json_strappend

Append a string to the string at path. Args: key: The name of the key path: The path in the JSON document value: The string to append Returns: New string length or error message

SERVERAWS SOURCE@awslabs/mcp

Medium RISK CLASS

Category Write

Parameters 00 required

Recommended Rate-limitedsee the rule below

Registry record Grade F, identity verified Pull the record →

This record as markdown: /tools/aws/json-strappend.md

WHAT IT DOES

What json_strappend does on AWS

AI agents use json_strappend to create or update resources in AWS, usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your AWS environment.

RISK

Why json_strappend is rated Medium

This tool modifies existing data by appending a string value to a JSON document at a specified path. It is a reversible write operation (the original value is changed but the key/document still exists). Misuse could corrupt JSON data stored in AWS, but it does not delete data or execute code.

From the tool's definition Append a string to the string at path

Attacks that exploit this kind of access

RECOMMENDED RULE

The rule that runs json_strappend safely

PolicyLayer is an MCP gateway: it sits between your AI agents and AWS, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For json_strappend, this is the rule to start with:

json_strappend Rate-limited

json_strappend 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.

View as policy code

policy.json

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

LIMIT THIS TOOL → Instant setup, no code required.

The button opens the PolicyLayer dashboard: create your workspace, connect AWS, apply this rule, and every json_strappend call is checked against it from then on.

FAQ

Questions about json_strappend

What does the json_strappend tool do? +

Append a string to the string at path. Args: key: The name of the key path: The path in the JSON document value: The string to append Returns: New string length or error message. It is categorised as a Write tool in the AWS MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on json_strappend? +

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

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

Can I rate-limit json_strappend? +

Yes. Add a rate_limit block to the json_strappend 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_strappend completely? +

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

json_strappend 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.

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