lemmy_block_user

THE RISK

Medium Risk

Why lemmy_block_user needs a policy

Blocking a user modifies account state (block list) in a reversible way — the block can be undone. It does not delete content or move money, but it does create a persistent relationship change. Severity is medium because misuse could disrupt the user's social experience by silencing legitimate users.

From the tool's definition Block a person (hide their posts + comments from your view)

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

POLICY

How to control lemmy_block_user

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_block_user:

policy.json

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

lemmy_block_user 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.

LIMIT THIS TOOL →

Free to start. No card required.

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.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about lemmy_block_user

What does the lemmy_block_user tool do? +

Block a person (hide their posts + comments from your view). Rate-limited: 5/hour. 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_block_user? +

Register the Crow MCP server in PolicyLayer and add a rule for lemmy_block_user: 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_block_user? +

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

Can I rate-limit lemmy_block_user? +

Yes. Add a rate_limit block to the lemmy_block_user 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_block_user completely? +

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

lemmy_block_user 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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