list_set

Set value at index in list. Args: key: The name of the key index: The index (0-based, negative indices supported) value: The value to set Returns: Success message or error message

ServerAWS
CategoryWrite

How to control list_set ↓

WHAT IT DOES

What list_set does on AWS

AI agents use list_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.

THE RISK

Medium Risk

Why list_set needs a policy

The tool writes/updates a value in a list data structure at a given index. Despite being named 'list_set', the description confirms it sets (overwrites) a value, which is a reversible write operation. Confidence is slightly reduced because the description contradicts the name ('list_set' vs 'Set value at index in list'), and the server context (AWS) is unclear for this Redis-like list operation.

From the tool's definition 'Set value at index in list' — modifies an existing list element at a specified index

Documented attack patterns abuse exactly the kind of access list_set gives an agent:

POLICY

How to control list_set

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_set:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "list_set": {
      "limits": [
        {
          "counter": "list_set_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

list_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.

Create a free account and register AWS — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

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EXPLORE

Related tools and policies

More AWS tools

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All 300 AWS tools →

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Go deeper

Govern customer-ops agents → PII redaction, approval on external messages, and no-bulk-delete rules on the records agents change.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →
PolicyLayer vs alternatives → How deterministic, per-call MCP policy compares with gateways, identity layers, and authorisation platforms.

FAQ

Questions about list_set

What does the list_set tool do? +

Set value at index in list. Args: key: The name of the key index: The index (0-based, negative indices supported) value: The value to set 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.

How do I enforce a policy on list_set? +

Register the AWS MCP server in PolicyLayer and add a rule for list_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.

What risk level is list_set? +

list_set is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit list_set? +

Yes. Add a rate_limit block to the list_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.

How do I block list_set completely? +

Set action: deny in the PolicyLayer policy for list_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.

What MCP server provides list_set? +

list_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.

Enforce policy on every AWS tool call.

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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