json_arrlen

THE RISK

Low Risk

Why json_arrlen needs a policy

The tool queries and retrieves metadata (array length) from a JSON document without modifying, deleting, or executing anything. It is purely informational, making it a Read operation with low severity since misuse only exposes data structure information, not sensitive data directly.

From the tool's definition Tool description states 'Get the length of array at path' with args for key and path, returning 'Array length or error message'. This is a retrieval operation with no side effects.

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

POLICY

How to control json_arrlen

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 json_arrlen:

policy.json

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

json_arrlen 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 →
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FAQ

Questions about json_arrlen

What does the json_arrlen tool do? +

Get the length of array at path. Args: key: The name of the key path: The path in the JSON document Returns: Array 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 json_arrlen? +

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

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

Can I rate-limit json_arrlen? +

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

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

json_arrlen 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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