append_to_file · Nexus Core MCP: Risk & Policy

WHAT IT DOES

What append_to_file does on Nexus Core

AI agents use append_to_file to create or update resources in Nexus Core — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Nexus Core environment.

WHY IT NEEDS A POLICY

Why append_to_file needs a policy

This tool writes data to files (appending to existing or creating new ones). It is a Write operation since appending is generally reversible (data can be removed). However, severity is high because an AI agent could append malicious content to critical system files, configuration files, or sensitive documents, potentially causing significant harm even if technically reversible.

From the tool's definition 'APPEND text to an existing or new file. Same security boundary as write_text_file.'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

append_to_file Rate-limited

append_to_file 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": {
    "append_to_file": {
      "limits": [
        {
          "counter": "append_to_file_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 Nexus Core, so you can run agents against it safely.

FAQ

Questions about append_to_file

What does the append_to_file tool do? +

APPEND text to an existing or new file. Same security boundary as write_text_file. It is categorised as a Write tool in the Nexus Core MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on append_to_file? +

Register the Nexus Core MCP server in PolicyLayer and add a rule for append_to_file: 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 Nexus Core. Nothing to install.

What risk level is append_to_file? +

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

Can I rate-limit append_to_file? +

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

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

append_to_file is provided by the Nexus Core MCP server (noumenon-ai/nexus-core). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.