json_strlen

THE RISK

Low Risk

Why json_strlen needs a policy

The json_strlen tool retrieves metadata (string length) from a JSON document without modifying, deleting, or executing any operations. It is purely informational and poses minimal risk of misuse by an AI agent.

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

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

POLICY

How to control json_strlen

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_strlen:

policy.json

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

json_strlen 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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FAQ

Questions about json_strlen

What does the json_strlen tool do? +

Get the length of string at path. Args: key: The name of the key path: The path in the JSON document Returns: String 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_strlen? +

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

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

Can I rate-limit json_strlen? +

Yes. Add a rate_limit block to the json_strlen 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_strlen completely? +

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

json_strlen 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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