json_arrappend · AWS Labs postgres MCP Server MCP

WHAT IT DOES

What json_arrappend does on AWS Labs postgres MCP Server

AI agents use json_arrappend 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 json_arrappend needs a policy

This tool modifies existing data by appending values to a JSON array at a specified path. This is a reversible write operation (values can be removed), not destructive. Misuse could corrupt data structures in the database, giving it medium severity.

From the tool's definition Append values to the array at path

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

json_arrappend Rate-limited

json_arrappend 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": {
    "json_arrappend": {
      "limits": [
        {
          "counter": "json_arrappend_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 json_arrappend

What does the json_arrappend tool do? +

Append values to the array at path. 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 json_arrappend? +

Register the AWS Labs postgres MCP Server MCP server in PolicyLayer and add a rule for json_arrappend: 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 json_arrappend? +

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

Can I rate-limit json_arrappend? +

Yes. Add a rate_limit block to the json_arrappend 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 json_arrappend completely? +

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

json_arrappend 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.