Append to string value. Args: key: The name of the key value: String to append Returns: Success message or error message
AI agents use string_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.
This tool modifies an existing string value by appending data to it. This is a reversible write operation (the original value can be restored), but misuse could corrupt stored data. Severity is medium because it can modify potentially important configuration or data strings in what appears to be a key-value store.
From the tool's definition Append to string value
Documented attack patterns abuse exactly the kind of access string_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 string_append:
{
"version": "1",
"default": "deny",
"tools": {
"string_append": {
"limits": [
{
"counter": "string_append_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}string_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 to string value. Args: key: The name of the key value: String 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 string_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.
string_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 string_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 string_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.
string_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.