copy_lines · Filesystem MCP: Risk & Policy

WHAT IT DOES

What copy_lines does on FilesystemMCP

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

WHY IT NEEDS A POLICY

Why copy_lines needs a policy

This tool modifies files by inserting copied content, making it a Write operation (reversible modification). It does not delete or permanently alter existing lines, so it is not Destructive.

From the tool's definition Tool description states it will 'Copy a range of lines and insert the copy after a given line number', which creates new data within a file by duplicating existing content.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

copy_lines Rate-limited

copy_lines 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": {
    "copy_lines": {
      "limits": [
        {
          "counter": "copy_lines_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 FilesystemMCP, so you can run agents against it safely.

FAQ

Questions about copy_lines

What does the copy_lines tool do? +

Copy a range of lines and insert the copy after a given line number. Original lines are preserved. Use with move_lines to reorganise content without rewriting. Only works within allowed directories. It is categorised as a Write tool in the FilesystemMCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on copy_lines? +

Register the Filesystem MCP server in PolicyLayer and add a rule for copy_lines: 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 FilesystemMCP. Nothing to install.

What risk level is copy_lines? +

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

Can I rate-limit copy_lines? +

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

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

copy_lines is provided by the Filesystem MCP server (rijadalisic/filesystemmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.