Append value to list. Args: key: The name of the key value: The value to append Returns: Success message or error message
AI agents use list_append 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.
The tool modifies a list by appending a value, which is a reversible write operation. It does not delete data or execute code. Severity is medium because misuse could corrupt or pollute data structures in what appears to be an AWS-connected service, but it is not irreversible.
From the tool's definition 'Append value to list' — creates or modifies data by appending a value to an existing list
Documented attack patterns abuse exactly the kind of access list_append 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 list_append:
{
"version": "1",
"default": "deny",
"tools": {
"list_append": {
"limits": [
{
"counter": "list_append_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}list_append 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 value to list. Args: key: The name of the key value: The value to append Returns: Success message 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 list_append: 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.
list_append 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 list_append 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 list_append. 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.
list_append 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.
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300 AWS tools catalogued and risk-classified — across an index of 43,000+ MCP servers.