Set a value using CAS (Check And Set). Args: key: The key to set value: The value to store cas: CAS token from gets() expire: Optional expiration time in seconds Returns: Success message or error message
AI agents use cache_cas 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 is a cache write operation that modifies data reversibly. CAS (Compare-And-Swap) is a conditional update mechanism, so the operation is Write category rather than Read (it modifies state) or Execute (it's a data storage primitive, not arbitrary code execution).
From the tool's definition Tool is named 'cache_cas' and described as 'Set a value using CAS (Check And Set)' which stores data conditionally. The parameters include 'key', 'value', 'cas' token, and optional 'expire', indicating atomic write operations to a cache system.
Documented attack patterns abuse exactly the kind of access cache_cas 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 cache_cas:
{
"version": "1",
"default": "deny",
"tools": {
"cache_cas": {
"limits": [
{
"counter": "cache_cas_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}cache_cas 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 a value using CAS (Check And Set). Args: key: The key to set value: The value to store cas: CAS token from gets() expire: Optional expiration time in seconds 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 cache_cas: 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.
cache_cas 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 cache_cas 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 cache_cas. 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.
cache_cas 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.