Append values to the array at path. Args: key: The name of the key path: The path in the JSON document *values: One or more values to append Returns: New array length or error message
AI agents use json_arrappend 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.
This tool modifies an existing JSON document by appending values to an array at a specified path. It is a reversible write operation (values can be removed), so it falls under Write rather than Destructive. Medium severity because it can modify data in a JSON store, potentially affecting application state if misused.
From the tool's definition Append values to the array at path
Documented attack patterns abuse exactly the kind of access json_arrappend gives an agent:
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_arrappend:
{
"version": "1",
"default": "deny",
"tools": {
"json_arrappend": {
"limits": [
{
"counter": "json_arrappend_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}json_arrappend 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.
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Append values to the array at path. Args: key: The name of the key path: The path in the JSON document *values: One or more values to append Returns: New array 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.
Register the AWS MCP server in PolicyLayer and add a rule for json_arrappend: 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.
json_arrappend is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the json_arrappend 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.
Set action: deny in the PolicyLayer policy for json_arrappend. 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.
json_arrappend 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.
Start from AWS, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
300 AWS tools catalogued and risk-classified — across an index of 43,000+ MCP servers.