append_file

THE RISK

Medium Risk

Why append_file needs a policy

This tool modifies files by appending content, or creates new files if they don't exist. This is a reversible write operation (content can be removed), making it a Write category tool. Medium severity because an AI agent misusing it could corrupt files or fill up disk space, but changes are not inherently irreversible.

From the tool's definition Append content to the end of a file. Creates the file if it doesn

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

POLICY

How to control append_file

PolicyLayer is an MCP gateway — it sits between your AI agents and Claude TypeScript MCP Servers, and nothing reaches the server without passing your rules. This is the rule we recommend for append_file:

policy.json

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

append_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.

Create a free account and register Claude TypeScript MCP Servers — 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 →

Free to start. No card required.

EXPLORE

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Write tools on other servers

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Go deeper

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 →

FAQ

Questions about append_file

What does the append_file tool do? +

Append content to the end of a file. Creates the file if it doesn. It is categorised as a Write tool in the Claude TypeScript MCP Servers 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_file? +

Register the Claude TypeScript MCP Servers MCP server in PolicyLayer and add a rule for append_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 Claude TypeScript MCP Servers. Nothing to install.

What risk level is append_file? +

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

Can I rate-limit append_file? +

Yes. Add a rate_limit block to the append_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_file completely? +

Set action: deny in the PolicyLayer policy for append_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_file? +

append_file is provided by the Claude TypeScript MCP Servers MCP server (ukkz/claude-ts-mcps). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Claude TypeScript MCP Servers tool call.

Start from Claude TypeScript MCP Servers, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →