set_cardinality

THE RISK

Low Risk

Why set_cardinality needs a policy

This tool only retrieves the count of members in a set (akin to SCARD in Redis). It performs a read-only operation with no side effects, modifications, or deletions. Severity is low as it only exposes a numeric count value.

From the tool's definition Get number of members in set... Returns: Number of members or error message

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

POLICY

How to control set_cardinality

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

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "set_cardinality": {}
  }
}

set_cardinality is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.

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

CAP THIS TOOL →

Free to start. No card required.

EXPLORE

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Go deeper

Roll out MCP under one policy → Per-person scoped tokens, one central policy, and full audit across every server your team connects.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Data exfiltration → How read access becomes an exfiltration channel when chained with untrusted content.
MCP token cost → What connecting this server costs in context-window tokens on every request.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →
PolicyLayer vs alternatives → How deterministic, per-call MCP policy compares with gateways, identity layers, and authorisation platforms.

FAQ

Questions about set_cardinality

What does the set_cardinality tool do? +

Get number of members in set. Args: key: The name of the key Returns: Number of members or error message. It is categorised as a Read tool in the AWS MCP Server, which means it retrieves data without modifying state.

How do I enforce a policy on set_cardinality? +

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

What risk level is set_cardinality? +

set_cardinality is a Read tool with low risk. Read-only tools are generally safe to allow by default.

Can I rate-limit set_cardinality? +

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

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

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

Enforce policy on every AWS tool call.

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

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