Set string value. Args: key: The name of the key value: The value to set ex: Expire time in seconds px: Expire time in milliseconds nx: Only set if key does not exist xx: Only set if key exists keepttl: Retain the time to live associated with the key Returns: Success message or error message
AI agents use string_set 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 creates or modifies data in a key-value store (likely Redis given the parameter names ex/px/nx/xx/keepttl which are Redis SET command options). It is reversible (values can be overwritten or deleted), so it is Write rather than Destructive.
From the tool's definition Tool description states 'Set string value' with parameters to create or modify key-value pairs (key, value, ex, px, nx, xx, keepttl options). Returns 'Success message or error message' indicating a write operation.
Documented attack patterns abuse exactly the kind of access string_set 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_set:
{
"version": "1",
"default": "deny",
"tools": {
"string_set": {
"limits": [
{
"counter": "string_set_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}string_set 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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Set string value. Args: key: The name of the key value: The value to set ex: Expire time in seconds px: Expire time in milliseconds nx: Only set if key does not exist xx: Only set if key exists keepttl: Retain the time to live associated with the key 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_set: 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_set 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_set 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_set. 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_set 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.