Pop a value from the array at path and index. Args: key: The name of the key path: The path in the JSON document index: The index to pop from (-1 for last element) Returns: Popped value or error message
AI agents use json_arrpop 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.
Popping a value from a JSON array modifies the data structure by removing an element, which is a reversible write operation (the data could be re-inserted). It does not irreversibly destroy data in a destructive sense since the popped value is returned, but it does mutate the stored JSON document.
From the tool's definition Pop a value from the array at path and index
Documented attack patterns abuse exactly the kind of access json_arrpop 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_arrpop:
{
"version": "1",
"default": "deny",
"tools": {
"json_arrpop": {
"limits": [
{
"counter": "json_arrpop_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}json_arrpop 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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Pop a value from the array at path and index. Args: key: The name of the key path: The path in the JSON document index: The index to pop from (-1 for last element) Returns: Popped value 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_arrpop: 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_arrpop 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_arrpop 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_arrpop. 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_arrpop 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.