cache_prepend · AWS Labs postgres MCP Server MCP

WHAT IT DOES

What cache_prepend does on AWS Labs postgres MCP Server

AI agents use cache_prepend to create or update resources in AWS Labs postgres MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your AWS Labs postgres MCP Server environment.

WHY IT NEEDS A POLICY

Why cache_prepend needs a policy

The tool modifies existing data by prepending a string to it. This is a reversible write operation (the original data can potentially be restored), but it alters existing stored values. The description is minimal, but 'prepend to an existing value' clearly indicates mutation of existing data without destruction. Severity is medium as misuse could corrupt cached data but is not irreversible in most cache contexts.

From the tool's definition Prepend a string to an existing value

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

cache_prepend Rate-limited

cache_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.

View as policy code

policy.json

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

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches AWS Labs postgres MCP Server, so you can run agents against it safely.

FAQ

Questions about cache_prepend

What does the cache_prepend tool do? +

Prepend a string to an existing value. It is categorised as a Write tool in the AWS Labs postgres 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 cache_prepend? +

Register the AWS Labs postgres MCP Server MCP server in PolicyLayer and add a rule for cache_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 AWS Labs postgres MCP Server. Nothing to install.

What risk level is cache_prepend? +

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

Can I rate-limit cache_prepend? +

Yes. Add a rate_limit block to the cache_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 cache_prepend completely? +

Set action: deny in the PolicyLayer policy for cache_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 cache_prepend? +

cache_prepend is provided by the AWS Labs postgres MCP Server MCP server (awslabs.postgres-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.