list_length

THE RISK

Low Risk

Why list_length needs a policy

The tool performs a read-only operation (getting the length of a list) with no side effects. It queries data and returns information only. No data is created, modified, deleted, or executed. This is a straightforward Read category tool with low risk—misuse would only result in information disclosure about list sizes.

From the tool's definition Tool description states 'Get length of list' with an argument 'key: The name of the key' and returns 'Length or error message'. This is a query operation that retrieves metadata about a data structure without modifying, deleting, or executing arbitrary code.

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

POLICY

How to control list_length

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 list_length:

policy.json

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

list_length 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.

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EXPLORE

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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 list_length

What does the list_length tool do? +

Get length of list. Args: key: The name of the key Returns: Length 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 list_length? +

Register the AWS MCP server in PolicyLayer and add a rule for list_length: 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 list_length? +

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

Can I rate-limit list_length? +

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

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

list_length 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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