's definition Subscribe to a community (local or federated)

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

POLICY

How to control lemmy_follow_community

PolicyLayer is an MCP gateway — it sits between your AI agents and Crow, and nothing reaches the server without passing your rules. This is the rule we recommend for lemmy_follow_community:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "lemmy_follow_community": {
      "limits": [
        {
          "counter": "lemmy_follow_community_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

lemmy_follow_community 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.

Create a free account and register Crow — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

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EXPLORE

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
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FAQ

Questions about lemmy_follow_community

What does the lemmy_follow_community tool do? +

Subscribe to a community (local or federated). Accepts numeric community ID or. It is categorised as a Write tool in the Crow MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on lemmy_follow_community? +

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

What risk level is lemmy_follow_community? +

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

Can I rate-limit lemmy_follow_community? +

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

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

lemmy_follow_community is provided by the Crow MCP server (kh0pper/crow). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Crow tool call.

Start from Crow, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

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