list_prepend | PolicyLayer

WHAT IT DOES

What list_prepend does on Amazon Bedrock Knowledge Base Retrieval MCP Server

AI agents use list_prepend to create or update resources in Amazon Bedrock Knowledge Base Retrieval MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Amazon Bedrock Knowledge Base Retrieval MCP Server environment.

THE RISK

Medium Risk

Why list_prepend needs a policy

Prepending to a list is a reversible write operation that modifies existing data. Confidence is low because the description is very minimal and the tool name/description don't clearly explain what list is being modified or in what context, making it hard to assess full impact. However, the operation is clearly a data modification (Write) rather than read-only.

From the tool's definition 'Prepend value to list' - modifies an existing list by adding data at the front

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

POLICY

How to control list_prepend

PolicyLayer is an MCP gateway — it sits between your AI agents and Amazon Bedrock Knowledge Base Retrieval MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for list_prepend:

policy.json

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

list_prepend 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 Amazon Bedrock Knowledge Base Retrieval MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

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EXPLORE

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Go deeper

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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_prepend

What does the list_prepend tool do? +

Prepend value to list. It is categorised as a Write tool in the Amazon Bedrock Knowledge Base Retrieval MCP Server 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_prepend? +

Register the Amazon Bedrock Knowledge Base Retrieval MCP Server MCP server in PolicyLayer and add a rule for list_prepend: 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 Amazon Bedrock Knowledge Base Retrieval MCP Server. Nothing to install.

What risk level is list_prepend? +

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

Can I rate-limit list_prepend? +

Yes. Add a rate_limit block to the list_prepend 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_prepend completely? +

Set action: deny in the PolicyLayer policy for list_prepend. 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_prepend? +

list_prepend is provided by the Amazon Bedrock Knowledge Base Retrieval MCP Server MCP server (awslabs.bedrock-kb-retrieval-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Amazon Bedrock Knowledge Base Retrieval MCP Server tool call.

Start from Amazon Bedrock Knowledge Base Retrieval MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

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